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December 28

What is the minimum amount of heat it takes to burn you?

I realize that the confines of this question are rather vague, but please bear with me as I try to give further specifications.

For example, when the average person touches a hot stove top, he or she immediately jerks away his or her hand. So, in a sense, my question is what is the minimum temperature the stove top needs to be in order to cause the human reaction. Nkot (talk) 03:27, 28 December 2008 (UTC)

Heat is relative. So, it isn't a minimum amount of heat. It is a minimum difference of heat you are looking for. If you soak your hand in cold water for a bit and touch a "warm" stove, it will feel very hot. If you soak your hand in hot water for a while and touch a "warm" stove, it will feel cool. -- kainaw™ 03:29, 28 December 2008 (UTC)

In fact, it's even more complicated than that. I saw an exhibit with coils of metal that could be brought to different temperatures, and it turns out that if you have alternating warm and ice-cold coils, the combination feels hot. It's as if there are some receptors that sense warmth, and other receptors that sense extreme temperature (either hot or cold), and if you have alternating warm and cold coils, both receptors are excited. —Keenan Pepper 04:41, 28 December 2008 (UTC)

According to our article "Withdrawal reflex", interneurons in the spinal cord pass a signal from pain or heat sensors in the hand directly to motor neurons to pull the arm back. The sensing organs are known as nociceptors, I suppose from the Latin for "noxious". According to that article, exactly how the reflex is triggered is unknown, but "Nociceptor#Thermal" says "The first [thermal nociceptor] to be discovered was TRPV1, and it has a threshold that coincides with the heat pain temperature of 42°C." --Milkbreath (talk) 03:47, 28 December 2008 (UTC)

Interesting article, but doesn't 42C seem a little too low to be the kind of reaction I am talking about?Nkot (talk) 04:49, 28 December 2008 (UTC)

It does sound low but isn't 70-80 degrees enough to kill bacteria? When reading the question I thought if you were cold enough 50-60 degrees could feel very hot. Also a small child will feel the effect of reddening or burning much stronger than an adult. If you spend too much time indoors, 40 degrees in the sun for a couple of hours will burn you easy. ~ R.T.G 11:25, 28 December 2008 (UTC)

But, 80 degrees is not enough to trigger an involuntary "that's hot" reaction for a normal human at normal body temperature. A "warm" shower is above 100 degrees. Below 100 is considered cold. Below 90 degrees is considered very cold. So, 70-80 degrees would produce a "damn that's cold" reaction, not a "that's hot" reaction. However, as I noted above, if the person getting into a 70 degree shower just got out of a tub of ice, the 70 degree shower would feel hot. -- kainaw™ 14:58, 28 December 2008 (UTC)

Fahrenheit and Celsius, friends. A human will get a "damn that's hot!" reaction to about 60C, but 60F is chilly. ~ mazca ^t|c 15:26, 28 December 2008 (UTC)

Indeed I had a real WTF moment when reading Kainaw suggestion 80 degrees isn't going to trigger a 'that's hot' reaction Nil Einne (talk) 09:47, 3 January 2009 (UTC)

Oh, I don't know. That temperature corresponds to about 108 degrees Farenheit, which could be uncomfortable if you take into account the phenomenon Kainaw and Keenan discussed above. And the way I understand the article, and I'm pretty sure I don't very much, you're not going to find a single threshold temperature that will cause the reflex, because it relies on a combination of triggers from various receptors in the limb, our TRPV1 being just one of them. It could be, for example, that TRPV1 is there only to help prevent false alarms, which would be deuced inconveient. --Milkbreath (talk) 15:56, 28 December 2008 (UTC)

60C is 140F, so that's hot enough to burn. The fact that staying in the sun on a hot day might give you a burn has to do with ultraviolet radiation, not heat. Staying in the heat will not give you a burn pain reflex, but it might cause heat stroke. ~AH1^(TCU) 17:27, 28 December 2008 (UTC)

I think I found what I was looking for here (at the very bottom of the page). Specifically, for a "uncoated metal" where the period of contact is one second, a burn will be caused by temperatures of 70C or greater. Nkot (talk) 19:03, 28 December 2008 (UTC)

I've always used 66ºC (150ºF) as a reference. My hand can tolerate that temperature (barely) and so I can estimate from there. hydnjo talk 22:05, 28 December 2008 (UTC)

"Ba humbugi"

Apparently this is a snail found on the island Mba (here)- could someone give a taxonomy (family / class) for this species? Nadando (talk) 08:15, 28 December 2008 (UTC)

Sounds like Humbug to me. 93.132.189.157 (talk) 10:22, 28 December 2008 (UTC)

No 'apparently' about it, there is a snail with this name on a Fiji island. Apart from confirming its existence [1]I can only suggest contact with Oceanic snail experts. Richard Avery (talk) 11:14, 28 December 2008 (UTC)

According to Alan Solem's original description (search for "Type species. Ba humbugi"), it's in the family Charopidae. Note that Solem has given gave other odd names to species of gastropods—see Aaadonta and Zyzzyxdonta, for example. Deor (talk) 16:22, 28 December 2008 (UTC)

Changed verb form above, since it appears that Solem, erstwhile curator of the Division of Invertebrates at the Field Museum in Chicago, died in 1990. Deor (talk) 17:17, 28 December 2008 (UTC)

In the lower orders where the new species to be named can outnumber the taxonomist's imagination, they will sometimes turn to whimsy, as seen in List of animals named after celebrities. --Sean 14:05, 29 December 2008 (UTC)

Freesat, free satellite, DVB-T, DVB-S and satellite dish mounting

Hi, I have a new TV and am looking at free satellite broadcastings. I wonder a few things which are not covered in the info available hopefully someone around here knows a bit about it (maybe once they are finished new years but ask now anyway). My new TV can receive digital. I am led to beleive it recieves Freeview and am sure it does but it does not say Freeview on it (says DVT for digital and AVT for normal aerial) so, it does not say Freesat on it either (I am in Ireland so Freesat is available here). Freeview is called DVB-T and Freesat is called DVB-S. Is my TV built in DVB-T Freeview decoder capable of decoding DVB-S? Shouldn't I be able to hook a dish up to it directly and just tune in what I want without buying a special box? Also wondering if anyone has experimented or knows of putting a dish in a loft? I am in an upper flat so would need to climb 30ft to my outside wall but can get into my loft easy. Shouldnt a dish work ok if put in the loft? Happy new year :) ~ R.T.G 11:02, 28 December 2008 (UTC)

Very very few TVs sold in the UK and Ireland right now are Freesat/DVB-S capable, and those few that do explicitly say "freesat" on them. So you surely need a special box. [I'll mention in passing that I've tried both, and that the Freesat service was inferior to Freeview, and the picture quality (over SCART) from my cheapo freesat box was much worse than both digital and analog terrestrial TV. Your results may, of course, vary]. Yes, it is possible to put a special satellite antenna inside a roof space, but doing so requires a specific antenna and can be done only depending on the construction of the roof. I don't know about other places, but in Scotland the building regulations say that external satellite dishes can't be fitted higher than 3 (or is it 4) storeys - beyond that the danger of them blowing off and killing someone is felt to be too high. So when new blocks of tall flats are constructed, they're fitted with a high-gain satellite antenna inside the roof space - I guess they specifically locate this in relation to a skylight or radio-transparent roofing panel, so as to not block the signal. I'd say this requires a professional install, and indeed fixing an external antenna so high as you describe is probably better left to someone who does that a lot too. Local TV installer companies should be able to supply and install the dish, and run the cable into your living room, for maybe 200 euros. 87.113.77.112 (talk) 17:38, 28 December 2008 (UTC)

My roof is the standard pine wood frame and stone based tiles. As I am on a coastal hill I must get a decent aerial for the local channels but some of these Freesat kits dont cost much more with a Sky type dish. Any quotes I saw for supply and fitting started at 400 euro and sourcing the cheaper kit myself for less than 100 makes it a bit much. Maybe I will fit it in the loft to see. It gets fairly windy around here lol. ~ R.T.G 23:50, 28 December 2008 (UTC)

Psychoactive drugs and human survival

Would it ever have been in the human species' survival interest to evolve, or not to evolve, an immunity to all psychoactive drugs? NeonMerlin 17:02, 28 December 2008 (UTC)

Chocolate can kill many animals and birds, and we also seem to be developing some protection against alcohol. I can't see any problem about immunity developing eventually for things like cannabis or ecstasy if people keep abusing them in such quantities but speculation is frowned on by WP refdesk. Dmcq (talk) 19:43, 28 December 2008 (UTC)

I wonder if chocolate is toxic to humans too - with the only difference being that due to our species' size, a human would need to eat a much larger amount (perhaps more chocolate than could be consumed in one sitting) before any ill effects occurred? --Kurt Shaped Box (talk) 01:38, 29 December 2008 (UTC)

That's an easy one: the major psychoactive component of chocolate is theobromine, which has an LD50 of about 1000mg/kg, or 80g for an 80kg human. Theobromine is approximate 0.5% (w/w) sweet chocolate and 0.15% (w/w) of milk chocolate ^[2], so the amount of to kill 50% of 80kg humans would therefore be 16kg of sweet and 53kg of milk chocolate. An interesting note: the LD50 of theobromine in dogs is significantly lower than humans (300mg/kg)^[3], but it would take about 60g/kg and 200g/kg of sweet and milk chocolate, respectively, per kg of the animal to be 50% fatal. – ClockworkSoul 02:08, 29 December 2008 (UTC)

Consider this: psychoactive drugs work by a vast number of mechanisms to affect how the CNS functions. For example, cocaine, MDMA, and LSD all increase the effects of dopamine in the brain, but do so via completely different mechanisms: cocaine inhibits serotonin reuptake, MDMA induces serotonin release, and LSD mimics serotonin. The same target can even be effected with similar effects by different mechanisms: ethanol and barbiturates are both agonists of the GABA receptor GABA_A but interact with different sites on the same receptor. There are dozens of neurotransmitters, each with one or more receptors that can be interfered with in some way. Considering the incredible number of drugs and variety of mechanisms, I would say that it's really, really, really unlikely that such a general immunity could evolve. – ClockworkSoul 01:47, 29 December 2008 (UTC)

And further to Clockwor's explanation the effects of psychoactive drugs either mimic or trigger natural processes that in their original form and intensity are rather essential or useful. Consider e.g. Parkinson's disease which is related to Dopamine which Clockwork cited. If you'd get something like that as a trade off for immunity to LSD or cocaine then I don't think evolution would favor that result. A human or animal that couldn't sleep or would constantly be in a state of "fight or flight" because it's "immune" to serotonin would end up dead pretty fast. 76.97.245.5 (talk) 10:26, 29 December 2008 (UTC)

Psychoactive drugs are generally present in plants to kill or frighten off animals that might eat them. It's therefore likely that in an area where many plants produce psychoactive drugs, organisms could evolve to deal with them. However this would have to take place in an ad hoc way, and immunity to everything is unlikely bearing in mind the costs it would have (mentioned above). Humans may have evolved their tolerance to alcohol because it is found in rotting fruit (and is largely tasteless and hard to detect), but if you have enough food it's easier to develop an aversion and just not eat the nasty chemicals: people are reluctant to eat bitter flavours, which stops us consuming many alkaloids (though many people can tolerate caffeine-full coffee). --Maltelauridsbrigge (talk) 15:52, 29 December 2008 (UTC)

Regarding psychoactives in plants, you could argue that we have partial immunity to some drugs, i.e. DMT, as monoamine oxidase in the gut prevents any significant concentrations from reaching our CNS (of course us clever humans found the plant that contains MAO inhibitors and mix them with DMT-containing plants). In fact first pass metabolism, P-glycoproteins and the blood-brain barrier could be considered an immunity to psychoactives, just not every one of them. --Mark PEA (talk) 19:25, 29 December 2008 (UTC)

Humans seem to have already evolved to tolerate higher doses of the poisons that appear on mouldy cereals. Compared to other animals anyway. I looked up ergot and ergotism but couldn't find this info.Polypipe Wrangler (talk) 11:34, 31 December 2008 (UTC)

chemical reactions

Are there any cool chemical reactions that I cause with things i might find in my house such as cleening suplize or cooking in gredence. Please dont give me any chemical reactions that will get me in trouble from my parents or blow up the house thanks. --76.235.179.169 (talk) 19:31, 28 December 2008 (UTC)

Read the article about toast to learn about the Maillard reaction. That shouldn't blow up the house and has the added benefit of having a tasty result. hydnjo talk 19:44, 28 December 2008 (UTC)

On this subject, Alton Brown's Good Eats program used to delve deep in to the chemical explanations for all of the cooking he did. For example, the Pancake Episode (2) may give you some insight into pancake chemistry - flour proteins, different types of fats and oils, baking soda, etc. "1 cup buttermilk + 1/4 teaspoon baking soda = enough carbon dioxide to lift 1 cup of flour"... Nimur (talk) 20:09, 28 December 2008 (UTC)

Adding household vinegar to baking soda releases lots of carbon dioxide. You can use this reaction to power a small "jet" boat. -- Flyguy649 ^talk 19:49, 28 December 2008 (UTC)

A pretty classic and cool thing is a non-Newtonian fluid, which you can create simply by adding corn starch into water -- so it's a cheap and completely harmless thing to do. (I believe a ratio of one part corn starch, two parts water will produce the desired results.) I don't know about you, but the idea of a liquid that turns into something resembling Play-Doh when you sequeeze it and runs right past your fingers when you relax your grip always blows my mind a little bit. It's not really a chemical reaction, though, but what the hell. -- Captain Disdain (talk) 02:39, 29 December 2008 (UTC)

I had trouble understanding your question, at first. Then I realized you meant "cleaning supplies or cooking ingredients", not "cleening suplize or cooking in gredence". StuRat (talk) 10:10, 29 December 2008 (UTC)

Baking soda boats are fun: take half a walnut-shell, fill with baking soda, add a short piece of straw, seal with a thing bit of playdoh or putty or wrap in aluminum foil, leave one end of the straw sticking out. Put it in water with a few drops of lemon juice in it. Your boat will fizz around. Magic color changes also can be fun: boil some water with a couple of leaves of red cabbage and put the liquid in a glass. It will turn red if you add lemon juice or vinegar. Write something with lemon juice on a piece of paper and let it dry. If you ask an adult to heat the paper over a candle the writing will turn brown. These sites have some more ideas [4] [5]. Not exactly a chemical reaction, but also fun [6]. Mix fresh green peas (not frozen or canned), 1/2 apple (peeled and cored) and 2 tablespoons of brown sugar in a blender. Have people close their eyes and sniff it, it will smell like strawberries. 76.97.245.5 (talk) 11:12, 29 December 2008 (UTC)

The red cabbage juice experiment is also a fun way to identify acids and bases: [7]. StuRat (talk) 13:15, 29 December 2008 (UTC)

Can I have some more Sun Conure advice, please?

How far does a Sun Conure usually fly every day in the wild, on average? At her last checkup, the veterinarian said that my SC was getting quite tubby, so I'm trying to get her to fly a lot more than she does at the moment. How far are they supposed to be able to fly before they get out of breath? --90.240.153.26 (talk) 19:37, 28 December 2008 (UTC)

I'm rather skeptical that you could possibly get a captive bird to fly as much as a wild bird. You will likely also need to restrict food quantities. Also, do they have low calorie bird food ? StuRat (talk) 10:02, 29 December 2008 (UTC)

Following EC:

Don't have info on their range in the wild, but if you'd like to give your bird more exercise, put some toys in the cage. Apart from ropes and things, you can DIY puzzles from wooden rings and balls and bells from your pet store or a crafts store (make sure the wood isn't treated with harmful chemicals if you use the latter). There are also standard parakeet toys available, but lots of birds find those boring. (Same problem as with little kids, cats etc. the stuff you spend oodles of money on is less interesting than a pot and a spoon or a crumpled up bit of paper.) A fresh branch for climbing and tearing off leaves and bark can also be both entertaining and energy consuming. Wrapping nuts in paper (check with your vet which types are safe, newspaper is definitely not recommended) before you feed them is also an idea that will make your bird spend a few calories before gaining them. 76.97.245.5 (talk) 10:05, 29 December 2008 (UTC)

You people look all the same to me!!

I started coming into contact with a number of asians lately, and I have to say that I do find them harder to distinguish. My question is: is this due to minor variations in appearance or does the brain get better at picking off differences of a race you are familiar with? Bastard Soap (talk) 21:27, 28 December 2008 (UTC)

I think it's experience. In countries where "non-asians" grow up with asians in the community, the "non-asians" have no problems picking the asians apart. I think you do not grow up with asians in your community. If you do not believe me, try picking penguins apart. 122.107.203.230 (talk) 21:43, 28 December 2008 (UTC)

Hi. I am inclined to say it's the latter, but I'm not 100% sure. Maybe you are having a harder time finding distinguishing caracteristics, since most north Asians have the same hair colour, eye colour, skin colour, etc, while Caucasians might have different hair an eye colours, freckles, etc. I think it's the same reason why sheep can tell each other apart but humans have a harder time doing so (by the way, I'm definitely not imagining things, your text is single spaced while my answer is double spaced in the edit window, but not anymore, because edit conflict). ~AH1^(TCU) 21:46, 28 December 2008 (UTC)

It's not a very politically-correct thing to say (I don't think that it's said with any malice, to be fair) - but I've definitely heard (white) people saying that they find it difficult to distinguish one black man from another. Personally, I think it's just a case of lack of familiarity with the facial characteristics of races other than your own. Then again, how many times have you been walking down the street and mistaken a person of your own race for someone else you vaguely know and had to double-take? --Kurt Shaped Box (talk) 01:34, 29 December 2008 (UTC)

Here's a simple test: ask a group of white persons from New York City and from rural Ohio whether they are able to easily distinguish between members of different ethnicities. I'll bet you dollar to donuts the you'll get far more affirmative responses from the New York group than the Ohio one. – ClockworkSoul 02:25, 29 December 2008 (UTC)

Well, if comparing to everyone in Asia, including Russians and Saudis, I'd say no. However, if you meant Orientals, I'd be more inclined to agree. If we limit a comparison of, say Chinese with Europeans, two groups of approximately the same population, I'd definitely say there's more variation in the Europeans. The Europeans have every hair color there is, including red, blond, brown, and black, while the Chinese pretty much only have black hair. The Europeans have every hair texture, including straight, wavy, and curly, while the Chinese pretty much only have straight hair. The Europeans also have more eye colors. So, based on these attributes, I'd say it's fair to say there's more variation amongst Europeans. Now, one could argue that I chose those attributes which vary more for Europeans, but, in this case, which attributes vary more for the Chinese ? If they had a variable number of fingers, or something like that, then I would accept that argument. StuRat (talk) 09:57, 29 December 2008 (UTC)

Yes, I would argue that you've been very selective in your attributes. An example of an attribute that varies more amongst the Chinese than among Europeans is skin pigmentation. I suspect also, the general shape of the face. mike40033 (talk) 04:49, 4 January 2009 (UTC)

Cecil Adams did a column on this. 24.91.45.12 (talk) 16:53, 29 December 2008 (UTC)

StuRat, it's a matter of cultural conditioning; in fact, and your own response above lists facial variations that you have been conditioned to notice. In other ethnicities, they notice other things, like hairline position, eye placement, cheekbones, lips, etc. A friend of mine from Laos once complained to me that "you white people look all the same." Sure, white people have a greater variety in their hair texture, hair color, and eye color — but those aren't the cues he was conditioned to look for, because in his ethnic group hair and eyes aren't perceived as differentiating features (people dye and perm their hair and wear colored contacts in his country too, so those features are useless anyway, he'd say). ~Amatulić (talk) 22:43, 29 December 2008 (UTC)

You seem to have ignored the last two lines of my post. In order to argue that Chinese don't "look more alike" than Europeans, you need to list some identifying characteristics that vary more for Chinese, just as I listed some that vary more for Europeans. Otherwise, we get that Europeans vary more for some characteristics, and by the same degree for others, which still leaves Europeans as varying more when you average those characteristics out. As for the argument that you can't identify people based on hair or eyes, as those can be changed, so can facial features, with make-up or plastic surgery. StuRat (talk) 15:32, 30 December 2008 (UTC)

Skin tone IMHO varies more in Chinese then it does in Europeans. It's also worth considering who we're talking about. Europeans is actually a very broad ethnic group. Chinese by which people probably mean Han Chinese is likely a less variable group then Europeans. Also I doubt you have enough evidence to rule out there being greater variance in facial features or even eye colour (brown eyes can actually vary a lot in colour). In other words, while you may be correct, you haven't presented close to enough evidence to suggest you are. The Cecil Adams column is a decent read Nil Einne (talk) 09:43, 3 January 2009 (UTC)

Skin colors vary more for Chinese than Europeans ? Europeans vary from extremely white in the Nordic countries to quite dark in Italy and Spain. And you say that the brown eye color of Chinese varies more than the blue, green, brown, and black eyes of Europeans ? I don't understand how that's possible. I was attempting to compare contiguous regions with similar populations, regardless of the ethnic groups contained therein. But, in any case, both Europe and China contain many ethnic groups. As for limiting the Chinese to the Han, there's no need to do that, go ahead and include Mongolians, Tibetans, and even Koreans, etc., if you think that will make a difference. Now, as for you wanting proof, what part of my statements exactly do you find to be questionable ? That many Europeans have blue eyes ? That almost no Chinese do ? Give me some facts that you dispute and I'll try to find some proof. StuRat (talk) 19:24, 3 January 2009 (UTC)

Oh wow! A speculation-fest!! Can I join in too? I think it's entrirely experiential. You get better at picking differences between (apparently) similar objects with practice:

• Ask any pathologist how s/he differentiates between similar cell types - many will respond "they look different."
• We have 2 (unrelated) black cats at home, they're both skinny and have yellowy-green eyes. We've had them both for about 10 years and to my wife and I they're like chalk and cheese to look at, but our kids (who are younger than the cats) and other family & friends who've known them as long have trouble telling them apart.
• Anyone who's ever hired a car can probably relate to the problem of not being able to differentiate your hire car from similar model cars of the same colour.

Happy NY! Mattopaedia (talk) 03:17, 1 January 2009 (UTC)

As we grow, we learn to distinguish faces based on certain features. Each person uses a different set of features to distinguish faces. For someone unfamiliar with Asian faces, or African, or Caucasian faces, members of that group "all look the same". I lived in Asia for a while, and on several occasions, had conversations with locals that went like this. Someone says - "That guy (referring to a Caucasian male) looks a bit like that other one." Then I look at the two, and seeing no resemblance, reply "not really." The phenomenon is both ways.... modulo Hollywood, of course....... when I was younger, I kept a beard. Then came a day I decided to get rid of it. I asked people to try to imagine what I would look like without a beard. Some had no trouble (call them group A), others just couldn't imagine it (call them group B). Of course, these are two ends of a spectrum. When I finally shaved it off, I got a range of reactions. Interestingly, the people in group B typically said "Oh my God, is that you? I can't believe it. You... you look completely different". The poeple in group A would say, after chatting normally to me for half an hour "You've changed something, haven't you. Is it your glasses?" My theory is that some people used the beard (or lower face) as a cue, others didn't. And these "experimental subjects" were all from the same ethnic group.... mike40033 (talk) 04:42, 4 January 2009 (UTC)

Methane clathrates, global warming, and such

Hi. Back in September, we found out that "peliminary findings" indicate that methane clathrates in the Siberian Arctic Ocean were melting. Now, what does that actually mean, and is there any chance that they aren't melting? If these are "peliminary" findings, then will they be confirmed, and when? I'm currently reading a book (four years old) that lists three possible scenarios with severe global warming: 1. The collapse of the ocean thermohaline circulation; 2. The collapse of the Amazon; 3. The release of the methane clathrates. The book lists the third one as the "least likely to occur this century"! So how could it be occuring now? Also, why haven't I heard of this on the news on TV? In fact, I only know one other person in real life who already knew about this before I told them about it. If the thermohaline circulation freezes the Norwegian sea, will the clathrates have less time each year to release, or will the melting of the whole Arctic Ice cap in summer (which could occur as early as 2013, from a LiveScience article) trigger a longer window? Also, this always confuses me: If the freshwater released by the Arctic could disrupt the Gulf Stream, how come it is supposed to "plunge Britain and Scandinavia into an ice age"? I mean, if the Arctic sea ice is going to melt anyway, then this will only have an effect in winter, since then the ice cap could extend to cover the North Atlantic between Greenland and Scandinavia in winter, which currently does not happen. Also, I have a hypothesis (and no, I'm not asking for opinions, just if it is valid), that the slowing and possible colapse of the thermohaline will cause smaller semi-permanent high and low pressure zones, that warm currents will flow near the surface, disconnected with cold currents, which will flow at lower depths, that this will cause a warming of the Humboldt current, which will therefore bring rain rather than fog, which will un-desert the Acatama, which will in turn solve Peru's water crisis. Also, a question about the Great Lakes. How much sea level rise is needed in order for either salt or brackish water to seep into the bottom of Lake Ontario (remember that salty and brackish water are denser than fresh, therefore it can seep into the St. Lawrence from underneath while fresh water remains on top or estuary, and therefore what is the bathymetric maximum height above sea level of the depth of the seafloor of the St. Lawrence river). Thanks. ~AH1^(TCU) 21:39, 28 December 2008 (UTC)

Sorry, no time to answer all of this, but read Methane clathrate particularly the section on climate change. As to why it's not a hot news item, apart from affecting other factors that influence global warming and thus keeping those deposits from melting there isn't anything we can do to prevent it AFAIK. You can't very well build freezers around it to keep it cold. So news focuses on things we can influence like emissions of greenhouse gases from industrial sources. The Gulf Stream moves warm tropical water from around the Bahamas to the North Sea. If you compare the temperatures there to the Hudson Bay which is at about the same latitude you'll see the North Sea gets heated by the waters from the Gulf Stream. If the poles melt, cold fresh water will flow into the salty sea and drive the salty water down. This will disrupt the convection pattern and change or even reverse the flow of the Gulf Stream. Since there will no longer be warm water from the tropics warming Europe, it's going to get colder there although this is an effect of Global warming. This has been shown by running models of the earth looking at possible effects of various climate factors.76.97.245.5 (talk) 09:33, 29 December 2008 (UTC)

As far as Lake Ontario goes, the St. Lawrence Seaway descends from 73 m a.s.l. to 46 m at the International Rapids section. Rapids generally being fairly shallow, this implies a minimum rise in sea level of 46 metres, so I'd think we're OK for a while yet.

On the cooling of Europe bit, while I've seen the prevailing theories, I've also seen the statement that there will be a Gulf Stream as long as the winds blow, since it is surface wind which creates the current, so I'm a little confused on that one. Franamax (talk) 23:59, 29 December 2008 (UTC)

@Framax If you look at Gulf Stream the wind action is only part of a rather complex mechanism that creates this specific convection pattern (contributing part of the North Atlantic Deep Water to be precise). As far as "there will be a Gulf Stream as long as the winds blow" that phrase is pretty much out the window since the Gulf Stream stopped for 10 days in 2004 to everyone's surprise. (Don't know if the winds stopped blowing ;-) One of the problems with making climate predictions is that there are natural patterns, both large scale and long term as well as local and short term, that either get emphasized or offset by global warming. Since human activities such as agriculture depend on lots of factors remaining stable (e.g. rainfall, temperature, planting seasons) within very small margins, one could say "any change is bad change". Lisa4edit (talk) 02:12, 30 December 2008 (UTC)

Do photons lose energy?

Does the red shift noted as galaxies rush away from our own represent any loss of energy in the photons, and if so, where does the energy go? --Halcatalyst (talk) 22:20, 28 December 2008 (UTC)

First, I should note the distinction between redshift from the Doppler effect due to "actual relative velocity" ("comoving velocity") of galaxies, and the redshift due to the expansion of space (see Hubble's law). From the wording of your question, I am fairly sure you are referring to the latter. The short answer is, the energy of redshifted photons goes into gravitational energy. However, since the theory of general relativity indicates that the gravitational field is more accurately represented as the metric of space. Basically as the light traverses a region of expanding space, it gives up some of its energy to the structure of space. If you happen to be fluent in 4-dimensional GR tensors, stress-energy tensor is the relevant concept. --Bmk (talk) 00:50, 29 December 2008 (UTC)

Oh right - I neglected to specifically answer your first question - yes, redshifted light has lost energy - it has a longer wavelength than when it was emitted, so its energy is lower. --Bmk (talk) 01:00, 29 December 2008 (UTC)

Are the photons then converted into gravitons? --Halcatalyst (talk) 04:24, 29 December 2008 (UTC)

No, I wouldn't say the photons are converted into gravitons. Gravitons are at best a theoretical possibility. It's currently not known whether gravitons are a valid way to describe gravity. A simplified answer to your question is that the energy of the photons is transferred from electromagnetic energy to the "energy of the shape of space". Unfortunately, a more accurate answer requires knowledge of general relativity. --Bmk (talk) 06:51, 29 December 2008 (UTC)

I believe the Hubble's law is due to the Doppler effect. If this is correct, there is no gain in energy. You are simply observing the photon from a different point of reference. It would be like jumping and asking what could possibly have given the Earth enough kinetic energy so that it's moving away from you at many feet per second. — DanielLC 03:27, 30 December 2008 (UTC)

There's only one kind of redshift in general relativity and only one kind of relative motion. Your description is correct and so is Bmk's. Redshift is often divided into "gravitational", "cosmological" and "Doppler" redshift, and the last might even be further divided into a quasi-Newtonian Doppler shift and a special relativistic "correction" to that, but general relativity makes no such distinctions. -- BenRG (talk) 05:46, 30 December 2008 (UTC)

December 29

consuption of electricity by refrigrator

when we have different speed of cooling on two same type of refrigrators ,is there any effect on the consuptions of electricity electricity .By adjusting the lower speed can we reduce our electricity bills. —Preceding unsigned comment added by Khubab (talk • contribs) 01:14, 29 December 2008 (UTC)

Since no one else has bitten yet. I'm not quite sure what you are trying to get at here. Please see Refrigeration. If we are talking standard refrigerator the setting you can choose is the temperature. The lower the temperature setting the longer and more often the pump will run to cool the air inside the fridge. This will cost more energy than selecting a higher temperature. A fridge with better insulation to the (warmer) ambient air and a more efficient pump can save energy. However the "speed of cooling" is not a term that can be applied here AFAIK. 76.97.245.5 (talk) 09:01, 29 December 2008 (UTC)

There could be two almost identical fridges, but where one has a more powerful compressor capable of cooling quicker. In that case, provided that the temperature was set the same, the energy usage should be about the same, although the more powerful compressor would use more power, but for shorter periods of time. StuRat (talk) 09:44, 29 December 2008 (UTC)

female virginity question

(Medical Advice request removed by JSBillings ) PitchBlack 03:38, 29 December 2008 (UTC)

Sounds like you're asking for Medical advice. Ask your doctor. -- JSBillings 03:52, 29 December 2008 (UTC)

Whilst I don't have an answer for you - if this question is really about you (or someone you've been sleeping with), please seek medical advice from a real, qualified, flesh and blood doctor. --Kurt Shaped Box (talk) 03:52, 29 December 2008 (UTC)

A woman I consulted, who was once a virgin, suggests you should see a doctor if the symptoms you cited are a concern. Edison (talk) 04:55, 30 December 2008 (UTC)

Differnece in growth of Neem tree and Banana tree

If we cut the stem of the Neem tree, Leaves and branches comes at sideways. but this is not in the case of the stem of banana. If we cut it new one grows near the banana tree. what is the reason. —Preceding unsigned comment added by 203.129.203.164 (talk) 05:54, 29 December 2008 (UTC)

Do look up Monocotyledon and Dicotyledon. Shyamal (talk) 06:29, 29 December 2008 (UTC)

What logical fallacy is this?

First, I know this is the science desk, and maybe this is more of a miscellaneous question, but science and logic are connected so... Is there a name for the following logical fallacy: "The earth is lucky, because it is in the perfect green zone, not too far and not too close to the sun, etc." The same mislogic is shown when a person says "you are lucky because some other sperm could have reached the egg and you would never have existed if that had happened." Is this a named logical fallacy? One thing, I am not here to discuss whether either example is a fallacy or not. You are not going to convince me either is not and I'm not interested in that debate. Thank you.--68.237.248.155 (talk) 06:46, 29 December 2008 (UTC)

Confirmation bias? Or is that backward? —Tamfang (talk) 08:09, 29 December 2008 (UTC)

Not specifically named as a fallacy, but it is an expression of the Anthropic principle - in particular, there are two ways of looking at it: you can either marvel at the fact that something as unlikely as a planet capable of supporting sentient life exists, or you can point out that if it didn't, we wouldn't be here to marvel at it so it's not really a surprise that it does exist. I suppose it's a little bit like confirmation bias as well, like Tamfang suggests. Confusing Manifestation(Say hi!) 10:39, 29 December 2008 (UTC)

Try here List of cognitive biases. I think it is probably more of a False analogy - usually these "you're lucky" are used to try compare apples and oranges. 194.221.133.226 (talk) 10:43, 29 December 2008 (UTC)

This is related to the argument from personal incredulity, which often comes up in discussions related to Intelligent Design. The argument that "the world is too complex so it must be designed" is similar to a puddle of water marvelling at the fact that it fits exactly in its container, so it must have been designed too. ~Amatulić (talk) 22:35, 29 December 2008 (UTC)

The problem for me is that the planet has no preference for life or no life - it's a non-living, non-sentient thing. It doesn't care whether life forms upon it's surface or that it's barren. So 'lucky' doesn't really apply here. Sure, there is a statistical probability of it being in the 'green zone' - and that probability seems to be rather small. But if you imagine a planet that passionately didn't want to be infested with amoeba, ants, aardvarks, anchovies...and zebra then you'd say that it was "unlucky" to end up where that infestation could happen. So was the earth "lucky" or "unlucky"? You can't give an answer to that unless you know what outcome it "wanted" - and it's a big lump of rock that really doesn't care either way! So I think we have to 'unask' this question - it's meaningless. If instead you are asking "Was it lucky that the planet on which we humans developed was in the green zone?" then we most certainly have to defer to the "anthropic principle which says that if the planet hadn't done so, we wouldn't be here to ask the question - hence it's 100% certain that whatever planet we'd call "Earth" must be in the habitable zone. So I think the best answer for you is "The Anthropic Principle". SteveBaker (talk) 23:22, 31 December 2008 (UTC)

Multiple near-death experiences and emotions

I'll let the wind blow out the light
'Cause it gets more painful every time I die.

— Children of Bodom

In people who have had multiple near-death experiences, are the subsequent ones more likely to be strongly negative than the first one? NeonMerlin 13:31, 29 December 2008 (UTC)

I have had three near-death experiences (assuming you don't consider having my heart completely stop to be a "death-experience"). I don't remember any of them. They aren't positive or negative. I believe it is simply how memory works. When you nearly die, there is no transition of short-term memory to long-term memory. So, the short term memory of the events right before and including the event are not sent to long-term memory. I remember riding my bike and thinking "I need to get to the other side of the road" and then I remember waking up on the side of the road and not being able to move. I remember riding my motorcycle (while rather drunk) through the Mojave at night and suddenly realizing that there was no bike below me and then waking up in a bunch of rocks at the bottom of a steep cliff. I remember riding my motorcycle along a small road in the marsh and thinking that the headlights ahead appear to be swerving and then waking up when an paramedic grabbed my shoulder. So, the "experience" is gone. I could make up stories about bright lights and angels, but all I remember is one thing before the event and something after the event. -- kainaw™ 14:33, 29 December 2008 (UTC)

Yet you seem eager to have the same "non-experience" over and over again... Lova Falk (talk) 17:27, 29 December 2008 (UTC)

Far be it from me to imply unsolicited advice (*cough*) but were you aware that emergency room physicians – when they think no one is listening – tend to refer to motorcycle riders as 'organ donors'? TenOfAllTrades(talk) 17:54, 29 December 2008 (UTC)

I know. Even motorcycle riders say: It isn't "will I have an accident", it is "when will I have an accident." -- kainaw™ 20:35, 29 December 2008 (UTC)

Not sure I'd label the actual near-death experience as positive or negative, but - while I have never had one myself - I know a devout Christian who says that with his, he doesn't remember details, but what troubles him most is the fact that - after having been in a perfect state where he could feel no pain, there was no suffering, he wasn't tempted by his flesh, etc., now he's back here on earth, where it's just so difficult, because of the limitations.

So, I would posit that the feelings around the experience can be recalled; but it probably depends on how long one was *gone*, etc.. Which makes sense, because even then, as you say, it's not memories of things, but simply of emotions/feelings, to him.

I'll note that the Apostle Paul seems to share your view more - he says it is not lawful that a man may utter the things in the "third Heaven," not because it is illegal, but it is against the laws of nature; the nature of the one experiencing it doesn't permit him to recall or utter them.Somebody or his brother (talk) 13:47, 30 December 2008 (UTC)

chances developing same affliction

What are the chances that 2 brother's-in-law that are married to twin sisters develop acromegaly with the same type of pituitary tumor at the same time? These men live in different cities. Could foul play be involved? —Preceding unsigned comment added by CAElick (talk • contribs) 14:42, 29 December 2008 (UTC)

Perhaps both sisters were attracted to their future husbands because the physical effects of the disease? Sometimes twins both have the same preferences in a partner. Either that or both sisters are injecting human growth hormone into their husbands. -- JSBillings 16:36, 29 December 2008 (UTC)

Regular, repeated injections of human growth hormone could cause acromegaly, but I'm not sure how the sisters could also produce a pituitary adenoma. (For that matter, I find it difficult to imagine how they would have been able to covertly give regular injections of HGH to their husbands.) Though the incidence of acromegaly is pretty small (call it about four cases per million people per year: [8]) someone has to get it. (A couple of factors do improve those odds slightly, however. Men are more likely to have somatotrophic adenomas than women, and if the brothers are still in early to middle age they're at the peak time for developing the disorder.) Meanwhile, acromegaly usually progresses very slowly, and it typically takes more than ten years from symptom onset to diagnosis unless the doctor is primed to look for it — as when the patient casually mentions his brother-in-law's case.

(From here on in, I'm going with the Fermi problem approach, so use these numbers with caution.) The naive estimate (based on disease incidence) of both brothers-in-law being diagnosed with acromegaly at roughly the same time is about 1 in 100 billion. Taking into account the factors I mentioned above, the actual odds may be a couple of orders of magnitude better: closer to 1 in a billion. (As JSBillings notes, the twins might also find similar mates attractive, which my imply an additional component of shared genetic risk and further increasing the odds.) That being said, before you congratulate the two gents on being so 'lucky', you might want to consider a slightly different question: What are the odds that two brothers-in-law happen to develop rare diseases within a year of each other? If you figure that there are a thousand 'rare' disease with roughly the same incidence as this one, you're up to (roughly speaking) a one-in-a-million shot at any given pair of brothers-in-law sharing some disease. Still not what I'd call good odds, but better than your shot at the jackpot in most lotteries — and people keep winning those.

To approach your question about foul play, I'd say the jury's hung (at best). It's an improbable coincidence, but still quite possible. And so far no one's come up with a likely method by which the disorder could be surreptitiously induced. Absent that, the twins are going to walk. TenOfAllTrades(talk) 17:19, 29 December 2008 (UTC)

Be careful with assigning causal responsibility where there may not be any. Detecting causality can be a good survival trait. (If the other guy who angered the big boss got whacked on the head, angering the big boss might be detrimental to health and long life.) But it's not unheard of for relatives to assign blame to the spouse if someone dies or suffers from a severe illness. People do statistically get ill and even at the very unlikely end of the curve. Nonwithstanding that I'll imprecisely quote a criminal investigator who said that our cemeteries are full of victims of undetected spousal murders. (aka. divorce alternative) [9] The question becomes one of cost vs. benefit when asking questions. Are you likely to prevent injury to yourself or others by accusing the twins of wrongdoing? On the other hand it is almost a given that your relationship with them will be impaired if you suspect them of anything ranging from the evil eye to criminal intent. In it's extreme such witch-hunts can lead to alienation and even kill people. No medical or legal advice intended other than to use ones head before pointing fingers. Lisa4edit (talk) 23:56, 29 December 2008 (UTC)

antibiotics

Are Antibiotics effective? —Preceding unsigned comment added by Shodson8 (talk • contribs) 16:56, 29 December 2008 (UTC)

With some caveats, most antibiotics – prescribed and used correctly – are generally effective against appropriately-diagnosed target organisms. When taken inappropriately (against a viral infection, for example) they don't do a damn thing that's useful and may cause a number of problems. TenOfAllTrades(talk) 17:25, 29 December 2008 (UTC)

Interesting question, I'd say yes when used properly. If everyone used their antibiotics correctly (specifically finishing the course given, going back if you need more and finishing again, and prescribed correctly by the doctor) then there's little chance that an organism will evolve to become resistant; however, there's always a chance that the bacteria will become resistant. The chance, however, is significantly reduced by correct use and prescription. —Cyclonenim (talk · contribs · email) 18:36, 29 December 2008 (UTC)

If as TenOfAllTrades and Cyclonenim indicated you are prescribed those antibiotics by a doctor, that would usually indicate that your body is battling an infection. Alternatively you might have been given the antibiotics as a prophylactic measure if your body is too weak to ward off infections (e.g. after an operation). In both cases there is an indication that your body needs help dealing with the invading pathogen(s) and would incur severe damage, or at the very least make you feel very miserable for an extended period, without. By comparison if the antibiotics are prescribed and taken correctly they will reduce the number of pathogens and give your body a chance to heal. As with everything in nature there are many factors affecting the efficiency of antibiotics and there are cases where they aren't effective or even make things worse. The best we can do is to keep such cases to a minimum. Nevertheless, even if there are more and more of the little buggers that develop resistance, and allergies are an ever growing problem, I'd say antibiotics are effective if taken as prescribed.76.97.245.5 (talk) 20:38, 29 December 2008 (UTC)

Genome: Both good and original, but...

Samuel Johnson is attributed with saying to an aspiring writer, "Your manuscript is both good and original, but the part that is good is not original and the part that is original is not good." Is it likely that my genome is like that manuscript? NeonMerlin 19:17, 29 December 2008 (UTC)

The analogy would be considered off from a scientific perspective (not counting these}. A writer follows a purpose when creating his/her work. Nature is generally considered to be a conglomeration of events that happen without an overall ulterior motive. Individual elements may pursue certain interests like survival, procreation and quality of life but the overall effect may actually be harmful rather than beneficial to either the individual, it's species or the environment. For writers there are people in certain positions that established guidelines as to what they considered "good". (One should note that even that is a qualification not everyone agrees on.) In order to draw this analogy you'd have to specify what makes a portion of your Genome "good" and by whose standards. Just a couple of pointers to illustrate: Noncoding DNA was considered "junk", however, we have since discovered that some of those portions serve a function. Some congenital disorders have "beneficial" side effects. Would you e.g. consider the alleles that cause Sickle-cell disease good because it protects the bearer from malaria or bad because it makes them die sooner? Is the superior immune system that seems to come with Huntington's disease worth the cost? The genes that enabled humans to store fat efficiently enabled our ancestors to make it though lean time, yet is currently causing millions of their descendants trouble. The list goes on.76.97.245.5 (talk) 21:36, 29 December 2008 (UTC)

Assuming you mean "good" as advantageous to your survival, then its not a particularly appropriate analogy. Your "not original" genome is the sum of that which you inherited from either parent. It can contain elements that are good or not good in this context. The original part of your genome is a result of genetic mutation. It can be good (a change that proffers an advantage) or not good (a change that proffers a disadvantage) or neutral. Rockpocket 21:41, 4 January 2009 (UTC)

Putting milk in coffee

Imagine you made two identical cups of coffee. Imagine you made them both at 2pm so that they started off at the same volume and temperature (say 300ml at 100C), and you added an amount of cold milk (say 30ml at 5C) to one of them at 2pm and the same amount of milk at the same temperature to the other at 2.10pm. At 2.20pm, I believe that the drink you added the milk to first would be warmer than the drink the milk was added to later (assuming that neither drink had yet reached room temperature). Is that correct? —Preceding unsigned comment added by Milk Please (talk • contribs) 20:18, 29 December 2008 (UTC)

For the milk-coffee cup, the start temp, T(0), may be estimated as 300*100+30*5/330.

The formula you want to calculate the temp at 10 minutes, t=10, is T(t) = Ta + (T(0) - Ta)e^-kt where Ta is the ambient room temperature.

Then, you have the temp of both cups at 10 minutes. Repeat the first step to calculate the change in the second cup's temperature.

Finally, recalculate the temp 10 minutes later using the formula from the second step. -- kainaw™ 21:01, 29 December 2008 (UTC)

(EC) The one you add the milk to last should be colder, yes. The reason is that the temp loss is proportional to the temp diff (relative to the environment), so the hot coffee will give off more heat than the cold coffee. Thus, keeping it hot as long as possible ensures the maximum heat loss. StuRat (talk) 21:04, 29 December 2008 (UTC)

We get this question (and variations thereupon) so often on the Science Reference Desk that I have decided to create a configurable Milk + Coffee Simulator.

Simulation Code, in MATLAB

clear;close all;
room_ambient_temp = 25;

tau_nlc = 60;   % Time constant (Seconds) for the cooling
r_nlc = 1/tau_nlc; % constant for Newton's Law of Cooling in the form dT/dt = -r(T-Tamb)

% Initial coffee and milk parameters
coffee_volume = 300;   % mL
coffee_temp0  = 100;   % degrees C
milk_volume   = 30;    % mL
milk_temp0    = 5;     % degrees C

% Initialize the coffee and milk parameters
coffee1_temp(1) = (coffee_temp0*coffee_volume + milk_temp0*milk_volume)/(coffee_volume+milk_volume)
coffee2_temp(1) =  coffee_temp0;

%%Loop over time, measured after 2:00 PM, in seconds
for t = [1:60*30]
    coffee1_temp(t+1) = coffee1_temp(t) - r_nlc*(coffee1_temp(t)-room_ambient_temp);
    coffee2_temp(t+1) = coffee2_temp(t) - r_nlc*(coffee2_temp(t)-room_ambient_temp);
    

    % At time 2:10, add milk to Coffee #2
    if (t == 60*10)
        coffee2_temp(t+1) = (coffee2_temp(t)*coffee_volume + milk_temp0*milk_volume)/(coffee_volume+milk_volume)
    end
    
end


%% Plotting, including time-stamped x-axis
time_labels = datenum(2008, 12, 29, 14, 0, 0) + 1/(24*60*60)*[0:60*30];
plot(time_labels,coffee1_temp, 'r'); hold on;
plot(time_labels,coffee2_temp, 'b');
datetick('x', 16);
xlim([datenum(2008, 12, 29, 14, 0, 0), datenum(2008, 12, 29, 14, 20, 0)]);
legend('Coffee 1 - Milk Added at 2:00', 'Coffee 2 - Milk Added at 2:10');
xlabel('Time');
ylabel('Temperature (Celsius)');
title(['Hot Coffee Simulation with \tau = ' , num2str(tau_nlc) , ' sec']);

I obtained this temperature curve set when I set the time-constant of cooling to 1000 seconds (just over 15 minutes). Nimur (talk) 21:53, 29 December 2008 (UTC)

Is this Matlab stuff supposed to do something? Not in my Safari browser. --VanBurenen (talk) 21:59, 29 December 2008 (UTC)

Now the show/hide thing is working Ok. Thanks. --VanBurenen (talk) 22:01, 29 December 2008 (UTC)

Well, I haven't yet written a web-interface, it's only been about ten or twenty minutes; maybe in the next few days I'll put a CGI site up where you can input initial milk and coffee parameters and auto-generate the temperature curves. In the meantime you will need to obtain MATLAB or the GNU Octave to run this code yourself. Nimur (talk) 22:01, 29 December 2008 (UTC)

Did you consider the cup when running the matlab? In case you are using a ceramic mug and not a styrofoam cup I would have expected the mug to absorb and retain heat and lead to a (very slightly) higher temperature for the milk added later.Lisa4edit (talk) 00:11, 30 December 2008 (UTC)

The cup is not modeled explicitly. If you check out the Newton's Law of Cooling article, you can see a formulation of the model which estimates the time constant as (Heat transfer coefficient x surface area)/(mass*heat capacity). You could then calculate an exact time-constant, accounting for conduction to the air and walls of the cup, if you know the dimensions of the mug. However, you would still need to estimate the heat capacity and heat-transfer-coefficients for the materials (or look them up from an empirical table). Instead I chose to simply estimate the time constant, because "coffee takes about 15 or 30 minutes to cool to room temperature," which I found a suitable estimate. I also found a Web Octave interface, so you can now run the code from your browser - you may need to modify some of the plotting parameters for MATLAB/Octave compatibility. Nimur (talk) 13:40, 30 December 2008 (UTC)

OK, so this is not quite as straightforward as I thought it mighe be, I think this is because the cup can act as some kind of heatsink. So the coffee initially without milk, will lose heat more rapidly, but some of that heat will be stored in the mug. The coffee with milk will lose heat less rapidly, but less of that heat will be stored in the mug. As the two coffees cool some of the heat from the mugs will pass back into the liquid, slowing the overall rate at which it cools. In theory the amount of heat stored in the "non-milk" mug, and the rate at which it passes back into the liquid, might counteract the increased cooling caused by the greater temperature gardient between the "non-milk" coffee and the air. Is that right? Milk Please (talk) 11:11, 31 December 2008 (UTC)

Incidentally, I had to force myself to not use the heading "Hot Coffee", but I see that Nimur couldn't resist! :) Milk Please (talk) 11:17, 31 December 2008 (UTC)

I believe that this "heatsink" effect that you are mentioning would only change the value of the time constant of cooling. I don't think it would have any more complicated effect. Nimur (talk) 14:13, 31 December 2008 (UTC)

Well, as promised, I have delivered a totally unnecessary web interface to my Hot Coffee Simulator. It's a little bit clunky but I figured it was a good opportunity to try out Octave as opposed to MATLAB. Of course let me know if anything breaks... Nimur (talk) 21:50, 31 December 2008 (UTC)

Ni-MH Battery Behavior (Nikon EN-4 knockoff for D1 camera)

My batteries for my Nikon D1 are relatively new (a few months old), and have always given me trouble. They randomly stop working altogether after sitting overnight, and the battery meter often shows no sign of a low battery. To investigate, I ran the following experiment, and I would appreciate analysis. Please note that the experiment for battery #2 has not been completed yet, and please also note that the time intervals between tests is not regular. Thanks in advance for your help!

Experiment: Test battery's voltage at intervals starting from when it first comes off the charger and ending when it no longer powers the Nikon D1 camera. Voltage tests are performed immediately after each camera test. Batteries are stored outside of camera in between tests.

BATTERY 1 (Powermart DNK004, rated 7.2 V)

2008-12-23 18:53 EST - 8.63 V, powers camera. (straight off of charger from normal Charge cycle)
2008-12-23 21:18 EST - 8.43 V, powers camera.
2008-12-24 00:04 EST - 8.36 V, powers camera.
2008-12-24 11:31 EST - 8.26 V, powers camera.
2008-12-24 20:17 EST - 8.21 V, powers camera.
2008-12-24 22:47 EST - 8.20 V, powers camera.
2008-12-25 01:09 EST - 8.18 V, powers camera.
(Experimental procedure change: Battery 1 is kept in D1 camera with power turned off instead of disconnected from camera.)
2008-12-25 10:39 EST - 8.14 V, powers camera.
2008-12-25 21:56 EST - 8.10 V, powers camera.
2008-12-26 12:09 EST - 8.06 V, powers camera.
2008-12-27 01:45 EST - 8.03 V, powers camera.
2008-12-27 13:35 EST - 8.00 V, powers camera.
2008-12-29 15:08 EST - 2.97 V, fails to power camera.

BATTERY 2 (Powermart DNK004, rated 7.2 V)

2008-12-23 14:41 EST - 8.57 V, powers camera. (straight off of charger from overnight Refresh cycle)
2008-12-23 15:15 EST - 8.46 V, powers camera.
2008-12-23 15:44 EST - 8.41 V, powers camera.
2008-12-23 16:13 EST - 8.38 V, powers camera.
2008-12-23 17:47 EST - 8.31 V, powers camera.
2008-12-23 18:52 EST - 8.28 V, powers camera.
2008-12-23 21:19 EST - 8.24 V, powers camera.
2008-12-24 00:05 EST - 8.21 V, powers camera.
2008-12-24 11:32 EST - 8.15 V, powers camera.
2008-12-24 20:17 EST - 8.11 V, powers camera.
2008-12-24 22:48 EST - 8.10 V, powers camera.
2008-12-25 01:09 EST - 8.09 V, powers camera.
2008-12-25 10:41 EST - 8.06 V, powers camera.
2008-12-25 21:57 EST - 8.04 V, powers camera.
2008-12-26 12:09 EST - 8.01 V, powers camera.
2008-12-27 01:45 EST - 7.99 V, powers camera.
2008-12-27 13:36 EST - 7.98 V, powers camera.
2008-12-29 15:09 EST - 7.93 V, powers camera.

Steevven1 ^{(Talk) (Contribs) (Gallery)} 20:36, 29 December 2008 (UTC)

Are you sure they are supposed to last for more than 6 days without charging ? The batteries for my walkie-talkies seem to only last a day or so off the charger, and my cell phone lasts maybe about a week. So, you seem to be right in that range. StuRat (talk) 20:57, 29 December 2008 (UTC)

Your walkie-talkie and cell phone batteries are completely exhausted after being off of the charger for one day and one week respectively, while the devices are turned off? This seems strange to me. Steevven1 ^{(Talk) (Contribs) (Gallery)} 21:35, 29 December 2008 (UTC)

Your results for battery 1 are surprising. If the battery is not under load, why would it drop from 8.00 volts on 12-27 to 2.97 volts on 12-29. Was here some load on the battery in the meantime? Otherwise I would call it a "dud." For battery 2, you did not provide a report of a voltage at which it did not operate the camera ok. More and better characterized data is needed. Edison (talk) 04:39, 30 December 2008 (UTC)

Since the experimental method for battery 1 was changed to leaving it in the camera midway through, I'd be inclined to suspect that the camera does not in fact turn "off" but instead has a vampire (standby) current that results in depletion of the battery. Depending on the physical state of the battery, it could deliver rated voltage then rapidly decline. If it's still in the warranty period, I'd say take it back with "doesn't show low-charge before it stops working". Franamax (talk) 01:13, 31 December 2008 (UTC)

Optimal Eyepiece Diameter

Hello. I read the binoculars article and wondered: What is the optimal eyepiece diameter for 7×50 binoculars so that I am not seeing two small dots and too much environment outside the binoculars? Thanks in advance. Have a happy new year. --Mayfare (talk) 20:53, 29 December 2008 (UTC)

Your question is unclear: "two small dots????" Why wouldn't you want the eyepiece diameter to match the dark-adapted iris?Edison (talk) 04:34, 30 December 2008 (UTC)

When you look at the eyepieces closely, the eyepieces should not be so small that they are two small dots. Matching the dark-adapted iris - that leads me to an idea. --Mayfare (talk) 17:34, 30 December 2008 (UTC)

The holes in which the light exits is called the exit pupil. It is calculated by dividing the lens diameter by the magnification[10]. So, your binoculars' exit pupils should be about 7 mm. However, the optimum size for an exit pupil should be around 5 mm, depending on what you're viewing. It doesn't have much to do with the eyepiece diameter, because adjusting it would affect the madnification of the binoculars. Actually, that's the telescope formula, but maybe try searching the website for more info. I'm thinking either they put rings in the eyepiece, or the prism affects it, or you're just underestimating what the exit pupil actually is. Either way, hope this helps. Thanks. ~AH1^(TCU) 23:33, 31 December 2008 (UTC)

What is this??????

Really cool invertabrae. http://www.flickr.com/photos/pokerchampdaniel/3136421527/

Looks like a sort of ribbon worm. Nemertea is the phylum, but I have no idea of the genus, or species. ~Amatulić (talk) 22:21, 29 December 2008 (UTC)

So when it "squirts" out of its...uh...skin?..is that its method for moving quickly? --71.117.36.29 (talk) 00:34, 30 December 2008 (UTC)

From the German page: The proboscis lies dorsal of the colon in a coelom (in this case rhynchocoel). It can be everted frontally if needed. [11]Lisa4edit (talk) 01:06, 30 December 2008 (UTC)

Looks to me like an animal from the Hemichordata.

December 30

salt vs. freshwater evaporation rates

Hi. I know this is probably an obvious question (and it's not homework). Say you have two bodies of water. They can be lakes or puddles, for example. The two bodies of water are the same shape, depth, volume, surface area, etc, and are placed under the same temperature with the same amount of sunlight and the same initial conditions. One body of water is freshwater, while the other is saltwater. They both have the same amount of water entering and exiting via rivers, if you are considering lakes rather than puddles. Assuming no other factors (and assuming that the initial conditions either don't change or change at the same rate), which body of water will evaporate faster, and in the case of puddles, leave a dry basin sooner? What about brackish, or hypopthetical stratified lakes with freshwater on top and saltwater on the bottom? How do they compare? Thanks. ~AH1^(TCU) 00:06, 30 December 2008 (UTC)

The fresh water will evaporate faster. See colligative property and especially Raoult's law. Basically, the vapor pressure of water vapor over a body of water is proportional to the relatve amount of actual water in that body of water. Since salt water is less "water" than fresh water is, it will evaporate slower, since it has a lower vapor pressure. --Jayron32.talk.contribs 00:21, 30 December 2008 (UTC)

Following EC:

Going by Evaporation: Concentration of other substances in the liquid (impurities): If the liquid contains other substances, it will have a lower capacity for evaporation. Thus saltwater should evaporate more slowly and increasingly so since the concentration of salt increases in relation to the remaining water. Pretty much the same as Jayron said. Lisa4edit (talk) 00:24, 30 December 2008 (UTC)

Arguing to the other side, and varying the stated initial conditions somewhat: I have observed that if ice is on a sidewalk, if salt or calcium chloride icemelter is added. it seems that changing the ice to water with chemical additions, causes it to evaporate, leaving dry sidewalk, compared to ice covered sidewalk where no salt/icemelter was added. Water ice does not appear to sublimate (ice phase to vapor phase) as rapidly as melted ice (water with dissolved solids) evaporates. Edison (talk) 04:32, 30 December 2008 (UTC)

Solid ice has a MUCH lower vapor pressure than does liquid water; in this case because you are looking at different phases (solid versus liquid) and thus different modes of molecular organization, you are really comparing apples to oranges. --Jayron32.talk.contribs 04:35, 30 December 2008 (UTC)

Fine. Then I will build a nice little machine: Ice is melted with salt. Then water evaporates into a vertical column (height h) above it. The partial pressure of water vapor will be the vapor pressure of the salt-water mixture (at some temperature below 0 degree Celsius) at the bottom of the column and will be lowered by a factor of exp(-m*g*h/(k*T)) at the top of the column (the mass of a water molecule is about 18 g/N_A; the factor is about 0.9999 for T = 270 K ad h = 1 m). If the column isn't very high then the partial pressure of water vapor is still larger than the vapor pressure of pure ice. I will place nucleation sites at the top of the column so ice crystals can grow. While the salt-water solution at the bottom has cooled during evaporation, the ice crystals will get a little bit warmer due to the heat of sublimation being regained. When the ice crystals are large enough, I'll drop them down and use some mechanism to extract energy from the falling. Then they'll meet the salt-rich (due to evaporation) solution at the bottom and melt, and I can restart the whole process ;-)

Icek (talk) 11:24, 30 December 2008 (UTC)

Something similar to the device you describe already exists and is widely available in a – non-energy producing – prototype form. It uses an exotic fluid called 'seawater', which falls as a crystalline solid known as 'snow'. This remarkable technology is entirely solar powered and carbon neutral.

Somewhat less facetiously, I assume that you're going for a tongue-in-cheek description of a perpetual motion machine (of the first kind): a device which extracts free energy from the environment. As with all such devices, the challenge is in locating the 'leaky' assumption. Here, the trick is that the gravitational potential energy of the ice crystals at the top of the column doesn't come for 'free'. That energy is pulled out of its surroundings, from the kinetic energy of the air in the column (heat). Since you're stealing that energy out of the system (by making the falling ice do work), everything cools down. If the column isn't fitted with an external heat source, then eventually it will be chilled to the point where it stops working.

In principle, you could put the device in a sunny location; that's how the Earth makes its snow, after all. But there are much more efficient ways to extract energy from sunlight. TenOfAllTrades(talk) 16:35, 30 December 2008 (UTC)

It is of course tongue-in-cheek, but my real point is that it's still a perpetual motion machine of the second kind. When starting the machine, the temperature may be equal in all parts of the machine. Yet mechanical energy can be extracted while it cools without heat exchange with the environment. Icek (talk) 17:11, 30 December 2008 (UTC)

If the temperature is equal in all parts of the machine, then why would ice crystals grow? Wouldn't they reach thermal equilibrium with the water vapour, adding and sublimating molecules at the same rate? Franamax (talk) 19:23, 30 December 2008 (UTC)

My description is to be read in the context of the 2 preceding comments by Edison and Jayron32. Icek (talk) 20:05, 30 December 2008 (UTC)

As nobody seems to care about disproving my perpetual motion machine, I will explain: TenOfAllTrades was of course correct that it doesn't generate energy from nothing, because the crystallization process takes out water vapor molecules with higher-than-average potential energy and if the remaining vapor equilibrizes (is that a word?) and reaches a Maxwell–Boltzmann distribution again, it will be colder. But if it is true that the vapor pressure of water above a liquid solution of water and salt at a temperature below the melting point of pure water is higher than the vapor pressure above pure water ice at the same temperature, then my machine is still a perpetual motion machine of the second kind. Whenever two substances, A and B, mix spontaneously, and the vapor pressure of substance A (salt in our case) is practically zero above the solution, then the vapor pressure substance B above the solution cannot be higher than the vapor pressure above pure B, or the second law of thermodynamics is violated. Icek (talk) 07:12, 31 December 2008 (UTC)

Collisions in an ideal gas

What is the frequency of particle collisions in an ideal gas, given the standard parameters (particle number, pressure, volume, temperature)? Is this substantially different in a solid? Thanks, *Max* (talk) 06:21, 30 December 2008 (UTC).

Frequency of particle collisions in ideal gas is zero. Ideal gas is a gas of non-interacting particles. It is, of course, an approximation; no gas is truly ideal. You have to specfy the chemical componsition and the thermodynamic parameters of the real gas to be able to determine the collision frequency. And, by the way, the "collision frequency" in a solid is a very poorly defined quantity. There are electron-electron, electron-phonon, phonon-phonon, electron-impurity collisions, and so on. Collisions may be with or without umklapp if the solid is ordered. Effective collision frequency for longitudinal momentum, transverse momentum, and energy transfer may be quite different; etc. Your question is very good; it is just not answerable in simple terms. Please be more specific. All the best, --Dr Dima (talk) 07:57, 30 December 2008 (UTC)

Why did this atom get together with that atom?

I mean to ask this on two levels. Re: the formation of the universe. What attracted the first atoms in existence to interact with the other atoms? Secondly,-today. If I point to a random object or even thin air, and in my direct geometrical line lies two atoms. One here, one there. One touches the other one. Why?--Mark L. Dowry (talk) 06:48, 30 December 2008 (UTC)

I don't know why about the second question, but my guess for the first is that it is still gravity which attracts too objects together. The very first atoms were likely attracted in this way. —Cyclonenim (talk · contribs · email) 09:22, 30 December 2008 (UTC)

It's not gravity that causes atoms to join to form molecules. There are many types of chemical bonds. Ionic bonds, for example, are electromagnetic attractions between positive and negative ions. StuRat (talk) 15:22, 30 December 2008 (UTC)

What attracted them? Well, the first atom looked across the rom in science class, saw the 2nd one's hair glistening in the sun, her eyes just sparkled so, and she looked at him with the most beautiful smile...sorry, I just had to say that after reading the title :-)Somebody or his brother (talk) 13:52, 30 December 2008 (UTC)

The OP may find Cosmogony somewhat interesting. Questions of the behavior of matter during the first instants after the Big Bang are quite difficult to answer, and our understanding of a small, hot universe during that time period is that it was a VERY weird place indeed. As far as what holds atoms together NOW (which is kind like asking what caused them to form molecules in the first place) is pretty simple. Atoms in all chemical compounds are held together by electrostatic forces, see electromagnetism. Its ALL held together, at the molecular level, by positive and negative charges attracting each other. Ionic bonds, covalent bonds, metallic bonds, all of it is simply the positive bits and the negative bits sticking together. Inside of the nucleus of the atom, forces such as the strong nuclear force and the weak nuclear force are at work. Depending on which model you are working from, gravity either is a fictional force, or it is simply too weak to be able to explain the strengths of the interactions at the molecular and sub-atomic scales. --Jayron32.talk.contribs 18:01, 30 December 2008 (UTC)

PROOF

09:29, 30 December 2008 (UTC)harshagg hi,

actually one asked in misc section that less of school teachers know how to prove like charges repel and unlike attract.Can anyone prove this.I don't think so that simply saying its because property of charges.There must be some trick behind that —Preceding unsigned comment added by 122.163.44.220 (talk) 09:29, 30 December 2008 (UTC)

I think it is a theorem in quantum field theory. See Chapter I.5 of Quantum Field Theory in a Nutshell by A. Zee, available here. In classicalgrade-school physics there's no reason for it to be true, so you can't fault grade-school teachers for failing to give an explanation. -- BenRG (talk) 12:10, 30 December 2008 (UTC)

Be a smart ass and say there are no proofs outside of logic and mathematics.--droptone (talk) 12:33, 30 December 2008 (UTC)

It's probably sufficient to say that centuries of consistent empirical observations lead us to believe that there are two classes of electric charge - positive and negative - and that objects with net positive charge attract objects with net negative charge, etc. You can go on to say that more detailed observations explain the subatomic structures (protons and electrons, generally) which contain this charge. To try and prove that decades or centuries of consistent observations are all correct is more of a philosophy question like Droptone implied. Nimur (talk) 13:46, 30 December 2008 (UTC)

What's wrong with getting two objects with the same general charge and seeing if they repel and doing likewise with objects of opposite charges? You'd have to assume, of course, that the objects are in fact electrically charged. --Bowlhover (talk) 17:31, 30 December 2008 (UTC)

See below. -- BenRG (talk) 18:55, 30 December 2008 (UTC)

The problem of induction.--droptone (talk) 18:41, 30 December 2008 (UTC)

True, but I would consider the fact that all known like charges repel to be an acceptable proof, especially since all electric charges are due to the same particles and those particles are exactly the same. --Bowlhover (talk) 23:46, 1 January 2009 (UTC)

The OP is looking for a strange definition of "proof". Its a meaningless question. How do you prove that the sun will actually rise tomorrow? How do you prove that when I let go of my pencil, it will drop to the floor? How do you prove that fire is hot? All of these things, and the entire scope of scientific thought, are "proven" in the same manner as electric charge dynamics. The explanation of "like charges repeling and unlike changes attracting" is consistant with all known observations, and the theory can be used to accurately predict future behavior with something like 100% accuracy. That's a proven as anything in the entire world can be. How do you prove it? Because it always happens that way, and it never happens the other way. What more proof do you need?!? If you can get two negatively charged objects to attract to each other, there are people in Sweden who would like to give you a shiny medal. --Jayron32.talk.contribs 17:53, 30 December 2008 (UTC)

Nonsense. There are lots of proofs in physics. Newton proved Kepler's laws from his laws of dynamics and gravitation. Before that Kepler's laws were raw empirical facts, but afterward they were necessary consequences of more fundamental laws. Kepler couldn't explain why orbits aren't square, but Newton could. Hundreds of years later general relativity showed that the exponent in the law of gravitation, which was an adjustable parameter in Newton's theory, can only be 2. In quantum field theory, like charges have to repel in a vector field theory—you can't have a theory that's like electromagnetism except that like charges attract. I think that's also true in classical electromagnetism (Maxwell's equations). These are all theorems in the mathematical sense. Proofs in physics rest on axioms that may or may not be right, but proofs in mathematics also rest on axioms that may or may not be right, and mathematicians aren't actually much more certain of their axioms than physicists are. They believe in the basic structure but there's a lot of uncertainty about the details. The accumulated knowledge of biology is to a large extent a brute collection of facts, but in physics it's largely a collection of theorems. -- BenRG (talk) 18:55, 30 December 2008 (UTC)

I disagree that, for example, Einstein's formulation of gravity "proved" the adjustable parameters any more than Newton's estimate for that parameter. The mathematics may be more elegant, but there is still a fundamental level at which we must say, "General Relativity seems consistent with every observation we have made thus far." The mathematics of Relativity may dictate a parameter of 2 - I don't dispute this. But it is an equally strong statement as Newton's assumption, because it is fundamentally saying that this equation can be used to model reality. The level of elegance of the derivation is irrelevant. Science is the process of modeling the real world, and we can develop any model we like - and any model we contrive can be useful if it fits our observations and makes our predictions more accurate. The notion of "proving" that the theory fits the experiment is only a matter of building trust through repetition. Nimur (talk) 22:37, 30 December 2008 (UTC)

Which is why, in the end, Popper embraced falsification, not verification, as the mechanism of creating reliable knowledge. It is easy to come up with verifiable claims that are not falsifiable; it is hard to come up with falsifiable claims that can be verified. --98.217.8.46 (talk) 04:37, 2 January 2009 (UTC)

imortal

If you are biological imortal. What would happen to you, would you age ? would your body still deteriorate. And what are your chances of liveing to 2200

Seeing as being immortal (as a human) is impossible it's an impossible question to answer. If science could develop far enough to make a person immortal it would only be worthwhile if it could ensure that people maintained a decent age of body (say full adulthood, mid 20s - though anything up to maybe 60 could have a market value - being immortal and being in the body of a 110 year old is not likely to be appaeling to consumers). Being immortal body-function wise is different to being immortal life-wise. You might be shot or run-over, or die in a freak magic trick - essentially it's one thing to make people's bodies not die 'naturally', another thing entirely to make someone immune from death itself. 194.221.133.226 (talk) 12:57, 30 December 2008 (UTC)

Considering that we're talking about make-believe here, any number of scenarios could happen. You might grow older and more decrepit with each passing decade, but never die. Or you might look like you're 25 forever. Perhaps you could be killed by violence or poison or a disease, but your body would never fail on its own. But you might as well wonder if a man who can fly can fly to Mars. That depends entirely on the internal rules of the fiction. -- Captain Disdain (talk) 14:25, 30 December 2008 (UTC)

Oh dear, an edit conflict ate my post. The links I wished to provide, if some kind soul would like to weave them together coherently then by all means: The Picture of Dorian Gray Vlad the Impaler Programmed cell death Free-radical theory cancer mutation solar radiation Ship of Theseus how many times can the same person step in the same stream? 98.169.163.20 (talk) 15:01, 30 December 2008 (UTC)

If biological immortality were possible, I'd say it would take the form of constant replacement of body parts (with replacements grown in a lab) as they wear out, so you would have some "new" parts and some "old" parts at any given time. The problem with this is the replacement of the brain. Perhaps it could be replaced a few cells at a time, so as to not wipe out your memories. StuRat (talk) 15:18, 30 December 2008 (UTC)

We have a very detailed article on immortality. What you asking is, however, a bit vague. If you refer to this reality, then there is no magic, and no immunity from death. You can expect the science to advance far enough in the next 50-100 years to allow replacement of any tissue damaged by aging or disease, without affecting the person's mind and memory too much. Severe enough trauma would still kill anyone; there is no way around it. You cannot make "backups" of people like you make backups of your files; that was only possible in Star Trek reality, if at all. Now, in fantasy books there are many kinds of immortality, too. Probably the most common one is where some races (as in LoTR) or some individuals (as in Juuni Kokki) do not age but can die by the sword. Another type of immortality is that of Koschei, who can only die if the inanimate object holding his soul is destroyed. Third type is immortality of the "living dead", often employed, for example, in vampire or shinigami fiction. Fourth type is immortality of non-biological or purely spiritual beings, but that's not what you are talking about. Ultimate type is the immortality of God; some religions describe "dissolution" in God as an ultimate fate of the righteous people. I hope this helps in any way. All the best, --Dr Dima (talk) 21:02, 30 December 2008 (UTC)

Turritopsis nutricula is an immortal jellyfish, it reverts to its polyp stage after becoming an adult. However, I don't know if if its cells deteriorate over time. ~AH1^(TCU) 22:24, 30 December 2008 (UTC)

To say living things cannot (in principle) be "backed up" is vitalism in my humble opinion. —Tamfang (talk) 06:05, 5 January 2009 (UTC)

Velocity of an Object under Gravitational Influence

What is the velocity of an object under gravitational influence at any time t?(Reminder:Velocity is dependent on acceleration which is dependent on distance which is dependent on velocity......(I attempted to solve this question a few tens of times but always failed(I even used calculus(I prefer calling differential calculus fluxions and integral calculus fluents because thats how Newton, the first discoverer of calculus called it) and other complex methods to try to solve this question)))----The Successor of Physics 14:56, 30 December 2008 (UTC) —Preceding unsigned comment added by Superwj5 (talk • contribs)

I'm not going to solve this, but will make some comments. First, I assume you want to neglect air resistance ? The velocity is given as v = gt, where g is the acceleration due to gravity. But, as you said, the value of g changes with distance from the Earth (or other massive object). In such a scenario we need to determine if the mass of the object is trivially small, such that the acceleration of the massive object towards it can be ignored. If not, then we need to decide what we mean by the velocity of the object. Do you mean the velocity towards the massive object, which is the sum of both accelerations, or do we mean the acceleration of the smaller object alone ? StuRat (talk) 15:06, 30 December 2008 (UTC)

I mean something more like the acceleration of the smaller object alone.----The Successor of Physics 14:39, 31 December 2008 (UTC)

Hmm. So, as is described in Newton's law of universal gravitation, the time derivative of the velocity of an object in a gravitational field is proportional to the strength and direction of the gravitational field. The gravitational field is determined by the distribution of masses. Therefore in order to figure out what the velocity of an object in a gravitational field is at any time, you must know the distribution of other masses in the system, as well as the initial velocity of your mass. There is unfortunately no way to write an algebraic expression for the velocity at time t without specifying the distribution of mass creating the gravitational field. Do you have a specific system in mind in which you wish to know the velocity of a mass? If so, we might be able to help you solve that specific differential equation and determine the velocity at any time. --Bmk (talk) 22:32, 30 December 2008 (UTC)

I would like an uniform sphere mass model and I would want it to allow for tidal forces too. Also, I would like to include the initial velocities of the objects. I hope I could get something for as much objects as possible, but if it doesn't work a system of two objects would do.----The Successor of Physics 14:39, 31 December 2008 (UTC)

It looks like you're trying to solve a complicated version of the n-body problem. Unless you know advanced mathematics and would accept a very slowly-converging infinite series as a solution, and unless one mass is much greater than that of all the others, it is only possible to solve a two-body problem. The solution for a two-body problem is the Kepler orbit; spheres can be treated as point masses located at their centers. Just make sure the distance between them is greater than the sum of the radii and this simplification and this simplification would be realistic. As for tidal forces, I'm really not sure how those can be taken into account, but they have an extremely small effect in reasonably short (with millions of years) timescales for planet- or moon-sized bodies at the usual astronomical distances. --Bowlhover (talk) 07:27, 1 January 2009 (UTC)

Thank you! It helped a lot!!!----The Successor of Physics 09:26, 1 January 2009 (UTC)

I'd suggest a point mass model first. Then maybe try a uniform sphere mass model. StuRat (talk) 03:37, 31 December 2008 (UTC)

Whatever you might be trying to calculate, remember that only two variables can be in any equation. If you have more, find a function expressing one of the variables with respect to another. For example, in the equation dv/dt = Gm1/r^2, v and r are related by the conservation of energy. If r is written as a function of v, the equation can be solved by integrating. --Bowlhover (talk) 01:54, 31 December 2008 (UTC)

Motion of an aircraft

If an aircraft travels against the Earth's spin, does it take a larger force for it to reach the same speed as if it were to take off in the direction of the Earth's spin. According to Newton's first law, shouldn't the aircraft taking off in the direction of the Earths spin have a higher initial speed? Thanks in advance. Clover345 (talk) 17:10, 30 December 2008 (UTC)

Interesting question, but I'd lean toward no. What gets the aircraft moving is the tires pushing against the tarmac and the turbine blades pushing against air molecules. Initially the force vectors resulting from earth's spin affect all those factors. It's not a case where earth spins fast enough for the forward force vector getting strong enough to overcome the aircraft's inertia and flinging it off the surface. Lisa4edit (talk) 18:22, 30 December 2008 (UTC)

Not tires pushing against the runway- airplanes do not have drive wheels. What matters for takeoff is airspeed. The earth and its atmosphere are rotating, so this is probably pretty close to being ignorable. Friday (talk) 18:28, 30 December 2008 (UTC)

Lisa, I recently watched a Mythbusters episode where they dragged a carpet opposite the direction of the airplane, and sure enough it took off like normal (much to the surprise of the pilot). Graphical confirmation of what Friday is saying - all the wheels do is support the plane, within a few percent. Franamax (talk) 01:05, 31 December 2008 (UTC)

Evidently not Albatross airlines. (My flight instructor had it wrong, too.) Thks. for putting the facts straight. Lisa4edit (talk) 01:44, 31 December 2008 (UTC)

Aircraft speed is more influenced by wind speed, but centrifugal force is important when putting rockets into orbit. Less fuel is needed if the rocket is launched at the equator and ends up going in the direction of the earth's spin. Dmcq (talk) 18:27, 30 December 2008 (UTC)

You might also want to read up on the jet stream: "Within North America, the time needed to fly east across the continent can be decreased by about 30 minutes if an airplane can fly with the jet stream, or increased by more than that amount if it must fly west against it."--Shantavira|^{feed me} 18:54, 30 December 2008 (UTC)

Fictitious forces due to the Earth's non-inertial reference frame on a jet plane are very small (less the one percent of a g) - see rotating frame of reference. Since the atmosphere mostly moves with the earth, aside from changing wind patterns, an eastward flying plane has no advantage over a westward plane; they are both pushing through a medium which is (on average) at rest relative to the earth's surface. --Bmk (talk) 22:44, 30 December 2008 (UTC)

And when I said fictitious forces, I really meant fictitious accelerations. --Bmk (talk) 03:14, 31 December 2008 (UTC)

Since the Earth, the plane, and the air are all moving at the same speed initially, due to the Earth's rotation, there is no net effect on the velocity as measured relative to the Earth. If you measured relative to a stationary object (let's say the Moon, as it's relatively stationary compared with the Earth), then it would, indeed make a huge difference which way you went. If flying East near the equator, you'd need to go around 1000 miles per hour (relative to the Earth) just to become stationary (relative to the Moon). StuRat (talk) 03:33, 31 December 2008 (UTC)

I think you've missed the point of the question, all speeds are measured relative to the earth. —Preceding unsigned comment added by 92.8.100.245 (talk) 11:41, 31 December 2008 (UTC)

No, I didn't miss the point of the Q. The whole point of my answer is that it doesn't matter if the jet was already moving in one direction, as that movement in one direction doesn't exist when measured relative to the Earth and air, which are all moving the same velocity as the jet initially. I then expanded on the answer to point out that the premise is only correct if viewing the jet's motion relative to a stationary observer (off the Earth). StuRat (talk) 07:59, 1 January 2009 (UTC)

But you are discussing simple Galilean mechanics, which doesn't work here as the earth is a non-inertial reference frame, the question is regarding the increment in centripetal force caused by the planes motion, and you cant explain this away by changing reference frame as the plane is accelerating, not moving, towards the centre of the earth, and is so in all inertial reference frames. —Preceding unsigned comment added by 92.1.148.100 (talk) 12:46, 1 January 2009 (UTC)

helium

If you inflate a rigid tank to it's maximum pressure with helium, will the tank get lighter? Thanks Bill —Preceding unsigned comment added by 208.100.237.48 (talk) 20:43, 30 December 2008 (UTC)

That would depend on what was in there before. If it was a vacuum inside, then it will be heavier (since helium weighs more than nothing). If there was already something in there, and you replaced it with helium, then it would depend on the densities involved. - Akamad (talk) 20:48, 30 December 2008 (UTC)

If the tank is a K bottle at 2500 psi of helium and room temperature then the tank will contain about 1.5 kilograms of helium. If it were full of air at standard pressure, it would contain about 50 grams of air. The answer is no, the pressure of the helium would be sufficient to make it more dense than air. In general, the answer will depend on whether you can sustain a pressure in the tank such that the helium (pressurized) is denser than air at standard pressure; that would yield

${\frac {P_{helium}}{P_{atmospheric}}}={\frac {molarmass_{air}}{molarmass_{helium}}}$

...or a pressure of about 7.5 atmospheres (110 psi) - not a very strong tank, if that's the maximum pressure it can withstand. Nimur (talk) 22:54, 30 December 2008 (UTC)

Passive solar building design

Why are west facing windows bad in passive solar building design? It is mentioned a few times in the article but no full explanation is given. All the other aspects I understand. 41.243.38.111 (talk) 21:31, 30 December 2008 (UTC) Eon

It's due to the fact that the sun sets on the west. For example, in the middle of summer, at 3pm, when it's hot enough as it is inside your house as it is, you don't want the added heat from solar radiation. - Akamad (talk) 22:21, 30 December 2008 (UTC)

Thanks, but that still doesn't quite explain it for me, because in the winter you'll want that extra solar radiation. Also, in a climate with very hot summer you might want to avoid the morning sun (east) just as much if enough heat enters the building through conduction. My problem is that when averaged out over 24 hours of earth rotation, I don't see what can be achieved by leaving out west facing windows that doesn't hold true for leaving out east facing windows. 41.243.38.111 (talk) 06:47, 31 December 2008 (UTC) Eon

Just don't forget that the air is colder in the morning than in the evening because it cools the whole night. Icek (talk) 07:00, 31 December 2008 (UTC)

Sorry if this is starting to become a discussion. If heat conduction from the air itself is the concern, then (ignoring solar radiation) does the direction that the window is facing make a difference at all? I don't know if it is realistic to assume that cold morning air gets trapped on the east side of the house for the entire day, but even if we did assume this, then preferring east facing windows only offers an advantage as a method of keeping the building cool on hot days and it would be a disadvantage during a cold winter. This is never mentioned in the article. So I'm still in the dark about this one. 41.243.38.111 (talk) 08:38, 31 December 2008 (UTC) Eon

Well, it's really not just the air that is colder, but also the house itself, and my point was that you generally need more heat in the morning as the temperatures are lower. Icek (talk) 09:58, 31 December 2008 (UTC)

From what you're saying it does make sense to me that, given the thermal delay surrounding day/night temperatures, east facing (as opposed to west facing) windows will help even out the 24 hour temperature profile. What it does not seem to do in my mind is to even out seasonal variations. Is my understanding correct? —Preceding unsigned comment added by 41.243.38.111 (talk) 16:18, 31 December 2008 (UTC)

I think it is. For correcting seasonal variations, one simple approach is to have large (vertical) south-facing or north-facing (in the northern or southern hemisphere, respectively) window panes. Then the amount of sunlight entering through these windows will be larger in winter, because the altitude of the Sun will be lower. Icek (talk) 16:35, 31 December 2008 (UTC)

Thanks, that all makes sense to me now. 41.243.38.111 (talk) 23:41, 31 December 2008 (UTC) Eon

in the northern hemisphere, in summer, the sun sets north of west, and in the winter, it set south of west. The total difference is large. Therefore, west-facing windows are a major disadvantage in summer and a trivial advantage in winter. -Arch dude (talk) 16:44, 31 December 2008 (UTC)

Thanks but, again, couldn't the same be said for sunrise and east facing windows? 41.243.38.111 (talk) 23:41, 31 December 2008 (UTC) Eon

I've wondered the same thing myself, and from my reading I eventually formed this opinion: The inside of your house gets hot by conduction and by radiation from outside. Whilst most of the heat gain is due to radiation (the sun roaring in through your window), this is either contributed to (or offset) by the conductive heat gain (or loss). In the morning, the ambient air temperature is lower, and offsets the radiant heat from the sun (the cooler air absorbs radiant heat energy). In the afternoon, the air is hotter so this offset is lost. That, plus what Arch dude said. Mattopaedia (talk) 03:40, 1 January 2009 (UTC)

Now I'm totally in the dark again. Looking at some average daily temperature profiles I see daily thermal lag isn't so high. The air temperature is lowest just as the sun rises. Due to conduction air temperature inside a building will tend to rise along with outside air temperature in the first half of the day to reach a maximum a few hours after noon. By allowing sun to also heat the building during the morning upward slope (with east facing windows), higher peak temperatures will be reached. West facing windows sound more optimal to me! They would also capture solar energy before sunset, allowing some stored solar energy (as heat) inside the building to help keep the temperatures slightly warmer at night. So no, I'm not convinced at all :( 41.243.38.111 (talk) 11:12, 1 January 2009 (UTC) Eon

In winter. It could well be better to have west facing windows, since this will warm your house in the evening. (However, it's more important to have east facing windows, since the morning is when you really need the solar radiation.) But I guess the argument is that the benefits you get in the winter are more than offset by the detriments you get in the summer (of course, your location has a big part to play in this). I am speaking from experience: my house has floor to ceiling windows and it get horrendously hot in the afternoon during summer. The benefits during the winter are not that great. - Akamad (talk) 11:27, 2 January 2009 (UTC)

Maybe it depends on where you live. I live in a subtropical climate, and I can tell you for certain the afternoon sun really heats the place up, which is nice in winter, but not so much in summer. Mattopaedia (talk) 22:39, 2 January 2009 (UTC)

Waiting for the water to turn hot

In my house, the water heater is on one side of the building and the kitchen is on the opposite side. When I turn on the hot water tap, I have to wait for up to 30 seconds before the water runs hot. If I'm wanting to waste as little water as possible (let's assume it's not possible to save it in a container), does it make any difference whether I have the water just trickling out while I'm waiting, or have it on greater pressure? Does this depend on the type of heater? I'm assuming that all the cold water in the pipes, between the heater and the tap, has to be let go first before any of the heated water can arrive, so if it makes no difference whether it flows out slowly or quickly, I may as well have it on full pressure and at least save some time. Am I thinking right? -- JackofOz (talk) 22:51, 30 December 2008 (UTC)

The more quickly the water moves from the heater to the sink, the less time it will have to lose heat through the walls of the pipe. If you reduce the problem to an absurd level, if you open the tap only very slightly, the water might move so slowly toward the sink that it will have lost all heat before it gets there. Get yourself some pipe insulation; you're throwing money out the window as we type. --Milkbreath (talk) 23:04, 30 December 2008 (UTC)

If you want to read up on this, the length of pipe between the heater and the tap is known by plumbers as a "dead leg", and the insulation as "lagging" (it's a noun and a verb). BrainyBabe (talk) 17:54, 31 December 2008 (UTC)

You can actually buy devices that continually send the cold water back to the tank rather than running it down the drain - these are used in some hotels where a room may be a LONG distance from the water heater. However, they put the cost of heating the water up because the insulation of the pipes is not perfect. A pretty decent alternative is to use a 'flash heater' that sits under the sink and heats the water up right there under the sink as you need it. The efficiency of those devices isn't as good as the big heaters in your water tank - but because it only heats exactly what you need and does it instantly, there is almost zero wasted water and no energy is wasted in keeping the tank hot just in case you might need it at 4am! I had one of those in our house in the UK - along with a shower fitting that did the same thing. Turning off the big immersion heater made HUGE savings. Sadly, when I built my house here in the USA, I couldn't get the 220 volt wiring to the bathroom in any manner that would pass the local building codes and 110volts isn't enough to get the water hot enough quickly enough to make the system practical. Yet another case where stupid laws make us waste energy. SteveBaker (talk) 23:05, 31 December 2008 (UTC)

Thanks for those helpful suggestions about how to improve the efficiency of the system. What I'm wanting to know, though, is about how it operates right now, today. Is it a better strategy to have greater pressure or less pressure while waiting for the cold water in the pipes to empty? My theory is that if I can't save any of that cold water, I can at least save some time by letting it out quickly as opposed to slowly. That's what I want confirmation about. -- JackofOz (talk) 01:44, 1 January 2009 (UTC)

More pressure: The water moves through the pipe faster, so there's less heat lost to the pipe. Although, I suspect the reality is you're only talking about a time saving of a few seconds. This I know because of the time difference to get hot between my pressure regualted bathroom taps versus my unregulated laundry taps. Either way, you're going to lose at least the same initial volume of water that's in the pipe & cold, but theoretically the less heat lost to conduction, the sooner the water gets hot at the tap, and the less water wasted. Mattopaedia (talk) 04:37, 1 January 2009 (UTC)

Thanks, Matto. -- JackofOz (talk) 04:59, 1 January 2009 (UTC)

No wuckers, Jacko! Mattopaedia (talk) 05:11, 1 January 2009 (UTC) ;-)

I have the same problem, Jack. I've noticed the water stays ice cold for 30 seconds, then gets warmer instantly, but only slowly gets hot after that. I believe that the 30 seconds was for the water in the pipes to be dumped out. After that, I believe I'm waiting for the pipes themselves to warm up. With the metal pipes I have, even hot water is only warm when it gets to the end of the pipe. So, if I get in the shower when it's in the warm phase, it then gets hotter and I burn my kibbles and bits. Therefore I have to wait several minutes. I'm also concerned about wasting all that water, so have been taking baths instead. The cold water that pours out at first then mixes with the warm and eventually hot water to make for a just-right bath. Too bad every tub ever made is apparently only long enough for munchkins, though. To answer your original Q, fast would be good for the first phase, but then slower is better to allow the pipes time to heat. However, if you have plastic PVC pipes, then the "warming the pipes" phase may not apply to you, as they don't hold much heat. StuRat (talk) 07:46, 1 January 2009 (UTC)

I very much doubt that you are saving water by taking a bath instead of a shower. You should take the scientific approach:

• When pipes and water are cold - run the water into a bucket until it gets hot enough to shower in. Count how many bucketfulls you'd waste. Now run the shower (again, into a bucket) for the typical duration for which you shower. Count how many bucketfulls you use usefully.

• Now run a bath to the depth you normally use - and empty it (using the same bucket) into a nearby sink or toilet - counting the bucketfuls.

I can almost guarantee that the shower will use less water (and hence also less energy to heat the water).

SteveBaker (talk) 17:01, 1 January 2009 (UTC)

Oh - and remember that when you take a bath, some of the energy in the water goes into warming up the bathtub itself. SteveBaker (talk) 17:03, 1 January 2009 (UTC)

One big factor that will throw off all your calcs is that I shave in the shower/bath, which adds a good 20 minutes to the timetable. I wonder if I'm the only one who does this ? If seems to work a lot better for me as the used shaving cream, bits of hair, and inevitable blood that falls off my face goes down the drain instead of on the edge of the sink, floor, my clothes, towels, etc. There's also a temperature concern. I leave it cold in my bathroom, but it gets nice and warm in the bath/shower, from the bath water/shower water. I have an electric space heater I could use to heat the entire bathroom, but that's got to cost more as it's expensive electrical heating and I'd be heating a much greater volume of air that way. Also note that, after I finish a bath, I don't drain the bath water immediately, but leave it to add heat and humidity to the cold, dry winter air inside my house, first. StuRat (talk) 16:52, 2 January 2009 (UTC)

I don't know about the shaving thing (I have a beard which comfortably - and naturally - circumvents that whole thing). An electric shaver seems like it would be more efficient (and certainly shouldn't take 20 minutes or involve large quantities of blood splatter!) But as for the warming & humidifying of the bathroom - I would suspect that the shower does that better because you have an aerosol spray that's mixing with the air rather than water just sitting there in a puddle. Much of the heat from your bathwater is almost certainly being conducted away through the plumbing. But in any case, heating water in order to subsequently heat the air can't possibly be energy-efficient when compared to a heater that's designed to heat air. So I think you're deluding yourself - by the time the bath water has given up it's heat, you're going to be dressed and outta there - so even if you do warm the bathroom that way, the benefit is slow in coming. But, again, a scientific approach is required. Take a thermometer into the bathroom - shower one day - bath the next - plot graphs - publish results. SteveBaker (talk) 19:46, 2 January 2009 (UTC)

I have to disagree with "heating water in order to subsequently heat the air can't possibly be energy-efficient...". Heating water and using it to subsequently heat the air is hydronic heating, which is quite common. The shower puts too much humidity in the air, so that it condenses on the windows and soaks the window sills. Bath water, on the other hand, adds heat and humidity slowly, so that it has a chance to distribute itself evenly throughout the house. As for electric razors, I've never found one that can give me a smooth shave; my skin feels like sandpaper after using one. StuRat (talk) 07:24, 3 January 2009 (UTC)

The Californian Energy Commission [12] says: "If your home was built before 1992, chances are your showerheads put out about five gallons of water per minute (gpm). Multiply this by the number of minutes you're in the shower, and the water adds up fast! An average bath requires 30-50 gallons of water. The average shower of four minutes with an old shower head uses 20 gallons of water. With a low-flow shower head, only 10 gallons of water is used. To test the amount of water used in a shower vs. a bath is to put the plug in the bath next time you take a shower (but not a stand-alone shower as you might spill over the lower shower wall). After you've showered, see how much the tub filled up."

That's useful data. If you have a half inch diameter water pipe going between hot tank and shower - and (let's guess) your hot tank is no more than 30' (360") from the shower (that would be a very large and spectacularly badly designed house!) - then the amount of cold water in the pipe is something like 90 cubic inches - which is only about a third of a gallon. If that takes 30 seconds to flow through your pipes and you have to wait another couple of minutes for the pipes to warm up - then you're probably wasting no more than a couple of gallons in letting the water heat up - and that's pretty negligable compared to the 30 to 50 gallons in a bathful. But that doesn't quite gel with a showerhead putting out 5 gallons per minute - because it's being fed from the same water source at the same speed. So I suspect that if your water takes 30 seconds to start getting hot - then your showerhead must be a more modern lower-flow head. Hence, I estimate that you're wasting 2 gallons waiting for the water to get hot - then using 10 gallons more for your shower for a total of 12 gallons...versus a minimum of 20 gallons for a bath. So showering is VASTLY more efficient...even if you have a long wait for the water to get hot. SteveBaker (talk) 19:44, 1 January 2009 (UTC)

I think it'd be fair to say that most Australian household taps and shower heads are low flow - since 80-odd % of the country's been in drought for the last 5 or so years. Now, to me, a gallon is about 4.5 litres. A showerhead that puts out 22.5L/min seems absolutely insane to me, but that's because we've had some pretty severe water restrictions here (140L/day/person) where a showerhead like that would use up your day's water allowance in a little over 6 minutes. My showerheads use 9L/min, which is pretty common, but you can by ones that use 6. I reckon it's not unreasonable to have more than 30' of pipe between your hot water service and the tap, when you allow for the course the pipe has to take from the HWS under the floor and/or through the roofspace and walls. But, for the sake of the discussion, I'll call it 10m (about 33') of 12mm pipe (Volume =πr²h, or 3.1415926536x0.6²x1000 = 1130cm³ or1.3L (~0.3 Gal)). So that would take a little under 9 seconds to empty the cold water from the pipe at 9L/min. That would mean the water volume needs to be replaced about another 2 times for the water to heat the pipe & become hot. You're still only talking about losing 3.9L though. If you use a 9L/min showerhead, then the water saving compared to a bath is enormous - a 4 minute shower would use no more than 45L (10gal) allowing for time for the water to get hot, probably at least half what you'd use in a bath. Mattopaedia (talk) 23:37, 2 January 2009 (UTC)

More pressure of course. If you can't get at most of the pipes, at least insulate where the hot pipe comes out of the heater for as long a distance as you can. That hot bit wastes a lot of energy. Polypipe Wrangler (talk) 04:00, 3 January 2009 (UTC)

Since all of the water that you use that comes from the hot water tank has to be replaced - and therefore heated, the amount of energy you consume is directly proportional to the amount of water you take from the hot tap. Insulation will help only in that it allows you to run the water for a shorter amount of time before jumping into the shower (but we've already decided that this is a small part of the problem compared to the time you spend showering or the depth of the water in your bath). What's more beneficial is that insulating the pipe will make the water arrive at your shower head at a higher temperature which allows you to mix in more cold water to get the temperature bearable. However, the driving factor is still the amount of water you use - and the shower beats the bath hands-down in that regard. SteveBaker (talk) 13:35, 3 January 2009 (UTC)

Depth and water pressure

If I went down an oceanic trench for thousands of meters aboard a sub, and all of a sudden found a cave on a wall of the trench, could I go in, come out the sub, and scuba dive inside the cave without dying? 190.157.120.42 (talk) 22:52, 30 December 2008 (UTC)

Why would it matter if there is a cave? The water pressure would still be at ambient (at thousands of meters depth, that would be a really high pressure, like tens of thousands of psi). Your scuba gear would probably be crushed - a normal scuba tank is only at 2000 psi, so air would not flow out of the tank (you'd be lucky if water didn't rush INTO the tank). Also, you would need some kind of hatch on the sub that wouldn't suffer the same problem - which is not likely. While exploring Challenger Deep [13], the Navy was not even able to use air for ballast, (it was not feasible to blow the tanks with air at lower pressure than the ambient water!). Instead they used gasoline or hydraulic fluid for ballast, to overcome the pressure problem. The crew compartments of DSV Alvin or Trieste are far below ambient water pressure, and you'd have a hard time pushing the door outward... Even if the hypothetical cave did exist, and hypothetically, if there were a trapped air space, that air would still be at ambient pressure and you'd have the same problems. Human biology is not well-suited for high pressures, as are found in the deep ocean. While scuba-diving, you cope because the air you breathe is kept at ambient water pressure via a pressure regulator, and so you are not crushed by the water. This limits you to only a shallow dive, though. A bathyscaphe or other deep-sea vehicle protects and isolates the humans from the pressurized water environment, by keeping them inside a hull which is kept at survivable pressure levels. Nimur (talk) 23:10, 30 December 2008 (UTC)

But doesn't the height of water determine its pressure? In other words, "the pressure at any point in a still body of water is the sum of the pressure due to the weight of the atmosphere and the pressure due to the weight of the column of water directly above that point" [14] So out in the open that would certainly be true, but if I get into a cave, don't I reduce the pressure from the column of water above any point inside the cave? 190.157.120.42 (talk) 23:27, 30 December 2008 (UTC)

Pressure of a fluid is due to the pressure of anything above it (or horizontally exposed to it, or below it), including pressurized air in a cave (or solid material "pushing back" on the water) (or the water outside the cave squeezing on the water inside the cave). Take a look at Hydrostatic equilibrium - sorry that this article is a little bit technical - but the idea is fairly simple. The water must be in equilibrium with the other water around it. This means that the water in the cave will be at the same pressure as the water outside the cave. Pressure acts in all directions - up, down, horizontally - and the unique case of a column of water results in increasing pressure with depth because of the weight of the water above. Nimur (talk) 00:13, 31 December 2008 (UTC)

Thank you for your replies. 190.157.120.42 (talk) 00:30, 31 December 2008 (UTC)

December 31

Best exercise for endorphins

What's the best type and program of indoor exercise for generating endorphins (and, ideally, continuing to do so for the next couple days)? I have access to a high-end campus gym. Will caffeine increase or decrease the effectiveness, taking into account that it will allow me to work out harder? Also, can I expect a change in endorphin levels to affect my academic performance? NeonMerlin 01:12, 31 December 2008 (UTC)

Have you read Endorphin? Lisa4edit (talk) 01:52, 31 December 2008 (UTC)

I have, and don't see an answer there. NeonMerlin 04:24, 31 December 2008 (UTC)

I think the whole "endorphin high" is sensationalised by the media. Ask an opioid addict what a real endorphin high feels like, because a bit of running isn't really going to make the difference to your brain that pop-science has made out. --Mark PEA (talk) 14:09, 31 December 2008 (UTC)

I agree with Mark and would also suggest that the type of exercise is going to make no noticeable difference how how you feel but how often you take exercise might do. Ideally you will move around at different times during the day. Itsmejudith (talk) 17:56, 31 December 2008 (UTC)

Horizontal folk-dancing? Mattopaedia (talk) 03:46, 1 January 2009 (UTC)

Do you mean sex, Matt? I agree. Axl ¤ [Talk] 12:16, 2 January 2009 (UTC)

Absolutely!! It's probably the only form of exercise most people would look forward to the prospect of doing almost continually for a couple of days. ;-) Mattopaedia (talk) 22:19, 2 January 2009 (UTC)

One way to feel the effect of endorphins is to cry (hear terrible news), then eat a bar of chocolate, then jog for 10 mins. Polypipe Wrangler (talk) 04:05, 3 January 2009 (UTC)

Pressure reducing devices in city water mains

My friend works for the water company in my city, and she was telling me about these large machines that they have at certain locations along the water mains pipelines, which reduce the pressure. So, as she says, water enters it at one pressure (she measures pressure in feet) and exits it at a lower pressure. She couldn't explain how it worked, unfortunately, which was frustrating because I can't understand how such a device could possibly work. Wouldn't some other property of the water also have to be changed, as all the various properties are tied together by Bernoulli's law and other such fluid mechanics principles? Cheers, Maelin (Talk | Contribs) 01:42, 31 December 2008 (UTC)

A venturi tube? Changing the diameter of the pipe can change the pressure in the pipe, if the net flow of water is conserved. Also, water pressure will drop as various feeds tap off from the water-main, splitting the flow. Also, changing absolute elevation of the pipe (such as following the contour of a hill, or just changing the depth that the pipe is buried) can decrease pressure. Nimur (talk) 01:48, 31 December 2008 (UTC)

Reduced pressure zone device isn't explicitly built into the mains to reduce pressure, but does so anyway. (It's there to keep your swimming pool water from flowing back into the line. Lisa4edit (talk) 02:00, 31 December 2008 (UTC)

Here are a couple of links that I found here and here that have some diagrams and animations. The basic principal appears to involve a valve that opens towards the high pressure (inlet) side, with some kind of diaphragm on the outlet side that controls the movement of the valve. As the outlet pressure increases, the diaphragm moves outward and the valve moves towards the closed position. As the outlet pressure decreases, the diaphragm moves inward and the valve moves towards the open position. I'm not sure if the same principles apply to large scale devices in water distribution systems. I'm no expert in this field (not even a novice). I'm surprised there is not a Pressure reducing regulator article. (To the fluid mechanics experts out there: Here's your chance to contribute!) -- Tcncv (talk) 04:02, 31 December 2008 (UTC)

There is a pressure regulator article, but it has few details on how they work. --Heron (talk) 18:01, 31 December 2008 (UTC)

The basic mechanism is pretty simple: on the low-pressure side, you've got a resevoir that, through a spring or other mechanism, maintains the desired pressure. Separating it from the high-pressure side is a valve that only opens if the pressure on the low-pressure side is below the target pressure. --Carnildo (talk) 23:50, 31 December 2008 (UTC)

[15] Kittybrewster ☎ 19:06, 2 January 2009 (UTC)

Laser based short sight correction-what is the downside?

This question has been removed. Per the reference desk guidelines, the reference desk is not an appropriate place to request medical, legal or other professional advice, including any kind of medical diagnosis, prognosis, or treatment recommendations. For such advice, please see a qualified professional. If you don't believe this is such a request, please explain what you meant to ask, either here or on the Reference Desk's talk page.

This question has been removed. Per the reference desk guidelines, the reference desk is not an appropriate place to request medical, legal or other professional advice, including any kind of medical diagnosis or prognosis, or treatment recommendations. For such advice, please see a qualified professional. If you don't believe this is such a request, please explain what you meant to ask, either here or on the Reference Desk's talk page. --~~~~

This probably falls under the category of medical advice. We can direct you to laser eye surgery and cataracts but you should primarily rely on your optician, optometrist, or other medical professional. If you need a second-opinion, you should definitely stick to trained medical professionals. Nimur (talk) 04:32, 31 December 2008 (UTC)

LASIK may have answers for some of your questions - WikiCheng | Talk 04:34, 31 December 2008 (UTC)

Myopia might also help you see clearer (In a purely proverbial sense ;-) Lisa4edit (talk) 04:48, 31 December 2008 (UTC)

Reactivity of Salt

Reactivity of Sodium formate and calcium chloride salt towards water? —Preceding unsigned comment added by HairulanuarMohdzin (talk • contribs) 09:40, 31 December 2008 (UTC)

At any normal temperature, the water will dissolve these chemicals. Calcium chloride can absorb water from a humid atmosphere to make a solution. Calcium chloride can form a hydrated salt. Sodium formate I don't know about. Graeme Bartlett (talk) 11:48, 31 December 2008 (UTC)

The article Sodium formate notes that it is deliquescent - that is, it will absorb enough water from the air to form a liquid solution. (Much like calcium chloride will.) -- 128.104.112.113 (talk) 00:06, 1 January 2009 (UTC)

Time

What is used as the physical point of reference for the time of day? Is it the orientation of the sun and the earth? In other words, if all of the clocks in the world stopped, would they be reset by (for example) saying that it is 12pm GMT when the Greenwich Meridian is directly in line with the sun (i.e. the sun is highest in the sky over the Meridian)? YaniMani (talk) 11:30, 31 December 2008 (UTC)

I'm not sure if I quite understand your question but if I do the first parts is answered at International Atomic Time and perhaps Leap second. It doesn't answer what happens if all the 300 clocks in various locations stop but I doubt there is any established contigency since it's so unlikely it's not worth considering. It's probably more likely that the earth will be hit by 3 large asteroids in one day and even that may not be enough to ensure none of the established clocks survive. And even if that happens there are still other atomic clocks that we could decide to use including many in space. If the earth is destroyed, the question is moot anyway. In the unlikely event somehow every single atomic clock stops working and yet there is still enough civilisation and technology to want an accurate global time standard I'm sure we will work out something but it would surely depend on what somehow cause every single clock to start working Nil Einne (talk) 00:40, 1 January 2009 (NZDT, UTC+13)

Thanks. I read them, but I am still not sure what the point of reference used is (they talk about correcting for rotation). I am of course not asking about whether there is a contingency for what happens if all the clocks stopped, it is just a theoretical situation to frame the actual question - i.e. what is the external point of reference that we would use? YaniMani (talk) 11:58, 31 December 2008 (UTC)

You have opened a great big can of worms. You'll never ask "What time is it?" again without wondering which time you're getting. When we ask what time it is, we mean civil time, which is really standard time, which is directly referenced to Coordinated Universal Time. In the United States, this time can be found on the air on WWV. The reference for UTC is indeed the Prime Meridian, but the position of the sun is too vague for really good timekeeping. The Wikipedia article on UT says "UT in relation to International Atomic Time (TAI) is determined by Very Long Baseline Interferometry (VLBI) observations of distant quasars." They fiddle with it as needed to keep Easter from moving into August, of course. --Milkbreath (talk) 12:30, 31 December 2008 (UTC)

Some further reference. Icek (talk) 16:01, 31 December 2008 (UTC)

When the sun is directly over the Prime Meridian, that should be the setpoint, conceptually, for all clocks, as it in fact was in the late 19th and early 20th century, before other standards such as atomic clocks. I understand that for reasons perhaps having to do with the sensitivities of the French to British standards, or the earth not being perfectly symmetrical, the GPS system places 0 degrees longitude a short distance away from the engraved line in a brass plate which equals the center line of the telescope at Greenwich formerly used for the determination of time. So the sun being directly overhead of one of the historic prime meridians could be used as the synchronizing standard of clocks. Edison (talk) 19:14, 31 December 2008 (UTC)

Because the earth's orbit is elliptic and not circular (and other factors), the apparent position of the sun will vary throughout the year if viewed at the same time each day. See Analemma. -- Tcncv (talk) 04:30, 1 January 2009 (UTC)

...And consequently, the basis for time is not that noon UT is when the Sun is at the zenith at the Prime Meridian; it's the average (mean) time of this event. That's why UT used to be called Greenwich Mean Time. Note that I said "UT" and not "UTC". UTC, the time we now use, applies a fine-grained correction to UT; see leap second. --Anonymous, 08:57 UTC, January 1, 2009.

To answer your question as asked: The definition of time has two components: the length of the second, and a nominal zero point. The current defintion for the duration of a second is no longer related in any way to the movement of the earth or other celestial bodies. The starting point is related to a particular event (actually, a statistical average of multiple events) in the past. Therefore, for purposes of definition, the answer to your question is "no." There is no formal definition of a particular future celestial event as being at a defined time. All future celestial events will have observed times, not defined times. Now to answer your hypothetical question: If by horrible mischance, we somehow lose track of time, How would we reset the clocks? This affects only the zero point, not the length of the second: we can create new clocks that accurately measure the length of the second. This means the new zero point must be agreed upon by convention. Depending on how long the clocks were stopped, teg new agreement might try to relate "new time" to "old time" by picking a particular celestial event that was predicted to high precision with respect to "old time" and agreeing that the event is the reference pont for "new time." But note that this would be a new agreement made by a comitee, and would have no more (or less0 absolute signigicance than our current definition. -Arch dude (talk)

Thanks for the correction about noon being subect to a correction depending on where in its orbit about the sun the earth is. Subject to that correction, the transit of the sun and the stars through the crosshairs of the Greenwich meridian could be used to re-establish exact time, down to the fraction of a second. The actual transit of the various stars was tracked each night, down to the fraction of a second, recorded, and used to determine standard time in the late 18th and early 20th century. By tracking many starts, and averaging the transits, the mechanical master clocks which sent out electrical signals on the hour, could be regulated to a fraction of a second., as it was over 100 years ago. Edison (talk) 06:10, 2 January 2009 (UTC)

January 1

Date

What year is it? ~ R.T.G 12:42, 1 January 2009 (UTC)

I'm pretty sure it's 2009 everywhere at this point. In the Gregorian calendar, of course. --98.217.8.46 (talk) 12:54, 1 January 2009 (UTC)

No time machine available in this age, sorry. This is year 2009; you better live with it... What year were you coming from?--PMajer (talk) 13:42, 1 January 2009 (UTC)

it's still the year of the rat until 1/26/2009 when the ox takes over --76.125.8.141 (talk) 16:47, 1 January 2009 (UTC)

It's also Year Heisei 21, in emperor era counting. Nimur (talk) 16:57, 1 January 2009 (UTC)

Take your pick, see year and list of calendars. I'd go for the 2008–2009 fiscal year. Also for date see date. Dmcq (talk) 22:31, 1 January 2009 (UTC)

@ Nimur wouldn't it still be Heisei 20 till the New Year somewhen in spring? Or do they use the Gregorian year for the era counting?76.97.245.5 (talk) 01:01, 2 January 2009 (UTC)

According to Japanese era name, and this citation: "In historical practice, the first day of a nengō (元年, gannen?) starts whenever the emperor chooses; and the first year continues until the next lunar new year, which is understood to be the start of the nengō's second year." I'm not actually familiar with the specifics but I can ask some Japanese colleagues and report back. Nimur (talk) 16:59, 2 January 2009 (UTC)

An optional use for leeches?

A was looking at some stupid stars advocating leeches as detoxifier (pfff). Anywayit got me mind rolling in a sort of similar direction, could those blood sucking MOFOs be used in conjunction with a diet? I would imagine that there are strick homeostatic processes that keep the volume of blood and number of cells constant, so if blood was removed energy would need to be dissipated to restore balance. Does this theoretically make sense?78.133.19.131 (talk) 13:00, 1 January 2009 (UTC)

Cut the leeches out and just ask if moderated blood loss could be a way to diet. I suspect the replenishment of a small amount of blood is not a huge caloric drain but I'm sure someone can answer for sure. --98.217.8.46 (talk) 14:05, 1 January 2009 (UTC)

I'd say yes, except for the usual problem that your body will detect that it's losing weight, think you're starving to death, then decrease your metabolic rate and increase your hunger tenfold, making you actually gain weight. Also, you'd need supplements to ensure that you're replacing everything which is lost with blood, such as iron. StuRat (talk) 15:50, 1 January 2009 (UTC)

Here is a British Medical Bulletin on metabolic rate changes as a result of injury (I'm not sure if controlled bloodletting counts as injury). It may provide some insight on the complex effects that StuRat mentioned. Nimur (talk) 17:02, 1 January 2009 (UTC)

Leeches have been used for curing diseases like paralysis with (supposedly good success) in Kerala, a southern state of India. Have a look at this and this and search for 'leech' - WikiCheng | Talk 03:53, 2 January 2009 (UTC)

That is totally unrelated to the question being asked. --98.217.8.46 (talk) 04:36, 2 January 2009 (UTC)

This rings my bullshit detector Wiki Cheng Bastard Soap (talk) 11:23, 2 January 2009 (UTC)

It certainly does! The amount of blood you could lose without all sorts of other health problems (such as aneamia) is pretty small - the blood donation people say that you can safely lose (and recover) only about a half-liter every month. The energy required to do that is tough to estimate - but it doesn't seem like it would be very significant. No - I think you should stick to using leeches for storm prediction. SteveBaker (talk) 14:30, 2 January 2009 (UTC)

The Mayo Clinic web site asserts that you 'burn' about 650 calories when you donate a unit of blood: [16]. While most jurisdictions limit donors to one unit every eight weeks, that restriction has a margin for safety built into it. If we assume blood loss at twice the permitted rate, that runs about 160 calories per week, or about 20 calories per day. It's a trivial reduction that would probably be wasted. (You're going to be hungrier than usual every time your body notices it's short on blood.) You'd save about the same number of calories by switching from cream to milk in one cup of coffee each day, or by taking your tea with one lump of sugar instead of two. TenOfAllTrades(talk) 15:24, 2 January 2009 (UTC)

I know someone who contracted giardiasis, then deliberately didn't get treated, to assist with dieting. I've heard of people use a similar trick with tapeworms. Medical advice: I wouldn't recommend any of these methods. Axl ¤ [Talk] 09:59, 3 January 2009 (UTC)

Tapeworms work by consuming food in your gut before you have a chance to absorb it. However, that just makes you more hungry and causes you to eat more - so they may not help as much as you'd hope. If you have the mental strength not to eat more as the tapeworms consume your food - then you'd probably have no problem sticking to a medically more reasonable diet. Dunno about giardiasis - but the list of symptoms described in our article suggests that it's not going to replace the South Beach diet anytime soon! SteveBaker (talk) 13:26, 3 January 2009 (UTC)

do the congenitally blind understand literary descriptions?

There are some things I read, for example colors I don't know the names of, even vaguely (Carmine? Puce? Bimini?), where I don't know what they're talking abut. But do the congenitally blind understand most visual descriptions? Do they know the same thing everyone else does, like what a mirror is, etc, or are these things like these weird colors are for me? —Preceding unsigned comment added by 79.122.79.41 (talk) 21:31, 1 January 2009 (UTC)

They understand as it is described to them only. This reminds me of Thomas Nagel's essay "What is it like to be a bat?" where he talks about the subjective nature of consciousness. Can you conceive of a bat's perception through echolocation? -- JSBillings 23:43, 1 January 2009 (UTC)

This is speculation but I would imagine that over time they pick up what visual based description might connote based on sighted people's reactions to them, although they cannot directly relate to it. If there is a gap in their knowledge about the appearance of something, they may miss an important visual indicator that would be apparent to sighted people. An example would be "the baseball player's shadows were long when they arrived at the field." This description would immediately indicate it a morning or evening setting, but a blind person who hasn't realized shadows change length over the course of the day will miss the time detail. If they were careful, they would ask themselves "what was the point about the shadows" and ask someone what it means. Something more difficult might be a description about a woman wearing a red dress to a party which would signal to a sighted person that she was trying to be seductive compared to say, the same woman wearing the same dress in blue. A blind person would probably realize that a red dress often indicates seductiveness only after connecting that detail with other description and then encountering the same literary red=seductiveness theme repeatedly. I would also imagine that the reverse hold true in that sighted people miss details that blind people would pick up on. A description about a blind person whistling a monotone as they walk through a doorway would not indicate to a sighted person that they were probably trying to get a feel for the size of the room by listening for the reverberations of the sound. (or something like that...) 152.16.15.23 (talk) 00:29, 2 January 2009 (UTC)

As this article shows [17] they sometimes can't make sense of the image even if they can see again. A blind acquaintance of mine said he got a much clearer idea about what an optical illusion was after someone had shown him a relief of the Necker Cube.76.97.245.5 (talk) 00:37, 2 January 2009 (UTC)

I came to different conclusions about the examples given. Red would be quite easy to associate with fire and then seductiveness. Shadows can be directly perceived by the heat of the sun. And going into a room I think they'd probably make a click or pip noise rather than a continuous tone to determine its overall size, it certainly works better for me. Interestingly on the last if it's quiet I seem to be able to hear which way a hall goes with my eyes closed quite easily even without making any noise. Dmcq (talk) 12:25, 2 January 2009 (UTC)

Also just as a note - A lot of blind people aren't walking around in total darkness (i.e. seeing nothing), but rather there are a wide degrees of what the blind can 'see' (changes in light, silhouettes etc.) - it really depends on the individual. 194.221.133.226 (talk) 10:36, 2 January 2009 (UTC)

I was told, ages ago now, by a cognitive scientist that it depends very much on the nature of the blindness. If it is due to "mere" problems with the eyes then the blind person thought in visual and spatial metaphors just the same as a sighted one (though clearly certain concepts like colors were going to be problematic). If it was due to a problem with the visual cortex of the brain, then the ability to visualize things in their head (and their understanding of visual metaphors) would also be affected. --98.217.8.46 (talk) 03:14, 3 January 2009 (UTC)

Mechanical device that keeps something spinning in exactly the right speed

Here's something I've been wondering since I was a wee lad (well, teenager, at least). I've asked a number of people this over the years (though obviously no engineer), but never gotten the answer. I've finally decided to find out the answer, once and for all, with the help of you fine refdeskers.

An old-timey mechanical wind-up clock works on this principle: you wind up a spring until there's lots and lots of tension in that spring. That tension is slowly released to the various cogs and gears inside the clock, and finally transmitted to the dials on the face of the clock. But here's my question: the spring can be wound with various degrees of tension, but the clock still goes at the same rate. If you wind the spring to half its capacity, the gears in the clock should only go at half the speed as if the spring was wound to full capacity (this is Hooke's law, no?) And yet, the dials on the face always moves at constant speed, regarless of how much tension there is in the spring. There must be some mechanical device doing this.

The reason I keep wondering about this is that there's all sorts of machines that seems to be using this same device, and whenever I see them, I wonder once more. Old movie cameras, for instance, were powered by springs or a hand-crank, yet regardless of how much tension there is in the spring or how fast you rotate the crank, the camera has to rotate the shutter and feed the film-stock at exactly 24 frames per second. Same thing with hand-cranked record players, they always have to move with the same rpm.

So, what is this magical mechanical device? How does it work? The only thing I can think of is that there somehow is something that applies a smaller counteracting force to the gears that is just big enough to ensure that they always rotate at the same speed, but increase and decrease at the same rate as the main gear (so if the tension in the spring doubles, the counter-acting force doubles too, but the difference between it and the main gear is constant), but I can't quite work out the details. Is that how it works? Do we have an article on how this device? Does it have a name?

I'd be most grateful for any answer, as I said, this has been bugging me for two decades, at least. Belisarius (talk) 23:25, 1 January 2009 (UTC)

Older mechanical clocks used a Fusee, which was later replaced by improvements in technology such as the Pendulum clock and Escapement. Clocks are pretty neat, you can make a fairly reliable pendulum clock out of tinker toys if you want. The articles I mentioned are actually very interesting reads, if you've been curious about this for a while. -- JSBillings 23:33, 1 January 2009 (UTC)

An interesting device (a bit off point) is the centrifugal governor ...used in a spring-loaded record player and a spring-loaded telephone dial to limit the speed. hydnjo talk 23:58, 1 January 2009 (UTC)

In clocks the wound spring is used as an energy store to power the clock, but not to govern its speed, (various other aforementioned mechanisms are used for that) and so the tension in the spring (until it approaches zero) has no affect on the speed at which the clocks mechanism ticks over. —Preceding unsigned comment added by 92.16.196.156 (talk) 00:10, 2 January 2009 (UTC)

For a rotating machine, Centrifugal governor is exactly right. For a time piece, you might look up Balance wheel and Pendulum.--GreenSpigot (talk) 02:28, 2 January 2009 (UTC)

Some devices like clocks had an escapement, which went tick-tock, and used a limited amount of energy for each incremental movement of a clock or watch. Other windup devices used a Centrifugal governor or flyball governor, similar to the one used on some earlier steamengines, to regulate or limite the speed. On a windup phonograph there was no intermittent escapement like on a watch, but a continuous rotation at a desired speed. If the speed tended to increase, as when the spring was wound tightly, the spinning caused the flyballs to move farther out and bend the springs to which they were attached, causing movement of the ring attached to tone end of the springs, and applying greater pressure to a braking mechanism, reducing the speed. On Watt's steam engine, the flyballs rotated about a vertical axis and governed the steam flow. On windup phonographs, the axis might be horizontal, and spring pressure rather than gravity was usually the force the centrifugal force of the balls worked against. The flyball governor worked well to prevent the fully wound spring from running the phonograph faster than was desired, but when the spring was nearly run down, it had used up all its range of regulation and could not prevent the mechanism from running slower and slower until it ground to a halt. Edison (talk) 06:03, 2 January 2009 (UTC)

The simplest governor to understand is a pendulum clock. At it's simplest:

• The spring is attached to the gear wheel.
• A lever arm with a single triangular tooth on each end is wedged into the gear to stop it rotating.
• A pendulum swings back and forth.
• At the end of the pendulum's travel it pushes on the lever arm - letting it up just enough for the tooth on one end to disengage from the gear while the tooth on the other end engages and grabs the next tooth in the gearwheel. This allows the wheel to turn by exactly one tooth for every swing of the pendulum.
• The gear wheel also drives the hands of the clock through successive reduction gears to get hours, minutes and seconds (and perhaps days, months, etc in fancy clocks).
• In the process of doing that, the gear wheel's rotation applies a small, fixed amount of energy to the pendulum to keep it swinging.
• Since the period of a pendulum swing depends on its length and not on how hard you push it, the pendulum swings at the same rate no matter how much energy is in the spring.
• This limits the amount of energy the spring can release for each swing of the pendulum - so the clock runs at the same speed until there is insufficient energy in the spring to keep the pendulum swinging against the forces of friction and air resistance.

In most long-case 'grandfather' clocks, the pendulum is set up to swing once a second because that makes a pleasing 'heartbeat' and makes the subsequent gearing that drives the hands of the clock a little bit simpler. The repeated release-and-grab cycle of the spring-driven gearwheel is what makes the characteristic ticking sound you hear in most mechanical clocks. There is generally a small adjustment you can make to the weight on the end of the pendulum to adjust the length - and hence to make the clock tick faster or slower.

Clocks that chime generally have another spring mechanism for doing that which is held in check with another lever arm that's tripped when a tooth on a wheel attached to the minute hand reaches the top of the hour (or whatever).

My long-case clock has a 30 day movement - you wind up TWO springs - one for the chimes and one for the hands. On my clock, the chime mechanism actually runs down after about three weeks - but the hands run for well over 30 days - which is a pretty amazing thing for such primitive engineering and a 100 year old mechanism.

On mechanical wristwatches, the pendulum is replaced by a wheel that is spun back and forth by a 'hair spring' - replacing the force of gravity with a spring force - but the principle is the same. SteveBaker (talk) 14:12, 2 January 2009 (UTC)

January 2

Electro magnetic theory

Find the vector magnetic field intensity in Cartesian coordinates at P2 (1.5, 2, 3) caused by a current filament of 24A in az direction on z axis extending from (i) z=0 to z=6 (ii) z=6 to z= infinity (iii) z= −infinity to z = infinity. (b) Given the electric scalar potential V=80z cos (x) cos (3 × 108 t) kV and magnetic vector potential A=26.7 z sin(x) sin(3 × 108 t) ax mWb/m in free space. Find fields E and H. —Preceding unsigned comment added by Antony salvin (talk • contribs) 04:57, 2 January 2009 (UTC)

Please do your own homework.

Welcome to the Wikipedia Reference Desk. Your question appears to be a homework question. I apologize if this is a misinterpretation, but it is our aim here not to do people's homework for them, but to merely aid them in doing it themselves. Letting someone else do your homework does not help you learn nearly as much as doing it yourself. Please attempt to solve the problem or answer the question yourself first. If you need help with a specific part of your homework, feel free to tell us where you are stuck and ask for help. If you need help grasping the concept of a problem, by all means let us know. Algebraist 05:07, 2 January 2009 (UTC)

OK, I've done it. Now what? Was there a question? Edison (talk) 05:49, 2 January 2009 (UTC)

What the Fuck is Fermet's Last Theorum?

Correct me if I'm wrong, but isn't it something along the lines of.... a mathematical proof? A proof that was "solved" by a Princeton mathematician (But not really)? He apparently had to "solve" some other math problem as well (Taniyama's Conjecture?) to untheorumize this. I'm a little confused about the whole deal. I'm not sure if this is some profound E=mc2 thing or just some obscure mathematical property that only mathematicians enjoy tinkering around with.

Sorry about the colorful language, by the way. I'ts just that when I typed this into your search box, it came up empty. I've found almost every other thing I've searched for, no matter how obscure, so I was a bit upset. And I figured it couldn't be that obscure if a country boy like me has heard of it. But for the most part, you guys do a pretty good job. God bless, and thank you for your patience. Sincerely, --Sunburned Baby (talk) 05:22, 2 January 2009 (UTC)

See Fermat's Last Theorem (note spelling). Algebraist 05:24, 2 January 2009 (UTC)

Also for future reference, Google Search (and possibly other search engines) would have spotted your incorrect spellings and offered the correct search term. See [18]. Abecedare (talk) 05:38, 2 January 2009 (UTC)

Wikipedia's inbuilt search does that also. Algebraist 05:48, 2 January 2009 (UTC)

Sometimes Wikipedia's search feature directs you to another article that doesn't exist. ~AH1^(TCU) 17:04, 2 January 2009 (UTC)

But not in this case Nil Einne (talk) 05:37, 3 January 2009 (UTC)

As for whether it's profound, no, but it is surprising, easily understandable to non-mathematicians, and seems deceptively easy to prove. That's why you have heard of it and not of, say, the much more profound NP problem. --Bowlhover (talk) 07:10, 2 January 2009 (UTC)

On the other hand, the modularity theorem, which was considered intractable before Andrew Wiles proved a special case in his proof of Fermat's Last Theorem, is profound. However, you need a gound grounding in analytic number theory to even understand the statement of the modularity theorem, whereas Fermat's Last Theorem can be understood with schoolboy arithmetic. Gandalf61 (talk) 13:13, 2 January 2009 (UTC)

The theorem itself is very easy to understand - it's a simple statement:

"If an integer n is greater than 2, then the equation aⁿ + bⁿ = cⁿ has no solutions in non-zero integers a, b, and c."

So, for example, we know that 3²+4²=5² because 9+16=25 right? In that case, 'n' is 2 - and there are plenty of equations that fit. But it's a bit surprising that there are NO equations that work when 'n' is bigger than 2...not for any non-zero values of n, a, b and c ! Wow!

That's something that mathematicians had long SUSPECTED (because they never found any equations that worked for n>2 - and people had used computers to test that up to very large numbers) but they couldn't PROVE it. That's a big deal if you're a mathematician. It's very annoying to have something that simple that seems to be true - but you can't actually prove.

So - this guy Fermat (who was/is a very respected mathematician) scribbles a note in the margin of a book that says he's found a really cool proof - but he doesn't have room in the margin to write it down. Then he dies without writing the proof down anywhere. Since then, many, many mathematicians have tried to work out this simple and elegant proof. None of them succeed until just a few years ago when FINALLY someone manages to prove it - but the proof is long, horribly complicated and maybe there are only a handful of people on the planet who understand it. Worse still, it relies on several other recent proofs that are just as complicated and perhaps even harder to understand.

Did Fermat really come up with a simple/elegant proof? No. That's very unlikely indeed. Did he manage to prove it the way modern mathematicians have finally proved it? No - that's pretty much impossible. Most likely, Fermat made a mistake in his simple/elegant proof...or he had some other motive for writing that margin note.

But let me make this clear - the PROOF is hard to understand. The thing it proves is really, really simple.

The consequences of having the proof are that formal mathematics can now build on the fact that there are none of these Fermat equations - but practical sciences have been not been deeply affected either way - it's not a particularly useful piece of mathematics in itself. However, it's likely that some of the things learned along the way will eventually prove useful. This isn't as useful as e=mc² (which is physics - not mathematics) - it's nowhere near as useful as (say) Pythagoras' theorem. What it is - is an incredible piece of mathematical reasoning - one of the most difficult things a mathematician has ever done - a stunning intellectual achievement.

IMHO, we should stop calling it "Fermat's Last Theorem" because it's really clear that Fermat did nothing to help solve the problem, and arguably (by making it seem that a simple proof existed) wasted more valuable mathematician's time than anyone else in history!

SteveBaker (talk) 13:47, 2 January 2009 (UTC)

Yes, it has wasted a lot of mathematicians' time, but it also likely intrigued and motivated many to enter the field in the first place. So in my subjective judgment the theorem and the romance that has surround the quest for its proof has been a net positive. What can take its place ? P = NP problem, "easy" Formula for primes ... Abecedare (talk) 10:00, 3 January 2009 (UTC)

With respect, I believe there's a proof by negation approximately 8 lines in length which doesn't stretch beyond high school algebra (which, I believe, is entirely within Fermat's toolset) that involves expansion of the binomial theorem (n-inifically so, but it cancels out in the next step) midway, and I think one of those sum/difference of squares/cubes shorthands, and the rest is fairly boilerplate grade school proof. 98.169.163.20 (talk) 07:20, 3 January 2009 (UTC)

No doubt you would have included this proof here, but the margin was too narrow. - Nunh-huh 09:47, 3 January 2009 (UTC)

I suggest that we IMMEDIATELY start discussion on Village pump, proposing that wikipedia page margins be expanded. We cannot tolerate any more such losses! Abecedare (talk) 09:52, 3 January 2009 (UTC)

I don't think 98 is in danger of dying soon. I could of course be wrong Nil Einne (talk) 11:20, 3 January 2009 (UTC)

penetration of UltraViolet A rays in flesh

when a 5 mm diameter spot of UV A rays is placed on human body part(by a UV laser); what amount of energy(mJ/cm^2) of UV spot is required for 5cm penetration of uv rays in flesh? 123.201.1.238 (talk) 12:13, 2 January 2009 (UTC)crony

5 cm ? That's 2 inches. You'd need enough energy to vaporize most of the flesh above it. Are you sure you don't mean 5 mm penetration ? StuRat (talk) 16:40, 2 January 2009 (UTC)

5 mm penetration? You must be great with the ladies. ok... y'all can go back to answering the question seriously now. I will stop. --Jayron32.talk.contribs 21:40, 2 January 2009 (UTC)

Solar cell production capacity

Hi

I have been unable to find any data on the worldwide solar cell production capacity ("how many square meters of solar cell can be produced per day (or per month or per year)?").

I am comparing various means of "green" energy production. Having arrived at the conclusion that covering almost all roofs with 10% efficient solar cells would provide enough electricity for a typical first-world nation, I wonder what it would take to actually do this. What would it take to produce hundreds of square kilometers of solar cell? What are the bottlenecks in solar cell production?

Thanks in advance —Preceding unsigned comment added by 81.11.170.174 (talk) 15:39, 2 January 2009 (UTC)

Check out Photovoltaics and Deployment of solar power to energy grids - and look especially at the references at the bottom of both articles. SteveBaker (talk) 15:43, 2 January 2009 (UTC)

Thank you, I found the total peak power produced by all solar cells produced in 2007 by the largest solar cell manifacturers in one of those references. It's not a surface/year, but I can calculate what I want to know from power/year just as well. —Preceding unsigned comment added by 81.11.170.174 (talk) 16:17, 2 January 2009 (UTC)

To answer your question about "bottlenecks", there are two main constraints to the take-up of PV. One is that the return on investment is rather low compared to other energy-saving measures available to households, e.g. extra insulation, solar water heating, condensing boilers. The other is that PV only generates electricity in daylight hours, so a form of storage is necessary. The usual method in domestic installations is to connect to the grid, so an inverter must be incorporated in the system, and not all jurisdictions allow for net metering, so again it may not be seen as cost-effective. Itsmejudith (talk) 17:28, 2 January 2009 (UTC)

Also, solar cells gradually lose power over their lifetimes and ultimately have to be replaced (which is costly). The business of sending unused electricity back to the grid during daylight and pulling it back at night is only working well right now because so few people do it. There could well be problems down the line when everyone (including the electricity generators themselves) have an excess of daylight power and a terrible lack of nighttime capacity. In a sense, all we're doing it pushing the storage problem back onto the electricity companies - which isn't exactly fair if 'net metering' prohibits them from charging us for the privilege of doing that. Hopefully, we end up with hydrogen powered cars or something - so we use excess daylight power to generate hydrogen and fall back on something else for nighttime supply. Certainly in a scenario where some country decided to do what our OP suggests, there would be a major 24 hour cycle storage issue. SteveBaker (talk) 19:36, 2 January 2009 (UTC)

One main bottleneck is the process of purifying the silicon. Polypipe Wrangler (talk) 04:16, 3 January 2009 (UTC)

DNA math

I need to know how many different combinations in human DNA are there before you actually get a human. I once was given this number and would like to know if 1) it is accurate,and 2) how that number would be arrived at (DNA = number times 10 to the 87th power)

I don't know what the number would be. As for how to calculate it:

Let n be the number of nucleotides in human DNA.

Let m be the number that you can change while keeping them human.

The number of possible strains of human DNA is 4^m*nCm

The number of possible strains that length is 4^n.

The fraction of strains at that length that result in a human is (4^m*nCm)/(4^n)=nCm/4^(n-m)

I don't know how high that would be, but I suspect it would be something more like one in 10^(10^(20)) — DanielLC 18:36, 2 January 2009 (UTC)

Our article on human genetic variation says "on average two humans differ at approximately 3 million nucleotides". The article goes on to say "Most of these single nucleotide polymorphisms (SNPs) are neutral, but some are functional and influence the phenotypic differences between humans. It is estimated that about 10 million SNPs exist in human populations, where the rarer SNP allele has a frequency of at least 1%". Even if there are only two alleles for each site, this gives 2^{10 million} possibilities, or about 10^{3 million}. There is a big assumption in this calculation that sites can vary independently. But it still looks as if 10⁸⁷ is a big underestimate. Gandalf61 (talk) 18:58, 2 January 2009 (UTC)

pittuitay tumors

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--Milkbreath (talk) 14:10, 3 January 2009 (UTC)

How many species are there?

Our Species article doesnt say. Well it gives a patheticly large possible range. Surely you can do better? Willy turner (talk) 22:47, 2 January 2009 (UTC)

new species are being discovered all the time, so it's impossible to give an exact number. These pages discuss it in detail [19] [20] [21] —Preceding unsigned comment added by 82.43.88.87 (talk) 00:14, 3 January 2009 (UTC)

Well... no. The thing is, it's impossible to know exactly how many species there are. New ones are discovered all the time while others go extinct. It's a big planet, and we simply don't know its contents well enough. (That said, maybe someone can be a little more exact than "between 2 and 100 million species", but I'm betting that not by much.) -- Captain Disdain (talk) 00:15, 3 January 2009 (UTC)

(Doh! Edit conflict)... I was going to say:

• Not really. The problem is that taxonomy is a human classification system (we do love to pigeon-hole things!), and by virtue of that, the rules surrounding definition of species, as well as higher and lower taxonomic levels, are pretty much arbitrary. See my recent edits and the associated discussion on the mycology of pityriasis versicolor for an example of how this makes life really difficult in medicine. Counting species is like counting grains of sand on a beach, by a committee, that can't agree on what a grain of sand actually is, and what size shell fragments should be before they're too big to be sand, and if the really small bits are sand or perhaps they should be called sub-sand, and if grains of sand should be differentiated by colour, or when they should stop for a cup of tea, or what type of tea to have, and whether there should be biscuits or cakes with the tea .... Mattopaedia (talk) 00:29, 3 January 2009 (UTC)

Well summarized, I concur with Mattopaedia. It's roughly arbitrary to define a species. I recall being taught in 7th grade science that "breeding capability" was the definite and indisputable indication of species boundaries, but since then, I have learned that it is significantly more subtle. See Species problem for discussion. Nimur (talk) 01:39, 3 January 2009 (UTC)

A related phenomenon are ring species: There are multiple populations of a species, and they can all interbreed (sustainably, i. e. the offspring can interbreed again, not like a mule which is the offspring of a horse and a donkey) with geographically neighbouring populations, but if you take two populations from different ends of the geographical distribution range, they cannot interbreed. Icek (talk) 08:57, 3 January 2009 (UTC)

The precise definition of "species" is certainly a problem - but just look at what you're asking here: I mean, sure, it's unlikely that there are more than a dozen large land animals that we don't know about - but that's a negligable fraction of the species on earth. Look at a single grain of dirt under a microscope and you'll see hundreds and hundreds of bacteria - all sorts of different kinds - more than you could easily count in an hour. If you live somewhere where there are a lot of people then maybe all of the little animals and plants (and other things) that you are looking at have been classified and named - but probably nobody did the necessary testing to see if they are all unique "species" or not...but probably that level of testing hasn't been done on all of them. Now consider that we haven't looked at grains of dirt from (say) every square mile of the earth's surface and classified every little squiggly dot that we see - we haven't looked at drops of water from every square mile of ocean and classified all of the diatoms, algae, bacteria and other little critters - we haven't taken air samples everywhere and done the same thing. We haven't looked at all of the species of gut flora in every kind of animal that we know. We're finding new species at the bottom of half-mile-deep coal mines - in deep ocean trenches and thermal vents - under the ice at the south pole there are lakes of liquid water with who-knows-how-many tiny animals and plants. The problem is simply immense - and we've barely scratched the surface of counting them all. There was a documentary on TV the other day about a bunch of cave divers exploring a cave in some god-forsaken place - they said that they expect to find a dozen new species of fish/insects/amphibians in practically every cave they visit! So it's not surprising that we have no real clue as to how many species there are. There could easily be another billion species living two miles underground where we've never even drilled - let alone started to count species. SteveBaker (talk) 13:09, 3 January 2009 (UTC)

I would say "Go to Wikispecies where you may find out all about species." but at the moment it is just a list of Latin names in need of people who can give translation and description to these latin names (stuff like Elephant and Fungus are not available on search yet but are all listed by latin with pictures). ~ R.T.G 13:29, 3 January 2009 (UTC)

The problem is, there are so many species of plants, animals, and microorganisms we haven't even discovered! There are also many species going extinct every year, some before we even discover them, so we might never know for sure. ~AH1^(TCU) 15:39, 3 January 2009 (UTC)

January 3

What is it eating?

Chewing on something?

This Nestor meridionalis meridionalis of the South Island of New Zealand was featured on the Main Page in the "Did You Know..?" section. When I first saw the photo, I thought it was eating a snake and I was consequently inclined to start a civilization; but on closer inspection, I found that the parrot in question doesn't eat snake. I'm not too sure, but that doesn't look like a Longhorn beetle grub, or a flower bud, or anything identifiable to me (maybe a melon fragment?). So, what is it eating? Nimur (talk) 04:43, 3 January 2009 (UTC)

Looks like a piece of capsicum to me.Mattopaedia (talk) 05:09, 3 January 2009 (UTC)

Given that New Zealand doesn't have snakes [22] and they would be another serious danger to our wildlife, it's a rather good thing it isn't eating one. Even more so on Stewart Island#Fauna. Nil Einne (talk) 05:29, 3 January 2009 (UTC)

Are we sure this is a wild parrot? Perhaps it's someone's pet and it's chewing on a red plastic toy of some kind? Just think of the kind of lens/camera you'd need to get that kind of a shot of a wild parrot - it doesn't seem very likely to me. But this is Wikipedia - we're a collection of real people, not some anonymous corporation. Hence you can go to the information page for the photo (just click on it) - look at the 'history' tab and see who posted the photo. Now you can go to that person's talk page and politely ask them all about their photo. This might not work - but on the whole we're a pretty friendly & helpful bunch here and I'm sure the photographer will be only too happy to talk about their (now very famous) picture. SteveBaker (talk) 12:54, 3 January 2009 (UTC)

Done. But again, it really looks like a slice of capsicum to me. Mattopaedia (talk) 14:54, 3 January 2009 (UTC)

Do we have any defense at all against meteors, black holes, etc.?

Like, big ones! In all seriousness, if astronomers detected a continent sized meteor headed our way with say, a year's warning, -couldn't we do something? Or if a black hole was heading our way couldn't we use some kind of antimatter type force to push it towards the moon? Or for that matter, if the moon ever broke free (from a black hole?) and started slowly drifting towards us, could we use some greater force to fastly push it away? When our sun expands during it's death throes, how will life forms of that era deal with it?--Dr. Carefree (talk) 09:49, 3 January 2009 (UTC)

No problem. We have loads of asteroid deflection strategies.--Shantavira|^{feed me} 10:15, 3 January 2009 (UTC)

Um what? Antimatter type force? Our current ability to produce antimatter is very limited and our ability to use it for practical purposes almost non-existant. And I'm not quite certain what the use of pushing a blackhole to the moon is. Plus the concept of us moving a black hole anytime soon, if ever, seem absurd to me. As to what humans will do, billions of years from now if they still exist and live on earth, I don't think many people have given it that serious thought although I'm sure there's something in science fiction Nil Einne (talk) 10:51, 3 January 2009 (UTC)

In practical terms, not only do we have no defense - we don't truly know what form that defense should take (because we don't know enough about the meteors themselves) and we don't have the ability to detect meteors early enough to make a difference.

Let's look at those problems individually:

• Detection: With a large mass on a trajectory that's pointing it at us, the earlier we do something, the easier it is. To take an extreme example: Suppose we only have a few minutes warning...we might have to try to deflect a meteor when it's one earth diameter away from hitting us square-on at the equator...we'd have to bend it's path by 30 degrees to make it miss us instead. To deflect a mountain moving at a thousand miles a second through an angle of 30 degrees in just a few seconds would probably take more energy than we have on the entire planet - it truly can't be done. However, if we know that the meteor is a threat (say) 20 years before it hits us - then the amount of deflection we need is a microscopic fraction of a degree and a really gentle nudge would suffice to save our planet. So early detection - and (most important) accurate orbital parameter determination - is a massive priority both because it gives us time (it might take 5 years to put together a strategy for deflecting this particular rock, building the spacecraft and getting it launched towards it's target) and it reduces the magnitude of our response.
• Analysis: There are many possible categories of threat. Comets are mostly ice. Meteors come in several varieties - some are essentially solid chunks of metal, others are solid chunks of rock, still others may be loose collections of small bolders, pebbles or even dust. Right now, we don't know which is which - which ones are the most common - whether large, dangerous objects are predominantly of one kind or another. We know (for example) that Comet Shoemaker-Levy broke up into a dozen pieces as it descended towards Jupiter - if we'd had to deflect that comet and we'd sent (say) a single large nuclear bomb then a whole range of disasterous possibilities come to mind: (a) The comet might break up before our rocket gets there and we can now only deflect one out of a dozen large, dangerous chunks. (b) Our bomb might actually do nothing more than break up the comet prematurely without deflecting it's course at all. So until we "know our enemy" - we're kinda screwed. We need to send lots of probes out to look in detail at a statistically reasonable collection of comets and meteors - and do lots of science to figure out what's out there.
• Deflection/Destruction: The problem is that breaking up a large meteor or comet without deflecting it's path doesn't help us. The total damage to the earth from a single rock that's the size of a mountain that weighs a million tonnes is precisely the same as for a million car-sized rocks weighing one tonne each or for a trillion rocks the size of basket balls weighing a few kilograms each. Simply smashing the meteor into pieces doesn't help at all! (The number of movies that get this fact wrong is truly astounding!) So we have to think in terms of deflection - not destruction. If we have enough time (see "Detection" above) then something as simple as a heavy spacecraft that flies along parallel to the course of the meteor for a few years and provides a REALLY subtle gravitational shift - might be enough. That's great because it works just as well with a flying rubble heap as it does for a mountain of nickle-iron or a million tonne dirty snowball. However, getting big, heavy things out of earth orbit and flying as fast as a meteor requires a heck of a lot of fuel and a huge amount of up-front planning. We certainly don't know about these threats early enough to do that reliably. So then we're left with hitting the thing hard with a big bomb, hitting it hard with an 'impactor' or nudging it more gently with rocket motors. None of those things will work for a flying rubble pile. For solid bodies - that'll work. We can build a probe with a rocket motor on it. Make a soft-landing onto the object and start firing our rocket. So long as the object is strong enough to take that pressure without breaking up - or without our rocket sinking into the surface or tilting sideways and deflecting the rock in the wrong direction - that could work. But it's more complicated than that if the object is spinning (as many of them are) - because now the rocket has to fire intermittantly when the rock is at the correct orientation or else the miracle of 'spin stabilisation' (which is what makes bullets fly in a nice straight line) will frustrate our efforts.

So it's safe to say that right now, we're defenseless on all three levels. Our detection ability is getting slowly better - we have surveyed some of the very largest rocks - and we're tracking their orbits. Perhaps we can now see mountain-sized rocks soon enough - but something a lot smaller than a mountain (like maybe a school-bus-sized rock) can take out a city - and we're nowhere even close to being able to track those soon enough or accurately enough. NASA have sent out probes to several meteors and comets to take a close look at them - and we've even tried firing an 'impactor' at one them...but we have a long way to go. A lot of people are thinking about deflection/destruction strategies...but no governments are building rockets and putting them into storage ready for the day when we'll need them - and funding for the entire process is distinctly lacking.

At some point we (as a species) need to seriously consider having a colony somewhere away from the Earth. There is always the possibility of the ultimate planet killer coming along that's too fast, too large, too unstable and/or too close to do anything about. Having a colony of humans living on (say) Mars with a self-sufficient life-style and a large enough gene-pool is the ultimate way to ensure the survival of the species come-what-may.

SteveBaker (talk) 12:43, 3 January 2009 (UTC)

If you'd like to learn more about it and help create "coursework", there's some activity on Wikiversity at v:Earth-impact events/Discussion, as well as an opportunity to use your imagination and research skills for colonizing off-planet at v:Lunar Boom Town. (And SteveBaker: mind if I copy over what you just wrote above? Good detail there!) --SB_Johnny | ^talk 12:56, 3 January 2009 (UTC)

All contributions to the ref desk fall under the GFDL - so long as you are using them under those same terms, you are welcome to take whatever you need. SteveBaker (talk) 13:45, 3 January 2009 (UTC)

Surely it would help to break it up as small objects can be burned up in the earths atmosphere, the amount of mass burned off an object will be proportional to the surface area of the object, which dramatically increases if the object is broken up. To take my point to its logical conclusion, an asteroid of a large given mass would cause significant damage, whereas the same mass of dust colliding with the earth would most likely just flouresce in the atmosphere as it "burned". —Preceding unsigned comment added by 84.92.32.38 (talk) 14:12, 3 January 2009 (UTC)

The net kinetic energy that has to be absorbed by the Earth system (atmo included) remains exactly the same. While a single large rock would probably cause more damage (at least more localized damage) based on impact, even the distributed vaporization of a massive asteroid would be a catastrophe. A school bus sized rock, sure -- vaporize it. A dino killer? That won't work. — Lomn 14:52, 3 January 2009 (UTC)

Our main capability in asteroid defense is early warning. Here is an incomplete list of early warning systems that I found:

• Pan-STARRS a (nearly complete) telescope which will continuously survey the sky for asteroid threats
• Near Earth Asteroid Tracking (NEAT) An organization which has several dedicated telescopes for asteroid detection and tracking.
• Spaceguard, a loose affiliation of asteroid detection programs/enthusiasts (including a formal program at NASA).
• Lincoln Near-Earth Asteroid Research (LINEAR) a NASA/USAF/MIT joint project for asteroid detection/tracking
• Lowell Observatory Near-Earth-Object Search (LONEOS) a completed 15-year project which detected hundreds of asteroids.
• Minor Planet Center an organization which is the recognized official center for collecting and publishing orbit data about all known asteroids and comets.
• Catalina Sky Survey another sky survey designed to identify asteroids.
• Spacewatch an University of Arizona program to discover near earth objects.
• Near Earth Object Program (NEOP) a NASA effort to study the list of known asteroids and determine the impact risk.

We currently know of around 5500 (see the NEOP page) Near Earth Objects, and hundreds more are being discovered every year. So as you can see, we humans have actually put a fair amount of effort into detecting impact threats from space. No one has ever tested any potential asteroid deflection systems, but as Steve says, early detection is key. --Bmk (talk) 15:44, 3 January 2009 (UTC)

That doesn't seem quite right... if gazillions of little pieces vaporize in the atmosphere or even hit the earth (somewhat more slowed down by atmospheric friction), they wouldn't also vaporize large amounts of actual earth materials as well (which would cause snowstorms in Havana, etc.). Am I missing something? --SB_Johnny | ^talk 15:33, 3 January 2009 (UTC)

(edit conflict)A meteor the size of a continent would be larger than the Moon. The largest asteroid we know of, Ceres, is only about 1/4 the diameter of the moon, and it's big enough to be called a dwarf planet. There are plenty of other potential risks, for example a rouge star passing through the inner solar system would disrupt the trajectory of many asteroids and comets, flinging some toward Earth. The chances for a large black hole to pass through the solar system are very small, since it would be more massive than the sun but smaller than New York. Small black holes could evaporate to Hawking radiation. Remember that the asteroid/comet that probably caused the Cretaceous-Tertiary extinction (the one that killed the dinosaurs) was only about 15 km (9 miles) in diameter. As for the sun expanding, we could maybe move to Mars, but chances are we won't even survive that long. Most estimates predict there is a "high probability" for us to become extinct within the next three million years or so. Anyway, an asteroid bigger than, say, Lake of the Woods, would probably crash through the Earth's crust, exposing its mantle, causing further problems. Anyway, there are plenty of other potential doomsday events that could affect us in the near future (try exitmundi, [warning, popups]), and many of those pose a larger threat to us than the likelihood of an asteroid hitting Earth (which will, with 100% probability, eventually happen). In fact, one potentially catastrophic scenario is already unfolding, and could affect us in our lifetime, yet many are refusing to do anything about it. It's called global warming. ~AH1^(TCU) 15:35, 3 January 2009 (UTC)

Pesticides

I was looking at pesticides recently and one thing which surprised me was the number of different products which are the same brand and seem to be the same thing.

For example there were a number of different products which were 12g/litre of Permethrin in the form of an emulsifiable concentrate, with the same total volume. Prices varied by a few cents in some instances. These were nominally intended for a different purpose, e.g. No Silverfish beetles, No cockroaches, No ants with appropriate instructions as to how much to dilute them (all with water only) and how to apply them although they sometimes gave instructions for some other purposes. Am I right that these are almost definitely the exact same thing but with a different label? Or is it likely they have other ingredients to aide in how they adhere to surfaces or whatever. There was one for spiders which was a higher concentration (50g/litre) which I can see the need for.

There were also a bunch of ready to use sprays for similar purposes most of which were 4g/litre of permethrin in the form of a ready to use liquid (why they recommended different dilutions for the concentrates but the RTU liquids are the same I'm not sure). These were the same quantity and IIRC the bottles were similar, I don't think they sprayed differently or anything. The prices varied more by about $2 or so. Similar to above, there could be differences in surfactants etc and particularly in this case what the containing liquid is although at least two of them said "Will remain active on inert surfaces for up to two months". Or is it likely these are more or less exactly the same product?

I do have photos of some of the products and you can also see them online e.g. [23] [24] [25] [26] and note they are registered as different products although under the same code HSR000265 (mentioned on the bottle) for the concentrates or HSR000263 for the RTU liquids.

Finally another thing I've noticed is certain wasp powders e.g. [27] have Permethrin in the same concentration (10g/kg or mg/g) as flea powder for carpets. Beyond perhaps difference in particle and in applicators on the bottle, am I right these are more or less the same thing? (Some are in higher concentration [28] but I'm guessing again there's little difference otherwise). Just to be clear I'm not referring to the stuff meant to be applied to pets, which are probably regulated differently (here in NZ they regulated as veterinary medicine as opposed to the carpet powder, wasp powder etc which are regulated as pesticides HSR000262).

P.S. It's possible some of the above have different cis/trans ratios for their Permethrin but any that did mention the ratio were 25:75 Nil Einne (talk) 11:55, 3 January 2009 (UTC)

It's certainly possible that the surfactants might be different for products labeled for indoor, horticultural, and veterinary uses. It's also possible that it's simply a marketing decision. I'm not sure how the regulators in NZ define the codes, but I know for the OMRI (wow, no article! OMRI rates products for the National Organic Program in the US) ratings in the US, it's a product-by-product registration. --SB_Johnny | ^talk 13:01, 3 January 2009 (UTC)