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July 3

Enthalpy-entropy chart

Hi. On this enthalpy-entropy chart, can someone please tell me what 'x' represents (ie x = 10%, x = 20%, x = 30%...).

The person who actually created the graph seems to have 'retired' from Wikipedia, and another user asked for a caption to be added to it.

Thanks in anticipation,  Chzz  ►  01:32, 3 July 2010 (UTC)

It's either Percent humidity, i.e. the ratio of actual vapor pressure to vapor pressure at the dew point, or some sort of meaure of proximity to the critical point. --Jayron32 01:38, 3 July 2010 (UTC)

I found a few descriptions that say that the x lines are "constant moisture or quality" lines. I don't know what that means, though. Looie496 (talk) 04:45, 3 July 2010 (UTC)

Constant moisture implies relative humidity, which is the link I speculated on first... --Jayron32 04:55, 3 July 2010 (UTC)

This is a Mollier Diagram for water-steam. It is named after Richard Mollier a German physicist. His water-steam diagram depicts the properties of steam and is used to estimate the performance of steam engines and steam turbines. X is called the dryness fraction of the steam. At Mollier diagram it says Lines of constant dryness fraction are drawn in the wet region ...

Dryness fraction represents the quality of steam for the purpose of doing work or supplying heat. X=90% refers to steam which consists of 90% (by mass) of dry steam and 10% colloidal liquid water. (Cloud is an example of colloidal liquid water mixed with gas. In a cloud, the gas is humid air. X is not referring to relative humidity which implies the presence of air. The diagram shows the critical point which is a property of water, not humid air.) Outside the boundary labeled as 100% the steam is superheated - it contains no colloidal liquid water, and the steam is hotter than the boiling point at the prevailing pressure.

Steam with X less than 100% is called wet steam. Steam with X=100% is called saturated steam or dry steam. When dry steam is heated it is called superheated. See Steam and Superheated steam. Dolphin (t) 06:48, 3 July 2010 (UTC)

Medroxyprogesterone versus medroxyprogesterone acetate

Hola,

I'm trying to figure out the difference between medroxyprogesterone versus medroxyprogesterone acetate. Anyone know? Anyone have any sources that can be used to distinguish the two? WLU (t) (c) Wikipedia's rules:^simple/_complex 01:57, 3 July 2010 (UTC)

Other than the obvious difference in chemical structure, what are you looking for? It appears that the 17-acetate is the only one used medically - both of the references in the MP article actually relate to MPA. The Merck Manual entry indicates that the former is "supplied as the acetate". The article in MedlinePlus uses the terms interchangeably. Many references discuss plasma levels of MPA, so my initial impression that the acetate is readily hydrolyzed is probably false. -- Scray (talk) 02:59, 3 July 2010 (UTC)

the following 4 posts were copied from WikiProject Medicine, as discussed there

(Original research alert) First, pregnanes and progestins are not mutually exclusive. Pregnane refers to the structure of the molecule, while progestin refers to its biological activity. Medroxyprogesterone is the active molecule. In order to stabilize medroxyprogesterone in a form that can be administered orally or intravenously, it's acetylated (see the final step in File:Medroxyprogesterone_acetate.png. In the body, the acetate residue is degraded and you get the active molecule back.

If you're talking about administering medroxyprogesterone as a pharmaceutical, then you're technically talking about MP acetate. If you're talking about steroid biochemistry in vivo, then it's more correct to refer to medroxyprogesterone, period. I'm not sure what implications this has for our article structure (and this is just me talking - it's been awhile since I took pharmacology, and I don't have a supporting source at my fingertips, so take it with a grain of salt because I have been known to be wrong). MastCell ^Talk 03:58, 3 July 2010 (UTC)

MastCell, thanks for correcting me (pending further corroboration/refs). That was my initial sense (see the RD/S history for my original comments if curious) but I couldn't quickly find anything reliable that supported me. I'll look again tomorrow if no one weighs in further. -- Scray (talk) 04:02, 3 July 2010 (UTC)

I wish I had some refs handy to back me up. BTW, I saw your comment at the Ref Desk about MPA levels ([1]). Are you sure those are actually looking at levels of medroxyprogesterone acetate? I'm not familiar with measuring levels of it. On the other hand, there's an extensive literature on monitoring plasma levels of mycophenolic acid (also abbreviated MPA), which is the active metabolite of mycophenolate mofetil (MMF). Like most immunosuppressants, MMF pretty finicky and has a narrow therapeutic window, so a lot of work has been done on MPA pharmacokinetics (mycophenolic acid) in the organ transplantation literature. You've probably already looked into this, but is it possible that the "MPA" levels you saw were actually mycophenolic acid, rather than medroxyprogesterone acetate? MastCell ^Talk 04:30, 3 July 2010 (UTC)

I'm very familiar with mycophenolate; the particular paper that made me doubt myself was this one, which led me to PMID 6457936, which is entitled "Medroxyprogesterone acetate in human serum". On re-reading the abstract, this may simply be sloppy terminology, since the RIA they're using might not distinguish medroxyprogesterone from the acetate. Clarity remains elusive, but I think you're probably right. -- Scray (talk) 05:01, 3 July 2010 (UTC)

end of material copied from WikiProject Medicine -- Scray (talk) 05:14, 3 July 2010 (UTC)

Can you clatify how or what property you want to distinguish, or what sort of test (chemical,biological,physical) you want to use to distinguish them.87.102.21.49 (talk) 11:27, 3 July 2010 (UTC)

I'm mostly interested in being able to find sources that distinguish between the two. Medroxyprogesterone and medroxyprogesterone acetate are both stubs and so far I haven't been able to find sources that are clear on the difference.

And my apologies for cross-posting this to the extent I did. To make things worse, I'm going to compile the posts at Talk:Medroxyprogesterone 17-acetate. WLU (t) (c) Wikipedia's rules:^simple/_complex 13:34, 3 July 2010 (UTC)

Can you expand on "distinguish between the two" - they are different compounds (the strcuture, CAS number, formula, name, molar mass etc are all different) : one is the acetate ester of the other. How do you want to distinguish between them? Chemically, physically, by effects?? 87.102.21.49 (talk) 14:03, 3 July 2010 (UTC)

Dude, the acetate probably helps with the bioavailability. When it gets to the active site, it probably gets deacetylated into your steroid. Think morphine versus heroin. John Riemann Soong (talk) 21:08, 7 July 2010 (UTC)

jail

can you have a electric space heater in jail if a relative brings it to you? —Preceding unsigned comment added by Alexsmith44 (talk • contribs) 08:15, 3 July 2010 (UTC)

It depends on the category of the jail, some low category jails may allow it, but not very likely due largely to the cost implications on the jail, I mean, just imagine all the prisoners having space heaters. In normal medium and high category prisons the possibility is zero for a range of security reasons. Richard Avery (talk) 10:11, 3 July 2010 (UTC)

what are the security reasons? —Preceding unsigned comment added by Alexsmith44 (talk • contribs) 10:17, 3 July 2010 (UTC)

The heater could have parts that could be used as a weapon.--Mr.K. (talk) 10:52, 3 July 2010 (UTC)

Or the heater could be used to start a fire. Really, I doubt any prisons would allow it. Maybe a halfway house, but that's about it. — Lomn 12:22, 3 July 2010 (UTC)

I doubt they would allow it - there are strict limits of the luxuries you are allowed in prison, and space heaters are a serious fire risk. They'll probably let your relatives bring you an extra blanket (in fact, the prison may well bring an extra blanket if you ask). A prison should be heated to a reasonable temperature anyway. If you have a medical condition that means you need a higher ambient temperature, they may well make allowances for you. (I'm assuming a prison in a developed country with a good human rights record - if either of those things don't hold, then there is no guessing what horrible conditions they might keep prisoners in.) --Tango (talk) 15:28, 3 July 2010 (UTC)

No jail in North America, at least, would permit a heater, due to fire risk, load on the electrical system, weaponization, and as a useful place to store contraband. Acroterion _(talk) 16:08, 3 July 2010 (UTC)

I imagine that's correct, but since this is the Reference Desk, perhaps someone would like to find a reference on the subject? --Anonymous, 17:15 UTC, July 3, 2010.

Do jail cells even have power sockets? This would seem unlikely to me Nil Einne (talk) 17:54, 3 July 2010 (UTC)

They might. Prisoners are sometimes allowed TV's or radios in their cells or reading lights. There might be somewhere to plug the vacuum cleaner. --Tango (talk) 18:28, 3 July 2010 (UTC)

I had a friend in a low security prison in the US. He was permitted to have some electronic devices (Specifically, I bought him a box fan). I don't know if he would have been allowed a space heater. It seems though that the heat was more of a problem than the cold, which is the case in many densely populated buildings. Buddy431 (talk) 21:08, 3 July 2010 (UTC)

It depends on the jail. In some, such as that described here, just about anythings goes. Zoonoses (talk) 12:35, 4 July 2010 (UTC)

Trapped in a fusion reactor?

Supposing a man got accidentally locked in a fusion reactor such as JET and it was switched on. Is the plasma dense enough and does it last long enough to do serious harm? We will assume he has a spacesuit so that the effects of vacuum can be ignored. —Preceding unsigned comment added by 80.1.80.3 (talk) 08:21, 3 July 2010 (UTC)

The temperatures are higher than the surface of the sun. The whole problem with fusion reactors of this kind is that the plasma has to be contained in a 'magnetic confinement' to avoid it vaporizing the walls of the reactor. So our poor test subject would likely be vaporized in a small fraction of a second. Still, that will prevent the huge amounts of radiation and energetic particles from being a problem. SteveBaker (talk) 14:39, 3 July 2010 (UTC)

We are talking about the same nuclear reaction that drives the sun, if a person could not survive on the sun, how could they survive in the nuclear reactor? The interior of the reactor has to be hot enough to keep the plasma ionized and it has to be compressed to undergo fusion, and considering people are made of matter, they would be vaporized just like the hydrogen and/or helium gases that power the reaction. In addition, some methods for heating the gases use high intensity microwaves, essentially turning the divice into a giant microwave oven. In other words, between the temperatures required for fusion, the radiation and energy produced by the reaction, and some of the ways the gases are heated, there is no way the unlucky person in the reactor would survive. —Preceding unsigned comment added by 74.67.89.61 (talk) 14:56, 3 July 2010 (UTC)

I apologise for my colleagues being so dismissive of this question. You are absolutely right that just because a plasma is very hot doesn't mean it would actually burn someone. Parts of the interstellar medium is plasma at 10,000K but obviously that wouldn't burn anyone because it is so sparse. Our article on the Joint European Torus says the plasma lasts 20-60s, so that would be long enough to burn if it is dense enough. So the question, as you say, is what is the density of the plasma? As the anonymous response says, the density has to be at least above a certain level in order for fusion to take place. Unfortunately, I can't find a description of what that density actually is. The JET website is full of interesting information, but apparently not that piece of information. The pressure inside the Sun is enormous, but the temperature there is more than 100 times the maximum temperature achieved at JET, so clearly the two situations are not comparable. --Tango (talk) 16:01, 3 July 2010 (UTC)

I believe exposure to neutrons would be a major problem, ignoring thermal energy flux for the moment, since the reaction liberates large quantities of fast neutrons (whose energy is harvested from the lining of the tokamak). Acroterion _(talk) 16:06, 3 July 2010 (UTC)

The neutron radiation could easily be a serious risk, you are right. Does anyone know the neutron flux for JET? --Tango (talk) 16:43, 3 July 2010 (UTC)

The original poster mentioned density. Is the pressure / amount of plasma sufficient? You can have very high temperature (= velocity) atomic particles, but if there are not too many of them, their combined punch will not be able to heat a cup of tea, much less vaporize anyone. I guess it's not so much about temperature than about total thermal energy. Also perhaps the type of particles matters; some kinds, while quite hot, might pass through our intrepid adventurer without sharing their kinetic energy? 88.112.56.9 (talk) 16:16, 3 July 2010 (UTC)

You can calculate the density from the pressure and amount of plasma, yes, but I can't find those numbers either. Total thermal energy would be a good way to estimate the harm, but the thermal conductivity of the plasma would be a factor too - a hot metal bar burns you more than a plastic bar with the same temperature and thermal energy, since more of the energy actually gets to you. --Tango (talk) 16:43, 3 July 2010 (UTC)

My intuition (= no sourceable fact content in this comment) suggests that gas/plasma uses a different mechanism to impart heat energy than a solid does. Gas goes: atom-skin. Solid goes: atom-atom-atom-atom-atom-atom*100000-skin. Atoms in tenuous gas/plasma do not conduct, they just hit you. 88.112.56.9 (talk) 19:54, 3 July 2010 (UTC)

Plus inside the plasma, the person would be crushed because the plasma is compressed.--dance (talk) 20:41, 3 July 2010 (UTC)

How much is it compressed by? I can't find any figures for the pressures they use. --Tango (talk) 21:15, 3 July 2010 (UTC)

The first thing they do before establishing the plasma is remove all the air with very good vacuum pumps. He will simply die of the lack of air. The person will ruin the vacuum, because the water in the body of a person takes ages to evaporate after some time hours or days they will open the chamber and find the person.--Stone (talk) 22:00, 3 July 2010 (UTC)

N.B.: per the OP: "We will assume he has a spacesuit so that the effects of vacuum can be ignored" -- Scray (talk) 22:09, 3 July 2010 (UTC)

They're going to let a multi-billion-dollar fusion reactor sit around for days with no vacuum before they bother finding out why? Boy, that fusion-workers' union — you don't mess with them, huh? --Trovatore (talk) 22:05, 3 July 2010 (UTC)

It won't be no vacuum, just a very slightly poorer vacuum. --Tango (talk) 01:07, 4 July 2010 (UTC)

For the purposes at hand, I imagine that's equivalent to no vacuum. --Trovatore (talk) 01:21, 4 July 2010 (UTC)

Thank you! I went to try and look that up (and didn't really succeed) and found just the information we need. This page says: "The densities of the hydrogen plasma that can be confined by magnetic fields are very low, about one million times lower than the density of air." I think that's low enough to be pretty harmless, even at temperatures of 100 million Kelvin. --Tango (talk) 01:33, 4 July 2010 (UTC)

No way! Just stop and think about what you're saying. The ENTIRE point of going to all that trouble with magnetic confinement is to prevent the plasma from vaporizing the walls of the reactor! If spacesuit material could comfortably withstand the heat/pressure then making a fusion reactor would be a pretty trivial problem - you'd make a simple spherical container and line it with 'spacesuit material' and you'd be done...so much for 60 years and billions of dollars of research! Our Tokamak article says "Magnetic fields are used for confinement since no solid material could withstand the extremely high temperature of the plasma."...so our intrepid experimenter is very, very dead from heat alone. You - and the others who say that the pressure is too low - need to explain why you'd need magnetic confinement if heat was not a major problem at the pressures the reactor is designed to run at. SteveBaker (talk) 03:26, 4 July 2010 (UTC)

That argument doesn't necessarily add up. I read the statement you referenced as "the magnets are needed to keep the plasma pure enough and at a high enough temperature" rather than "the magnets are needed to protect the structural integrity of the device". "Plasma vaporizing the walls of the container" isn't the same thing as "completely destroying the walls". From Magnetic confinement fusion, it appears that the magnets are to prevent "sputtering", where a portion of the wall is vaporized, thus lowering the temperature of the plasma. It's unclear how much of the container wall is actually vaporized, though. It's conceivable that a very small vaporization of the container (or man in a spacesuit) could ruin the experiment (by lowering the temperature and adding heavier element contamination), but still not compromise the structural integrity of the container (or health of the man in his spacesuit). Additionally, it appears that the plasma is only contained, heated, and compressed for a couple of seconds at a time. I don't know how much material would be vaporized in a couple seconds, and whether an ordinary spacesuit would stand up to it. Buddy431 (talk) 04:21, 4 July 2010 (UTC)

Right, that jibes with a vague memory of something I read somewhere, that the reason it is so essential to keep strict magnetic confinement is to keep the plasma hot; any failure of confinement, and consequent contact with a solid object, means your temperature instantly drops out of the working range for fusion. What happens to the solid object might also be a concern, of course, but keeping the plasma hot is a sufficient reason for confinement in itself, even if the damage to the torus were tolerable. --Trovatore (talk) 05:48, 4 July 2010 (UTC)

That's correct. If there were contact between the plasma and the walls, heat would be conducted from one to the other. A few wisps of plasma (which is all there is) will only remove a very thin layer from the walls. See the answer to the question "Q: I recently heard a sort-of scientific urbanmyth that at some point during its operation the plasma inside thetorus touched the top and the whole thing jumped up in the air. Isthere any truth to this story?" on the FAQ on the JET website (here). --Tango (talk) 06:25, 4 July 2010 (UTC)

Your person probably won't get hurt. The small amounts of plasma might heat his suit up, say 1 degree, while the plasma is no more plasma (just a gas). --Chemicalinterest (talk) 15:37, 6 July 2010 (UTC)

If the person would be fine, what's the point of making a fusion reactor? How to you use the heat if the heat is neglible?--92.251.137.196 (talk)

Metric system

What's the metric prefix for 10²⁷? --75.149.70.22 (talk) 14:28, 3 July 2010 (UTC)

There isn't one. The official list is at SI prefixes, and stops at 10²⁴. Physchim62 (talk) 14:34, 3 July 2010 (UTC)

Our article on SI prefixes mentions hella as an option (not, it seems, used outside the USA). --88.117.82.138 (talk) 14:36, 3 July 2010 (UTC)

"Hella" is an idea at the moment – it isn't used anywhere, and doesn't seem likely to be adopted anytime soon (if at all). If they chose "hella" for 10²⁷, what would be the prefix for 10³⁰???? Physchim62 (talk) 15:11, 3 July 2010 (UTC)

hellalotta- you asked for it 87.102.21.49 (talk) 15:19, 3 July 2010 (UTC)

I was thinking more along the lines of "f*ckloadsa", but we can always save that for 10^33... Physchim62 (talk) 15:38, 3 July 2010 (UTC)

I wouldn't say that it isn't used anywhere. Dismas|^(talk) 00:37, 4 July 2010 (UTC)

Isn't "Hella" the Norse name for the underworld? 67.170.215.166 (talk) 01:20, 4 July 2010 (UTC)

_{try the search box?} —Preceding unsigned comment added by 77.86.10.42 (talk) 12:19, 4 July 2010 (UTC)

That was a rhetorical question on my part -- and FYI, I knew all along that the answer is "yes it is". 67.170.215.166 (talk) 00:04, 6 July 2010 (UTC)

the sun

approximately how many fusion reactions occur within the sun every second? And how much light from it hits the moon?80.47.187.29 (talk) 17:22, 3 July 2010 (UTC)

The energy released by the creation of one Helium-4 nucleus by the Proton–proton chain reaction is 23.4MeV. The luminosity of the sun is about 4x10²⁶W. If we divide one by the other, we get about 10³⁸ Helium-4 nuclei being created every second. The amount of light hitting the moon is the solar constant multiplied by the cross-sectional area of the Moon. That's 1.4kW/m²*(pi*1700km^2)=7.5x10¹² W=7.5 TW. --Tango (talk) 17:39, 3 July 2010 (UTC)

Presuming of course that nothing is between the sun and the moon, like during an eclipse. Googlemeister (talk) 13:37, 6 July 2010 (UTC)

nucleus diameter

How scientists measure the diameter of a nucleus? —Preceding unsigned comment added by Lukyshubham (talk • contribs) 17:58, 3 July 2010 (UTC)

You can get an estimate from the Geiger–Marsden experiment. Physchim62 (talk) 18:04, 3 July 2010 (UTC)

As Rutherford himself might have illustrated, imagine you are Jesse Ventura in Diamonds Are Forever (film). Jesse "The Body" has to find a single diamond hidden in a soft-brick wall in 10 minutes, with only his minigun in hand. He fires millions of bullets spread randomly on that wall, slicing cleanly in-and-out of the clay for 8 minutes until finally he hears a loud "ping" as one of the bullet literally bounces back. "Bullseye", he says with a smirk, then spits his tobacco on the floor... before being skinned alive by The Predator.

The moral of the story is that Rutherford fired alpha particle bullets into a metal foil wall to find the size of that diamond, the nucleus, as a ratio of the clay brick wall, the electron cloud. He fires millions of alphas, and millions go clean through the foil, but occasionally one or two bounce back. The cross-sectional area of the nuclei is then the ratio of #particles bounced back divided by # particles fired, multiplied by the area of the foil wall. Jesse Ventura fired a million bullets, one bounced back, onto a 10x10-meter wall, so the length of the diamond is about sqrt(100m^2 / 1000000) meters = 1cm. SamuelRiv (talk) 03:18, 4 July 2010 (UTC)

Which Diamonds Are Forever (film) are you talking about? Jesse Ventura never was in that movie -- it was Sean Connery as James Bond, and there wasn't any scene with a Gatling gun or a diamond hidden in a brick wall (I know, I've seen it on DVD)! 67.170.215.166 (talk) 04:19, 5 July 2010 (UTC)

It's in the "deleted scenes". Actually, I thought I made it quite clear that this was a remake of Diamonds Are Forever starring Jesse The Body and replacing Kidd and Wyndt with The Predator and requiring Bond to find a diamond in a clay wall using only his minigun for some reason. AND YET IT'S STILL MORE REALISTIC THAN THE ORIGINAL! SamuelRiv (talk) 08:19, 5 July 2010 (UTC)

In case the OP wasn't talking about an atomic nucleus and instead wanted to know about a cell nucleus, you can use a microscope. Smartse (talk) 21:12, 4 July 2010 (UTC)

Decreasing metabolism?

Right now, there is great interest in increasing metabolism to help combat obesity (including interest in leptin and uncoupling proteins). I was wondering whether there is any research being done about how to decrease metabolism. Why is it that when we talk about solutions to global starvation, we never talk about metabolism – we always talk about growing more food? I should be particularly interested in any effect that would help the body burn several hundred calories less each day.174.131.45.82 (talk) 18:09, 3 July 2010 (UTC)

That's easy: do less. That's not usually an option for people facing starvation, though. --Tango (talk) 18:26, 3 July 2010 (UTC)

Doing less would burn fewer calories and prevent metabolism from increasing as a result of exercise. I was looking for a way to decrease metabolism other than inactivity or caloric restriction, however.174.131.45.82 (talk) 18:59, 3 July 2010 (UTC)

Along the lines of medication that decrease metabolism? As far as I know, that's usually considered a side effect rather than a benefit... Ks0stm ^(T•C•G) 20:08, 3 July 2010 (UTC)

I doubt that using medications would be safe, and I do not know of any medications that decrease metabolism as a side effect (many drugs cause unwanted weight gain - insulin can cause some people to gain weight, but it actually increases the metabolic rate).174.131.40.152 (talk) 20:58, 3 July 2010 (UTC)

I think the human body already has pretty well developed famine modes which it goes into when near starvation. I doubt tinkering with a few chemicals would improve what happens naturally. --BozMo talk 21:38, 3 July 2010 (UTC)

We do have drugs that can decrease metabolism, but if your metabolic range is within normal range, it is not a good idea to use these drugs. They would cause someone to develop hypothyroidism which does have have very desirable effects. —Preceding unsigned comment added by 76.91.30.156 (talk) 02:03, 4 July 2010 (UTC)

I already knew about ATD's (antithyroid drugs). What I am wondering is this: why do some people have slow metabolisms (excluding people with hypothyroidism and Cushing's syndrome)? How are they different from us? What could we learn from them? One answerer also talked about the body adapting to starvation. It is true that metabolism does slow down a little during starvation, but this occurs, of course, only when a person does not consume enough calories. Would it be possible to decrease metabolism so that a person would need fewer calories in the first place, without ever actually reaching a negative energy balance.174.131.68.138 (talk) 02:20, 4 July 2010 (UTC)

Wouldn't that just make them fat? 67.170.215.166 (talk) 07:57, 4 July 2010 (UTC)

[Geography] Locate or name the lake by picture

I've seen some different wallpapers that picture one lake.

These are links to 3 different wallpapers with this lake: (Warning: links spend traffic, these are large pictures (1920x1080 or larger))

http://i8.fastpic.ru/big/2010/0704/2a/56aa011b5871afdeece9e24644d4682a.jpg

http://www.goodfon.ru/download.html?id=39203&rash=1920x1080

http://www.goodfon.ru/download.html?id=30598&rash=2048x1536

Where's this lake, does anybody know? And does it have a name?

--Grue12 (talk) 22:13, 3 July 2010 (UTC)

Moraine Lake in Banff National Park, Canada. Deor (talk) 22:16, 3 July 2010 (UTC)

Thanks a lot! --Grue12 (talk) 22:35, 3 July 2010 (UTC)

Superball dynamics

Hi, I was reading an article on Superball dynamics, and they said that during a collision with a table or something, the normal component of the velocity will change sign without justification. Why would this be true? 74.15.137.192 (talk) 23:01, 3 July 2010 (UTC)

It's basically reflection. The kinetic energy gets converted into elastic potential energy as the ball compresses and then that elastic potential energy is converted back into kinetic energy in the other direction as the ball rebounds. --Tango (talk) 23:54, 3 July 2010 (UTC)

But why wouldn't this potential energy get converted to rotational kinetic energy or tangential kinetic energy? 74.15.137.192 (talk) 00:12, 4 July 2010 (UTC)

It just doesn't. That's the way physics works. You can't get rotation with a torque and you can't get tangential movement without a tangential force. The force between the ball and the table will always be normal to the table. --Tango (talk) 00:18, 4 July 2010 (UTC)

But if the ball is spinning and has an initial component of its velocity parallel to the table, then surely there can be some torques and frictional forces, no? 74.15.137.192 (talk) 00:31, 4 July 2010 (UTC)

Ah, if it's spinning that's a different matter entirely. A non-spinning ball with an initial parallel component to its velocity will end up with a slightly slower parallel component due to friction - that's completely independent from the normal component, though, which is why I didn't mention it. --Tango (talk) 01:05, 4 July 2010 (UTC)

Sorry for not being clear. Is your explanation still valid? 74.15.137.192 (talk) 01:36, 4 July 2010 (UTC)

Wait a minute, Tango - I think IP74 is also asking why all the force is normal, even without a rotational component but with a component parallel to the table, since that parallel part gets a bit "stuck" on the frictional table surface and thus decreases the angle of reflection. That would lower the final velocity but conserve both energy and momentum since the table is pushed in the process.

By the way, Tango, not sure if you teach undergrad or high school physics labs ever, but if you ever get to use those collision carts on the low-friction airtrack, have your students model the curve that the computer outputs at the half-second during collision - it's a nice spring-storage SHM, illustrating that springs are everywhere, even in supposedly-rigid metal carts. SamuelRiv (talk) 03:05, 4 July 2010 (UTC)

Well, the OP was asking about the normal component of velocity, which is effected by the normal component of the force between the ball and table. It's always difficult to answer "why?" questions in physics - there always comes a point where you just have to say "That's just the way it is.". --Tango (talk) 06:15, 4 July 2010 (UTC)

I was trying to answer OP's question after your first explanation, about rotation components. Regarding the great "why" of the Normal Force, I defer at that point to "this is where we introduce the Conservation of Momentum". Indeed, the Normal Force can be explained in terms of smallball-bigball for most purposes where the Normal is actually useful. When it's not useful, it's a convention issue (we can use Lagrangian mech instead - that gave one class some relief that there was some arbitrariness in this grandiose of subjects). SamuelRiv (talk) 06:31, 4 July 2010 (UTC)

What I understood from this was, the normal force doesn't depend on any tangential or spin velocity (that just adds frictional forces), and because the normal velocity reverses in a no-spin/no-tang. velocity scenario, it must always do so. Is that the right reasoning? 74.15.137.192 (talk) 07:01, 4 July 2010 (UTC)

Time Dilation

The faster something moves, the slower time passes for it, but doesn't this prove something is moving which indicates absolute space? Assume that there is a train passing a train station and bob is on the train and alice is observing the train pass on the station. If alice watches bob's actions through a window she will see what bob is doing in slow motion. Likewise if bob watches alice's actions, it will seem everything is happening super fast (assuming the train was traveling at a signifigant proportion of the speed of light). According to relativity, as long as neither person is accelerating, both alice and bob could argue that they aren't moving and the other person is, but if time changes based off speed and time was passing slower for bob, wouldn't that prove that he is the one moving because if he was trying to argue that alice was the one moving, then she would seem to be acting in slow motion. Because it is bob that time is passing slower for doesn't that prove he is the one moving which violates relativity? —Preceding unsigned comment added by 74.67.89.61 (talk) 23:53, 3 July 2010 (UTC)

Actually, both Alice and Bob see the other moving in slow motion. This seems like a contradiction, but because of the finite speed of light it does actually work. If you do the maths, it all works out. --Tango (talk) 23:56, 3 July 2010 (UTC)

It's only if they come back to compare there clocks that there's a problem. But that's hard to do. Either it takes light from one clock a finite amount of time to reach the other person (and everything works out, as 74 asserts), or one (or both) of them have to accelerate to meet the other... Which means he (or she, the one who accelerates) is at some point in a non-inertial reference frame, where special relativity is no longer sufficient, and general relativity is needed to explain the time dilation effects. Buddy431 (talk) 02:51, 4 July 2010 (UTC)

No, general relativity is not necessary to understand the correct answer to that question. There is no gravity in that question and no gravity means no need for GR. Just because you used the word acceleration does not mean that GR is required for a correct answer. What Tango said is essentially correct. The solution to the apparent paradox passes by the definition of proper time. More on that later (it's late and I'm tired). Dauto (talk) 03:49, 4 July 2010 (UTC)

One of the central principles of GR is that the effects of acceleration are indistinguishable from the effects of an equivalent gravitational field. So I don't see why you say that a problem that involves acceleration but no gravity does not need GR, since we can rephrase the problem in terms of a gravitational field with no acceleration, and the results must be the same. Surely acceleration and/or gravity => non-inertial reference frame => GR, not SR ? Gandalf61 (talk) 13:16, 4 July 2010 (UTC)

There is no acceleration involved, and no gravity. The question refers to special relativity. --Wrongfilter (talk) 15:07, 4 July 2010 (UTC)

The original question is just SR, yes. But if you read the thread you will see that Buddy431's extension, where Alice and Bob meet up again to compare clocks in the same reference frame, introduces acceleration and hence GR - it is a version of the so-called twin paradox. And this was what Dauto's reply and my own reply to Dauto were referring to. Gandalf61 (talk) 17:22, 4 July 2010 (UTC)

You don't need GR to deal with nongravitational acceleration. SR is fine. You can, if you like, use general relativity and say that in an accelerating reference frame there is a gravitational field, but that's just words. The GR predictions are the same as the SR predictions, and the mathematical framework in which they're derived is the same. There are no GR corrections. It's true that you can't accelerate an inertial reference frame, but that's an intrinsic property of inertial reference frames, not an inadequacy of the theory describing them. You can accelerate objects of finite extent, like rocket ships, and SR correctly describes the acceleration. -- BenRG (talk) 19:17, 4 July 2010 (UTC)

But if that were true, then we wouldn't need GR at all, as we could convert any problem involving gravitational fields into an equivalent problem with accelerations and no gravity, and then solve it using SR alone. Is that what you are saying ? Gandalf61 (talk) 22:51, 4 July 2010 (UTC)

No, that's not what he is saying. The only problems that can be solved that way are the ones where there is no source of gravity. GR is a theory of gravity. When there are no sources of gravity GR reduces trivially to SR. In GR the presence of Energy (or momentum) density (or flux) leads to the warping of space-time. In the absence of any (large) energy-momentum density-flux GR predicts a flat spacetime AKA Minkowski space-time. The theory that describes the motion of objects within that flat space-time is SR. The objects that are moving within that flat space-time may be accelerating but the space-time is still flat and therefore SR is enough to describe that kind of situation. Just because I used the word acceleration in that last phrase doesn't mean that GR is now required since the space-time remains flat. I hope that helps clear that point. Dauto (talk) 01:58, 5 July 2010 (UTC)

Hmmm. So an observer who notices that spacetime in the frame of reference in which they are at rest is not flat can either (a) assume they are in a gravitational field, and use GR in their own frame of reference to handle this non-flat spacetime or (b) assume they are accelerating, and use SR in an inertial frame of reference (which is accelerating relative to them) in which spacetime is flat ? And they will get the same results in either case ? Gandalf61 (talk) 09:19, 5 July 2010 (UTC)

Not quite. A curved space will be curved in any frame of reference (No matter what system of coordinates you chose to use the surface of the earth will never be a plane). But there is always a flat reference frame that is tangential to any given point of the curved spaca-time which is good enough an approximation for small enough a region around that point. The better the approximation you need the smaller the region will have to be. Within that region gravity can be replaced by an accelerated motion in that tangential flat space. That's why the equivalence principle is said to be correct only locally, but not globally. Globally the non-flatness becomes evident and shows up as tidal forces that cannot be removed by a change in reference frame. The equivalence principle is a consequence of the fact that the gravitational forces really are pseudo-forces that come about because of the choice of a non-inertial reference frame. In that sense gravity is a force similar to centrifuge force or to Coriolis force. And that's also why gravitational forces are proportional to the mass of the object in which it acts (Centrifuge and Coriolis forces also are proportional to the mass). Gravity actually belongs on the right side of the equation F=ma. The difference is that because of the curvature of space there isn't a single inertial reference frame capable to cover the whole space-time. different inertial reference frames have to be used at different points in space-time. Dauto (talk) 13:28, 5 July 2010 (UTC)

The misunderstanding is at the very beginning of the question: For the thing that is moving there is no change in "how fast" time passes. Your wrist watch almost advances at the same pace, whether you're lying in bed, riding on a train or on an interstellar spacecraft or even sliding through a wormhole (hypothetically). Your wrist watch (alternatively, all the physiological processes in your body) runs according to your personal time, so called "proper time" (as hinted at by Dauto). If Bob rides on the train, then Alice, standing on the platform, will see Bob's wrist watch run slow compared to her own watch. Conversely, Bob will see Alice's wrist watch run slow compared to his own. The situation is symmetric, which means that there is no absolute time. Sounds weird but that's the way it is and there are no logical contradictions. --Wrongfilter (talk) 15:07, 4 July 2010 (UTC)

Ah, I think this is actually where I meant to put this comment, rather than at the "neutron star" thread above.

It is important to distinguish what Alice and Bob see from what happens in Alice's and Bob's coordinate systems; these are not necessarily the same thing at all. What they see is affected by the passage of light between them, and is subject to the Doppler shift and to various distortions that are not purely relativistic (I'm sure we have an article somewhere, but the title is too long to remember; something like visual appearance of fast-moving objects?).

The safe, if slightly laborious, way to keep track of it all is to imagine that Alice and Bob have both set up a network of space probes that extend as far as necessary in all directions. The probes make sure they are at rest with respect to one another by shining laser beams at one another and making sure there's no Doppler shift, and they keep their internal clocks synched up by the laser beams as well (they transmit the reading on their clock; the neighboring probe makes sure that it's correct after subtracting the time the light takes in transit). Alice's probes are at rest with respect to Alice, and Bob's with respect to Bob. Then Alice's probes take pictures of Bob's watch as Bob passes them, and Bob's probes take pictures of Alice's watch. The "slowdown" is computed by comparing the pictures with their timestamps.

That's all in flat spacetime, pure special relativity. When you have gravitational effects it all gets more complicated, and there are more assumptions that need to be specified. Near the event horizon of a black hole these all become especially bad. --Trovatore (talk) 03:19, 5 July 2010 (UTC)

July 4

Forward-swept wings

How stealthy are forward-swept wings? And would a forward-wing fighter make a good carrier aircraft? And how efficient are they at high speeds? --The High Fin Sperm Whale 00:39, 4 July 2010 (UTC)

(1) Not very, they tend to focus radar waves when viewed from head-on. (2) Forward-swept wings alleviate the boundary layer separation problems associated with swept-back wings, which allows a higher critical angle of attack and thus a lower landing speed. (3) About as efficient as swept-back wings, the reduction in wave drag is a function only of the absolute value of sweep angle without regard as to which way the wing is swept. FWiW 67.170.215.166 (talk) 01:35, 4 July 2010 (UTC)

Stealth: I would have thought that stealth wise it would be much the same as a back sweep, but not as good as 'current' steath aircraft (thinking of the B2 here) where blending the wing/body reduces reflective surfaces. However this site enemyforces.net says of the Sukhoi Su-47 'Berkut', "The forward-swept wing has a lower radar signature from the front hemisphere"

Carrier: What makes a "good carrier aircraft"?. Low take off speed would be one factor, which is why some (F-14 Tomcat) had variable-sweep wings, (though back-swept and which also seem out of fashion now, see [2]) Swept wings have better stall charateristics at high angle of attack, which is relevant for all fighter aircraft. Supposedly, no forward-swept wing(FSW) planes are in production, so I don't think anyone are contemplating Variable forward sweep, yet. Then again, see this at dreamlandresort.com which purports to show a Grumman Northrop design for a FSW with variable sweep! Here too [3] more recently. And Northrop Switchblade on Wikipedia!

This site century-of-flight.net says "Aircraft with forward-swept wings are highly manoeuvrable at transonic speeds". --220.101 (talk) ^\Contribs 01:54, 4 July 2010 (UTC)

Variable FORWARD sweep?! Sounds like a perfect recipe for structural failure! 67.170.215.166 (talk) 05:08, 4 July 2010 (UTC)

Why? I can't see why it would be any more of a problem than variable backward sweep. --Tango (talk) 06:18, 4 July 2010 (UTC)

More twisting loads on the wing pivots, for one thing -- what do you think is the reason why swept-back wings can be made from ordinary aluminum but swept-forward wings require fancy-schmancy composites? 67.170.215.166 (talk) 08:00, 4 July 2010 (UTC)

"More twisting loads on the wing pivots" - I can't see any justification for that statement.77.86.10.42 (talk) 12:18, 4 July 2010 (UTC)

If you experiment with a piece of paper, then you'll see why... 67.170.215.166 (talk) 23:48, 4 July 2010 (UTC)

You know I have absolutely no idea what that means, please read Wikipedia:Reference_desk/Guidelines#Guidelines_for_responding_to_questions - it's quite counterproductive to make unverifyable claims.87.102.23.18 (talk) 01:37, 5 July 2010 (UTC)

What I mean is, take a long, fairly narrow strip of paper and a portable fan, and hold it at an oblique angle to the fan's airflow at the end nearest the fan (as if it were a swept-back wing with the wing root facing into the airflow); the strip of paper should flutter a little about the horizontal, but come back to the horizontal after each oscillation. Got that? Good, now reverse the strip of paper so that you're holding it at the end farthest from the fan (as if it were a swept-forward wing with the wing tip facing into the airflow); now the strip of paper should twist one way (either up or down) as far as it will go, and stay that way. This is such a basic experiment for illustrating the aeroelastic properties of swept-back vs. swept-forward wings that I'm perfectly shocked that the three of you (Tango, 77.86 and 87.102) have no idea about it or about the implications of its results. Seriously, if the three of you had at least a little basic visualization skills between you, then you would've got my point about twisting loads without me having to digest the very basics for you. 67.170.215.166 (talk) 04:38, 5 July 2010 (UTC)

Yes but a wing isn't a completely unrigid structure - in the experiment you describe the strip of paper just bends back completely and flaps about in both cases - there is an article aeroelasticity and I can't see a mention of what you are describing it's not flutter. I originally thought you were talking about up/down (flapping) torsion, but I'm not sure if you mean torsion force that attempts to force the wing back.. (The link to 'responding to questions' was to explain the usefulness of supplying interwiki links or external references to help confirm or explain what you are saying)'.87.102.23.18 (talk) 13:30, 5 July 2010 (UTC)

Doesn't matter- I think Acroteroin has explained it below.87.102.23.18 (talk) 13:35, 5 July 2010 (UTC)

sorry for the inconveinience - it just wasn't clear what you were saying87.102.23.18 (talk) 13:49, 5 July 2010 (UTC)

Yeah, the Goa'uld really loved their death gliders. nerd joke. --mboverload@ 07:12, 4 July 2010 (UTC)

The WP article on FSW makes only passing mention, but FSWs exhibit divergent aeroelastic flutter characteristics which amplify wing twist in certain flight regimes, leading to failure. It can be controlled using composites, but the structure necessary to deal with it increases weight unacceptably in metal construction. Digital flight controls are also helpful. In general, in stealth design, re-entrant angles are avoided, as they focus reflections (think of a headlight's shape) rather than diffusing them. Acroterion _(talk) 16:35, 4 July 2010 (UTC)

Tin allotrope conversion

Why didn't the tin in my pewter spoon change from the beta form to the alpha form when it was in a freezer for a couple weeks? --Chemicalinterest (talk) 12:20, 4 July 2010 (UTC)

Firstly the change doesn't always happen immediately - ie the reaction can be delayed like crystallisation from a supersaturated solution.

Secondly Pewter contains metals that inhibit the allotrope change - see also Tin_pest#Modern_tin_pest_since_adoption_of_RoHS 77.86.10.42 (talk) 12:40, 4 July 2010 (UTC)

Yes it contained antimony and that is one of the substances used to prevent the conversion. --Chemicalinterest (talk) 18:24, 4 July 2010 (UTC)

Only pure tin is subject to tin pest -- that's why these days you always find it alloyed with other metals. 67.170.215.166 (talk) 23:33, 4 July 2010 (UTC)

No. Please stop giving incorrect answers. (Some forms of) Alloyed tin is also subject to tin pest. eg [4] p17/64_{what happened} google books. 87.102.23.18 (talk) 01:38, 5 July 2010 (UTC)

Fine, from now on I will not answer questions about metallurgy, since my knowledge of this subject is admittedly not up to date. As for questions about aerodynamics, I would recommend that you refrain from answering, considering the obvious lack of knowledge about the subject that you demonstrated during the discussion about swept-forward wings. 67.170.215.166 (talk) 10:23, 5 July 2010 (UTC)

Most tin alloys are not subject to tin pest. --Chemicalinterest (talk) 11:56, 5 July 2010 (UTC)

So the tin buttons on the French tunics which caused Napoleon to retreat from the Russian winter (tall story from chemistry teacher that one) were supposed to be pure? Does anyone know if its actually true? --BozMo talk 11:38, 5 July 2010 (UTC)

It would be hard to tell whether that story was true. --Chemicalinterest (talk) 11:58, 5 July 2010 (UTC)

The story is actually not true -- Napoleon had to retreat because his food supplies ran out -- but AFAIK there was a similar incident involving a Russian army unit posted in Siberia (in peacetime) during a winter that was so exceptionally cold (I believe it was some time during the early 19th century, but after the Napoleonic wars) that the tin buttons on their uniforms crumbled and they all had to be issued replacement uniform buttons. I think this was the source of the story about Napoleon. FWiW (OK, no more metallurgy answers from me for the next couple months at least -- I've realized that much of what I learned about the topic way back when has been disproved, and I gotta update the huge database in my cranium with reliable, up-to-date info on this subject.) 67.170.215.166 (talk) 00:17, 6 July 2010 (UTC)

Clarify.. As far as I know nobody makes and sells an alloy that is more susceptible to tin pest than pure tin for obvious reasons (it would be useless). So to say 'tin alloys' don't get tin pest is sort of true - since alloys of tin will be produced with an eye on retarding or eliminating tin pest.94.72.242.84 (talk) 01:54, 6 July 2010 (UTC)

Piano Lid Prop angle

Most modern grand pianos' lid props appear to form a 90º angle where they meet the underside of the piano's lid. It seems logical to me that the lid prop is less likely to slip at that angle because there would be a direct load transfer of the weight of the piano's lid to the support stick. That is, grand piano manufacturers intentionally use a 90º angle for safety reasons. Could someone show me the mathematics, perhaps using vector analysis, to prove my hypothesis? The reader may want to visit http://en.wikipedia.org/wiki/Grand_Piano to see a couple of pianos that do not appear to use the 90º angle. Note the Louis Bas grand piano of 1781 and Walter and Sohn piano of 1805.Don don (talk) 16:52, 21 June 2010 (UTC)

here with an angle less than 90 degrees downward slippage is impossible without raising the mass of the lid

You already asked, [5] as Dmcq might have hinted(?) - your hypothesis is wrong - at 90 degrees the only thing that is stopping slippage is friction. Whereas at more acute angles slippage is impossible since the mass of the lid needs to be raised for the support to be able to fold into the resting position.Sf5xeplus (talk) 13:59, 4 July 2010 (UTC)

Yes, after I posted my question on the mathematics page I thought that it should have been placed in the physics section. How can the original entry be deleted?Don don (talk) 18:25, 5 July 2010 (UTC)

I don't think it's necessary to delete it, I'll add a link to here. [6] 94.72.242.84 (talk) 01:57, 6 July 2010 (UTC)

I assume you didn't mean upward slippage of the support which should be prevented by a block.Sf5xeplus (talk) 13:59, 4 July 2010 (UTC)

Perhaps the most sensible solution would be to have slippage prevention blocks on both sides of support? Sf5xeplus (talk) 14:00, 4 July 2010 (UTC)

Also aren't some fixed and hinged at the lid ? Sf5xeplus (talk) 14:05, 4 July 2010 (UTC)

I know of no piano that has a hinge at point C. (See following paragraph.)Don don (talk) 20:21, 5 July 2010 (UTC)

Perhaps I should clarify the angle to which I am referring in my original question. Looking at the picture of a grand piano found at the right, let point A be the center of the piano lid hinge pin to the left of the pianist; point B is the center of the support stick’s hinge pin; and point C is where the support stick contacts the piano lid. Currently, I have in my possession data (segment lengths AB, BC, and CA) from more than one hundred pianos. Using trigonometry, I have found that the average for angle C is approximately 90 degrees, plus or minus one degree.Don don (talk) 18:45, 5 July 2010 (UTC)

The premise of your question is wrong; most concert grand pianos don't have a fixed angle for the lid. Generally the lid angle can be adjusted either by elongating a telescopic prop (like this one) or by using a replacement prop (this article on recording gives 38° for the long prop and 10° for the short one). The piano lid functions as a reflector for mid- and high-frequency sound; Music, physics, and Engineering by Harry F. Olson shows a 15dB differential of 4kHz volumes horizontally rightward (outward) of the lid vs. the hingeward side. In performance, concern grands have to function well in a variety of settings - on the level with the audience, above them (e.g. on the stage in a multi-use hall where the audience is seated on a flat floor like a basketball court) or below them (e.g. on the stage in a concert hall where the audience on raked seating). So the musician will want the lid adjusted to best present the piano's sound to the audience (further diagrams in Olson suggest this effect is marked for at least the top half of the piano's range). So they'll set the lid angle to suit the performance space. It's not going to slip because it's not relying on flat friction to hold it up (there's a hole, or several). -- Finlay McWalter • Talk 15:34, 4 July 2010 (UTC)

I have been unable to find the "telescopic prop" on the website to which you refer above (beethovenpianos.com). However, I thank you for your comments on angle A. (See my clarifying paragraph above.) Using the data that I have collected so far, angle A is approximately 32 degrees, plus or minus 2 degrees, when the longest lid prop is used on a grand piano. (Some concert grands have as many as four different lid props!) As far as I know, there are only three lid prop sizes readily available commercially, 31", 30" and 22".Don don (talk) 19:01, 5 July 2010 (UTC)

The prop resists the moment of the lid tending to turn on its hinge. The value of the moment is MGR.cosØ where M=mass of lid, G = gravitational constant, R = distance from hinge to center of gravity of lid, Ø = angle of elevation of lid. The force P exerted by the prop on the lid may have two components: P.sine@ and P.cos@ where @ is the angle between the prop and the lid. P.cos@ is a force parallel to the underside of the lid and, if present, it will cause the top of the prop to slide against the lid unless this is prevented by a block, notch or hole. P.sine@ counteracts the lid turning moment which implies that MGR.cosØ = PS.sine@ where S = distance from hinge to prop contact. The situation @ = 90 degrees is interesting because P.cos(90 degrees) = 0 meaning the prop has no tendency to slide against the lid. Nothing here requires R = S so the lid angle Ø can be chosen by the prop length and the OP's hypothesis is not unreasonable. I add the diagram below at request of the OP.

File:Piano lid.png Cuddlyable3 (talk) 13:05, 6 July 2010 (UTC)

Yes angle=90 is a Saddle point Local extrema, but saddle points local extrema aren't really safe at all.77.86.10.42 (talk) 16:50, 4 July 2010 (UTC)

No. It's just a local extremum. Cuddlyable3 (talk) 13:31, 5 July 2010 (UTC)

thanks corrected 87.102.23.18 (talk) 13:38, 5 July 2010 (UTC)

Surely the most logical reason for choosing the angle they do is to best project the sound from the piano towards the audience as it reflects off of the heavy lid. I doubt it has anything whatever to do with the angle of the prop. SteveBaker (talk) 04:46, 5 July 2010 (UTC)

Respond to DonDon - one advantage of having an angle of 90degrees is that it minimises the compressive force in the support making it less likely to break. I still don't entirely agree that it is done for safety reasons since there are safer angles (ie internal angle less than 90) that work in combination with a block stopping the support slipping upwards. Nevertheless your point has a lot of truth in it.94.72.242.84 (talk) 19:31, 5 July 2010 (UTC)

That's possible - but piano lids just aren't that heavy and there is little or no penalty in having a totally over-engineered strut that could comfortably withstand ten times that weight. It's utterly trivial for the piano maker to design a strut that could place the lid at any angle he pleases. Hence the crux of the design here is most certainly not about saving 10 cents worth of wood in the strut and placing at the safest possible angle! It's about getting the lid to the optimum angle for sound reproduction...and all of this stuff about what is the least force and at what angle and slippage versus longitudinal force is totally irrelevant twaddle! SteveBaker (talk) 13:40, 6 July 2010 (UTC)

After the optimum angle of the lid is determined it remains to design the point on the lid where the strut is going to touch, and the angle between the lid and the strut. The tangential component of the force from the strut provides the torque to prevent the lid from closing, while radial and axial components of the force are of no use and can be chosen to be zero. That's why the strut should be perpendicular to the lid. Bo Jacoby (talk) 19:35, 6 July 2010 (UTC).

SI prefixes (redux)

The earlier question about 10²⁷ got me wondering... Among the SI prefixes, why was deca- given the two-letter prefix da? I realize it couldn't be d because that is for deci-, but why couldn't it have been D, just as we have m for milli- and M for mega-? Thank you kindly. — Michael J 16:41, 4 July 2010 (UTC)

Did you spot SI_prefixes#Proposed_changes - it seems they are aware of the inconsistences, but haven't done anything yet.77.86.10.42 (talk) 17:32, 4 July 2010 (UTC)

I saw that. I just wondered why it wasn't done in the first place. — Michael J 17:43, 4 July 2010 (UTC)

I see what you mean. The page Kilo- has the answer - the odd ones out hecto, deca, kilo (and centi, deci, milli) are original Metric system prefixes (year 1795) so they were already introduced and in use when the SI-system was started. I suppose it would have been confusing or impossible to get people to change from lower case prefixes which they had been using for over 100 years

Clearly the original metrix prefixes don't follow the 'same word root/ upper or lower case pattern' as do the later SI type.

In fact the metrix prefixes use a greek derived word for 10,100,1000 and a latin derived word for 0.1,0.01,0.001 .... Metric_system#Prefixes 77.86.10.42 (talk) 18:27, 4 July 2010 (UTC)

Adidas Jabulani

Please see http://commons.wikimedia.org/wiki/Category:Adidas_Jabulani - as far as I can tell all official 'jabulani' balls are the '8 sided' truncated tetrahedron design, yet there are other designs in commons

Can anyone confirm that this is a fake? Thanks.77.86.10.42 (talk) 16:46, 4 July 2010 (UTC)

I can't directly confirm, but I would note the presence of the word "Replique" in the name of the file (replica?), and also the fact that the match balls seem to be smooth, whereas there is distinct stitching on the one shown in the file. --TammyMoet (talk) 16:49, 4 July 2010 (UTC)

The Replique's are the cheaper version for sale to the mass market. See http ://hubpages.com/hub/Which-adidas-Jabulani-World-Cup-2010-official-soccer-ball-is-right-for-you (url split as for some reason that site is on the blacklist) for a truncated list of the different available Jabulani's. Nanonic (talk) 17:35, 4 July 2010 (UTC)

Thanks, the replicas seem to be more expensive than the ones with the new design.. odd

Resolved

Diopters

magnifying lens

magnifying and a diminishing lens

In corrective lenses, what does the unit diopter determine? Is this to do with how curved the lens is, how thick it is? Clover345 (talk) 18:39, 4 July 2010 (UTC)

Please read the article Dioptre. Cuddlyable3 (talk) 18:45, 4 July 2010 (UTC)

Thank you. I read that before I asked but its too scientific for me to understand. Clover345 (talk) 21:26, 4 July 2010 (UTC)

It's a measure of the strength of the lens - a higher value means a more powerful (either magnifying or diminishing lens)

A higher power lens is more curved, and being more curved means it is thicker. For a magnifying lens it's thicker in the middle, for a diminishing lens it's thicker at the edges.

So a higher dioptre lens will be thicker, and more curved than a lower dioptre lens.

I found some useful images at Eyeglass prescription. Note that a negative value is used to denote a diminishing lens.77.86.10.42 (talk) 21:35, 4 July 2010 (UTC)

If it has a positive diopter value you can use a lens as a Burning glass, as some boys discover to the discomfort of a few ants. Cuddlyable3 (talk) 22:15, 4 July 2010 (UTC)

If the sun is bright enough, you can even use a lens to light a campfire (like in The Mysterious Island). 67.170.215.166 (talk) 23:30, 4 July 2010 (UTC)

Though note that in Lord of the Flies, William Golding got it completely wrong by describing the short-sighted Piggy's spectacles being used in this way, even though lenses to correct myopia (such as my own) are diverging, somewhat diminishing my respect for the author when I read the book as a child. I thought I also remembered him describing a crescent Moon rising at sunset, but have subsequently failed to find such a passage - anyone else recall this? 87.81.230.195 (talk) 23:59, 4 July 2010 (UTC)

I've read the Lord of the Flies in high school, but it was a long time ago and I don't remember the details; the one thing I do remember is the conflict between Ralph and Jack, and the gradual transformation of all those civilized schoolkids into savages. 67.170.215.166 (talk) 04:44, 5 July 2010 (UTC)

You (87.81) are right about the crescent moon mistake - I remember it being mentioned in one of Martin Gardner's books. I'll try to find a reference later. AndrewWTaylor (talk) 08:14, 5 July 2010 (UTC)

Reptiles' sense of time

NOTE: I added the above "null" sections so that everyone's links will still work. ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:59, 6 July 2010 (UTC)

Reptile's sense of time

Would a reptile, being cold-blooded, subjectively experience a cold day as passing very quickly, but a hot day as going on for much longer? 92.15.12.165 (talk) 19:24, 4 July 2010 (UTC)

There is no reason why it would. Cold-blooded does not mean it has cold blood; it only mean that it is poikilothermic. --Chemicalinterest (talk) 20:07, 4 July 2010 (UTC)

Do I really need to point out to you that "cold-blooded" is a common synonym for poikilothermic and that I and other people using that phrase do not mean that it literally has cold blood?

I'm wondering if, since its body would be hotter on a hot day, therefore its neurones should be faster, therefore its gets more thinking done within a constant time period compared with a cold day. Hence subjectively objective time seems to go more slowly for it on a hot day. 92.15.12.165 (talk) 21:49, 4 July 2010 (UTC)

Haha I thought you meant that since it had cold blood it would see the "good" cold days passing quickly and the "bad" hot days (that heat up his blood) passing very slowly.--Chemicalinterest (talk) 10:59, 6 July 2010 (UTC)

This is an interesting question. We don't fully understand how humans keep time neurally on a minute-to-minute basis, much less reptiles. We do have a good understanding of how circadian rhythms are implemented in the brain, and I believe there is evidence that the day-clocks in cold-blooded animals are temperature-compensated to some degree, but do run a bit slower when body temperature drops. In short, the answer is not known. (And I haven't even addressed whether "subjective" actually means anything for a reptile.) Looie496 (talk) 21:55, 4 July 2010 (UTC)

Right, the question as phrased is about reptiles' qualia, which in principle are not amenable to objective analysis. The only way to know them for sure is to be the reptile in question. --Trovatore (talk) 22:12, 4 July 2010 (UTC)

Yes, but sleep passes very quickly for people (?) and there must be some objective difference in brain activity between the two.. Surely someone must have done a study on 'nerve activity in hot and cold reptiles' for us to be able to draw some sort of conclusion about the 'level of awakeness' or 'quality of experience' in reptiles as they get colder, if not whether times passes quick for them.77.86.10.42 (talk) 22:26, 4 July 2010 (UTC)

Nerve activity certainly slows down as body temperature drops. But there is no guarantee that timekeeping has a simple relationship to neural activity. Circadian timekeeping, for example, doesn't depend on neural activity -- the timekeeping process is driven by gene transcription. As I said, we really don't know at this point what mechanism determines subjective time, even in humans. It is probably neural activity at some level, but there are many types of neural activity, with different temperature dependencies, some of them pretty shallow. Human time estimation is definitely affected by body temperature, but since we are warm-blooded it is possible that there is less evolutionary pressure to counteract this in mammals than in reptiles. Looie496 (talk) 22:52, 4 July 2010 (UTC)

Discussion moved to Wikipedia_talk:Reference_desk#Grammitical_edits_to_question_header_on_science_desk_causing_offence please discuss there, and do not edit the questioneer's section heading further. Thank you.

Is there a behavioral explanation?

Is there a behavioral explanation for why the toilet-trained cat in this video is putting toilet paper in the toilet bowl? Bus stop (talk) 20:03, 4 July 2010 (UTC)

Yes, the cat has probably seen the human "parents" using paper after doing shit. So it has recorded the scene and trained itself to do so though of course it cannot understand why they do this. For the cat it is a ritual that its "parents" (who are humans of course) but to cat they are just big cats who are leaders of the pack who provide the cat food and protection, and what they do must be followed as a sign of respect  Jon Ascton  (talk) 21:57, 4 July 2010 (UTC)

No. It is instinctive behaviour of a cat to scratch earth over its stools. Tame cats try to do this even when there is nothing to scratch, as when the one in the video scratches at the plastic seat. At some point it discovers that a paper roll gives endless scratching satisfaction, that's all. Training a cat this way does not involve the owner performing for the cat to imitate. Cuddlyable3 (talk) 22:08, 4 July 2010 (UTC)

No, I have seen dogs and cats trying to involve the acts they have seen humans doing, and trying to reach out for electric switchs etc for no reason  Jon Ascton  (talk) 23:04, 4 July 2010 (UTC)

I could understand cats and dogs hitting switches for things because there's an immediate "reward" in that some action occurs that they desired. I find it harder, nigh impossible, to believe that a cat would paw at toilet paper just because they had seen a human do it. There's no "reward" for the cat. If the paper is close enough, as in the video, they may paw at that. Notice also that the cat in the video paws all around the seat. It's not unusual for a cat to paw at the walls and other vertical surfaces around a litter box either. (WP:OR warning) Around one of our litter boxes, we have a piece of hard plastic because we found the cats were scratching at the wall and causing damage. They still scratch at the plastic sheet but now it doesn't damage the walls. PS That's a good sized deuce for a cat! Dismas|^(talk) 00:19, 5 July 2010 (UTC)

Maybe pet psychology is different in India than in the West (!) Perhaps the evolutionary relation between human and cat/dog followed a different route to evolve in India, you know due to we-are-spiritual-you-are-materialistic factor  Jon Ascton  (talk) 03:50, 5 July 2010 (UTC)

I very much doubt they do it out of respect. Animals do things to show respect to the alpha, but I've never heard of them doing things to show respect to their parents (unless the parent is the alpha, of course). They probably do it because they have learnt from their parents and assume their parents were doing it for a reason. Animals don't do detailed analyses of the reasons for the things they learn (humans often don't either), they just imitate it when they want the same result (in this case, to not need the toilet any more). It could just be the usual cat instinct to scratch after defecating, as Cuddyable says, though - they certainly will scratch and things other than dirt if there isn't dirt around for no reason other than instinct. --Tango (talk) 19:47, 5 July 2010 (UTC)

Strange beetle

I took a picture of a beetle and uploaded it to Flickr. Can anyone help me identify it?

Americanfreedom (talk) 22:03, 4 July 2010 (UTC)

Reminds me of a Cockchafer, (which is probably isn't) - maybe you could look at the family Scarabaeidae while you wait.. or http://commons.wikimedia.org/wiki/Category:Species_of_Scarabaeidae (there's only 287 to go through..)

I think you need to tell us where you are to aid the indentification, eg region of the country you're in, not your address :) 77.86.10.42 (talk) 22:07, 4 July 2010 (UTC)

And how big is it, _{and what is that thing in the background - a tray with a picture on or something?}77.86.10.42 (talk) 22:10, 4 July 2010 (UTC)

This is definitely not a cockchafer, look at the antennae. Kinda hard to tell from that angle, but my guess would be that this is a fairly large longhorn beetle, maybe Cerambyx sp. or Neocerambyx sp., family Cerambycidae. Some of those guys, although of course not the largest of the beetles in general, tend to get impressively large. Let me know where you took the picture and maybe I can narrow it down to the species. --Dr Dima (talk) 23:08, 4 July 2010 (UTC)

I can't help directly, but here is a site [7] that may help you now or in the future. Caesar's Daddy (talk) 07:27, 5 July 2010 (UTC)

I took a closer look at the longhorn beetle picture you've got, it may be Prionus sp.. --Dr Dima (talk) 08:04, 5 July 2010 (UTC)

Without giving away to much (My dad's worried about security) i live in Northern California.

Americanfreedom (talk) 03:02, 6 July 2010 (UTC)

The beetle is not a security threat :) . Northern California is precise enough, so no need to tell us more than that about your location. I will look up the species tomorrow when the library opens. It does not look like Prionus californicus (AFAIR, P. californicus has prominent spines on the sides of the pronotum, which I can't see in your specimen). Anyway, tomorrow I will probably have a better guess of what it is. All the best, --Dr Dima (talk) 05:40, 6 July 2010 (UTC)

This is probably Prionus lecontei. There are only two Prionus species found in California: P. californicus and P. lecontei. The former has 12 antennal segments, the latter has 13. Looks like your one has got 13 ... --Dr Dima (talk) 03:08, 7 July 2010 (UTC)

abbe or v number

I haven't been able to find a thing on this - I was wondering what would be a typical abbe number (or some other comparative figure at other wavelengths) (whatever measure of chromatic aberration is relevant) for a camera lens assembly - ie is it higher than 59 as per crown or CR39 glass. I was thinking about a mid priced lens rather than super expensive, or super cheap..77.86.10.42 (talk) 23:02, 4 July 2010 (UTC)

Mig-29

Does the Mig-29 require a starter cart to start its engines? Or does it have self-start capability? Thanks in advance! 67.170.215.166 (talk) 23:37, 4 July 2010 (UTC)

This article says it has a "GTDE-117 gas-turbine starter-APU". Moreover this article has a "RD-33 Engine Start" section which says that stored air (from engine bleed) is normally used to start the engines, with the APU or battery-only as fallbacks. I dread to think how quickly you'd flatten your battery starting (presumably just one) engine. -- Finlay McWalter • Talk 00:04, 5 July 2010 (UTC)

Don't know about this Mig thing, but think I heard very interersting thing about the starting-of-engine process of a large propellor (probably a pre WW2 model) aircraft. They put a 12 gauge shotgun cartridge and fire it to start the engine ! Is that true...? Jon Ascton  (talk) 03:20, 5 July 2010 (UTC)

Yep - they really exist. See Coffman engine starter. SteveBaker (talk) 04:37, 5 July 2010 (UTC)

I knew that the Canberra bomber used a cartridge starter; didn't know that piston-engine aircraft also used this system... 67.170.215.166 (talk) 04:48, 5 July 2010 (UTC)

Well, talking of novel uses of 12 gauge shotgun cartridge, is there any machine which exploits the rush of its blast to dig a hole in ground when you have no time for a shovel ? Jon Ascton  (talk) 05:56, 5 July 2010 (UTC)

That would not be very effective, a shovel would work much better. Now if you want to get at water underneath a layer of ice an inch or so thick, maybe you could have more luck with that. Googlemeister (talk) 13:12, 6 July 2010 (UTC)

Submarine aircraft carriers of Japan employed pre-heaters for aircraft engine oil so that minimum time would be spent while vulnerable on the surface for starting and launching floatplanes. Cuddlyable3 (talk) 13:18, 5 July 2010 (UTC)

July 5

Giant isopod-- survival outside of water

Can a giant isopod survive on land? If so, for how long? (I ask because I've seen photos of them on land and apparently alive, such as [this], unless it's fake). 68.123.238.146 (talk) 03:22, 5 July 2010 (UTC)

That photo is definitely a fake. The trilobites vanished in the Permian extinction about 250 million years ago. The ICanHazCheezburger.com site is all about faked photos of one kind or another. Modern isopods are things like wood lice - but there is the Giant isopod Bathynomus - that can grow to a half meter or more in length. They live in very deep ocean areas though - I don't think they'd do well out of water. SteveBaker (talk) 04:35, 5 July 2010 (UTC)

So I can buy realistic plastic Trilobites?

...Excellent... --mboverload@ 04:39, 5 July 2010 (UTC)

I don't think the OP is confused about trilobites. The animals in the picture are clearly giant isopods despite the caption, and the OP asks about the survival of giant isopods out of water. Rckrone (talk) 07:19, 6 July 2010 (UTC)

They're images of giant isopods (which are common enough) who've been exposed to snack foods and Photoshop. Acroterion _(talk) 20:03, 6 July 2010 (UTC)

Considering that our Giant isopod article has a couple of pictures of them out of water (apparently alive), I would imagine that they can live a short time out of water. They're related to crab and lobsters (being crustaceans), which are also known to be able to survive outside of water (in some cases, for extended periods of time). A giant isopod probably couldn't function very well out of water (being used to having water help support its weight), but they might be able to avoid the quick suffocation that most fish would experience. Buddy431 (talk) 23:46, 6 July 2010 (UTC)

On which general topic see Gérard de Nerval. 87.81.230.195 (talk) 09:17, 7 July 2010 (UTC)

Ancient Machines

This site seems to be deliberately laid about yarn. They mention a "University of Chandigarh" which does not exist. The guy who made this up knew that there is a town called Chandigarh in India allright so thought that must have uni too ! There may be other loopholes too that need to be exposed  Jon Ascton  (talk) 03:42, 5 July 2010 (UTC)

Sorry, don't understand your question. Laid about yarn? --mboverload@ 04:13, 5 July 2010 (UTC)

I think he's stringing you along:) Myles325a (talk) 08:10, 5 July 2010 (UTC)

(EC) My guess is that the university is a self-made non-accredited diploma mill, as pretty much no credible scholar would entertain the so-called ancient astronauts theories. Not even the champion of the hypothesis in Chariots of the Gods? has much of anything in the way of credentials. In other words, academia is immune to this nonsense, but the public isn't, especially when this kind of trash appears on National Geographic channel's Is It Real? see critique. Alas, pseudoscience runs strong still, but we can take comfort that cocaine/opium or worse brain tonics aren't sold every which-way in every CVS nowadays, as would be in the late 19th century. SamuelRiv (talk) 04:16, 5 July 2010 (UTC)

Excuse me but there is no need to begin to bad mouth a university or an individual because a 'new-ager' has mentioned it/them in one of their 'new-age' articles. Please do some research before jumping to conclusions, both SamuelRiv and JonAscton. Thank you.87.102.23.18 (talk) 15:28, 5 July 2010 (UTC)

Just to clarify the scholar was Dr. Ruth Reyna who obtained a PhD from University of Poona 1961 , [8] (also written books eg amazon link). The Chandigarh university appears to be Panjab University, and the documents are supposedly held there. I don't know where the documents come from or anything about their authenticity. 87.102.23.18 (talk) 15:23, 5 July 2010 (UTC)

I assume these are the documents [9] ?

Hypothetical Example I have a crackpot theory that the observable universe is explained by tiny silver men inside my eyeballs casting shadows on my retina, and by invisble golden mice jumping up and down on my head to simulate gravity.. In my article I reference Isaac Newton's work on optics and gravity .. Science is not advanced by attempting to discredit the work of Newton, or Cambridge University simply because I mentioned them in my blog..87.102.23.18 (talk) 15:44, 5 July 2010 (UTC)

I don't understand the question here. The "strange artefacts" site is obviously just another conspiracy theory site, not to be read when sober. anything else? Physchim62 (talk) 16:01, 5 July 2010 (UTC)

I think the aim is to show some flaw - I would guess that dating the documents would be one obvious route to disproving the 'ancient astronaut' theory. If the documents prove to be old then that would be a suprise too.87.102.23.18 (talk) 16:08, 5 July 2010 (UTC)

To IP87: fair enough, the fault is mine. Regarding the original question, I hope I answered that with the links in my first response which discuss the whole history of the ancient astronauts kookery. Regarding the "documents" linked, they are actually modern drawings of what are assumed to be descriptions of flying machines in one of the Vedas, if I recall from a badastronomy.com debunking. It's like if I were to draw a picture of Noah's Ark, except I assume beforehand that it's actually a spaceship. SamuelRiv (talk) 16:12, 5 July 2010 (UTC)

Additionally there's no shortage of flying machines/people/creatures in many other ancient texts (along with gold growing rings, cloaks of invisibility etc etc) . Many people require more than just some text describing space travel to make them believe that it's literally true. The usual requirement for a historical document to be taken seriously as containing elements of fact is supporting evidence, usually archaeological remains, or secondary independant documents corroborating (eg such as matches in historical events/people in the bible and in eqyptian historical sources). 87.102.23.18 (talk) 16:47, 5 July 2010 (UTC)

Could you clone a human female from male cells?

As a male cell contains both an X (female) chromosome and a Y (male) chromosome, would it be possible to pair up two of the X chromosomes and create a human female from the male's genome? And if a boy was cloned (XY) as well as a girl (XX), could the two of them theoretically breed and start a population that derives from a single parent? I am supposing that human females can't do this because they do not have a Y chromosome. We are used to thinking that only females can create life, but the whole cloning idea is reminiscent of the Genesis story in which Eve is created from Adam's body, and not vice versa.Myles325a (talk) 08:28, 5 July 2010 (UTC)

I am not used to thinking that only females can create life. Cuddlyable3 (talk) 13:11, 5 July 2010 (UTC)

I suppose it might be possible, in theory, to remove the Y chromosome and duplicate the X chromosome to create a female "clone" from a male cell. But that cell now has two identical X chromosomes, so any recessive genes on the X chromosome have been duplicated, which will reduce the chances of the "clone" developing normally. Similarly, mating clones will result in a population with very low genetic diversity, which leads to low fertility rates. Gandalf61 (talk) 13:27, 5 July 2010 (UTC)

Was it Asimov who sang, "Oh give me a clone, of my own flesh and bone, with its Y chromosome changed to X. And when it is born, my own little clone, will be of the opposite sex."? -- 58.147.52.176 (talk) 12:17, 6 July 2010 (UTC)

Another science fiction reference is that of Lapis Lazuli and Lorelei Lee, daughters by this process of Lazarus Long in Heinlein's Time Enough for Love. -- 58.147.52.176 (talk) 23:40, 6 July 2010 (UTC)

The uniparental disomy article may be of interest, although it does not specifically address this issue. -- 58.147.52.176 (talk) 23:40, 6 July 2010 (UTC)

Strange hydrostatic aroud Australia

Hello dear thinkers. Please excuse my uneasy English, I'm French. For those who prefer the French version, here it is [10].

On this page Australie : nouvel état des lieux du réchauffement climatique they are explainations concerning the rise of water of the oceans linked to the global warming. My question is about a single strange sentence: It's written that "This rise of water is not uniform (all aroud Australia ) : on the south, the sea has risen at a speed of 3 mm per year during the recent years, but has risen at a speed of (from 7 to 10) mm per year on the north coast."

Question : How can the sea rise at a different speed on 2 shores opposite the same island ? (even if this island is a near-continent). With the difference between the 2 speeds (7-3)=4 mmm per year for 10 years long it makes 4 cm (more than 1 inch).

When I asked this question on the Reference desk in French for the 1rst time some months ago I got answers refering to earth gravity changes, but I think it's not conclusive. I also got answers sending me to Geoid but I also think that this geoid doesn't change enough to explain this strange difference between 2 "sea rise speeds".

Sorry for being so long asking my question, thank you very much for your "cogitations". Joël DESHAIES- Rheims-France---90.18.59.162 (talk) 14:07, 5 July 2010 (UTC)

Surprising as it may sound to you, the answers you were given are the correct ones. Dauto (talk) 14:38, 5 July 2010 (UTC)

The mistake you seem to be making is to look at the issue as a problem in hydrostatics, when the oceans are a dynamic system with continual energy input from the Sun. The height of a column of water depends on other factors apart from gravity, for example the temperature and the salinity. Just to give one well known example, the mean sea level (MSL) of the Pacific Ocean at that end of the Panama Canal is 20 cm (8 inches) higher than the MSL of the Atlantic Ocean just 77 km (48 miles) away. Physchim62 (talk) 15:42, 5 July 2010 (UTC)

Yes, there are 'anomalies' in sea level height, but how to explain the anomaly in the rate of sea level rise ?? 87.102.23.18 (talk) 16:28, 5 July 2010 (UTC)

Well, I think the question has a point. It is possible to have different sea level changes at different points, but it clearly isn't possible for the differences to build up continuously at a steady rate for very long. And the explanation wouldn't be in terms of earth gravity changes, but more likely in terms of changes in wind patterns and ocean currents, which push the water from place to place. As Psychim62 pointed out, there is a difference of 20 cm between the Atlantic and Pacific sea levels at Panama, caused by winds and currents. Looie496 (talk) 16:31, 5 July 2010 (UTC)

Perhaps the answer is related to average changes in wind/current direction then , happening at the same time as (and probably related to) the overal sea rise/warming effect. I don't know how to search for such data though.87.102.23.18 (talk) 16:40, 5 July 2010 (UTC)

The Australian tectonic plate[11] moves North (causing seismic activity Java - Indonesia). Might this induce a N-S tilt in Australia? Cuddlyable3 (talk) 18:44, 5 July 2010 (UTC)

Hello, I'm the French OP. Indeed, my title is wrong using "hydrostatic", we have clearly to think in terms of hydrodynamic. As some of you pointed, the hard thing is not the difference of "altitudes" but the difference of "speeds" in rising. After reading your explainations and the French ones hear [12] I'm still at the same point : or I don't understand how it can be possible, or (and it's my opinion now) there's a mistake in the data, according to me it just scientificly impossible. In anycase, I thank you all for helping me understanding sciences. Rheims - France ---90.18.59.162 (talk) 13:28, 6 July 2010 (UTC)

The important part is "in recent years". Obviously the difference can't continue forever, but by your own calculations, it can continue for 10 years without doing anything impossible. Looie496 (talk) 20:11, 6 July 2010 (UTC)

Difference between mud and loam

What is the difference between mud and loam, please? I am interested mainly in their use as a construction material.

Wikipedia says that loam consists of sand, silt and clay and that mud consists of soil, silt and clay, but can also contain sand. Thus, to me not being a native speaker both terms seem to have the same meaning. --146.107.3.4 (talk) 14:11, 5 July 2010 (UTC)

In British English, "loam" is used to describe a type of soil (a type that often becomes "muddy" in the British climate): "mud" is a more general term. Physchim62 (talk) 14:31, 5 July 2010 (UTC)

(ec)In agriculture Mud generally refers to ground matter/earth/soil that is clay rich - (ie not really peat, topsoil, humus though they will be described as mud when very wet). it is generally clay like in nature, but may be more liquid. Loam is generally more particulate, and so has better drainage.. though when wet it can be described as mud. It is not as slicky as clay.

In argricultural terms they are different. In building terms they are less distinct see http://www.thefreedictionary.com/loam : "A mixture of moist clay and sand" which is very similar to mud as used in Mudbrick.87.102.23.18 (talk) 14:38, 5 July 2010 (UTC)

You might find this useful http://www.tbe-euro.com/downloads/SustainableBuildingConference-Austria1.pdf : in construction terms I haven't found an example where the two can't be used fairly interchangably.87.102.23.18 (talk) 14:42, 5 July 2010 (UTC)

More Were you thinking of mudbricks and walls or more in terms of groundwork/subgrades? In terms of foundations etc this book makes some useful distinctions [13] 87.102.23.18 (talk) 15:04, 5 July 2010 (UTC)

The difference is a matter of water content mainly. Mud always means soil that is saturated with water enough to be sticky and squishy. If it dries out, it isn't mud any more. Loam is simply a type of soil with no particular implication about water content. If you saturate loam with water, it turns to mud. Looie496 (talk) 16:20, 5 July 2010 (UTC)

Is it Gun cotton?

This is what we found inside, besides shot itself, when we open a of 12 gauge shotgun cartridge in India. Is it so in the West ? I mean what exactly you find in yours in real life (as opposed to Wikipedia article)? What exactly is the stuff I have shown ? Is it gun cotton, or cordite what ever ? (the scale in centimeter) Jon Ascton  (talk) 17:24, 5 July 2010 (UTC)

I think it is either cordite, or, more likely, Smokeless powder. Guncotton isn't black. --The High Fin Sperm Whale 17:36, 5 July 2010 (UTC)

No, it ain't black either (only seems in pic).It is in fact dark green  Jon Ascton  (talk) 17:42, 5 July 2010 (UTC)

It's not pure gun-cotton nitrocellulose which is white like cotton. Nor cordite (image in article). It's almost certainly a form of smokeless powder. You might find this useful [14] - the powder is similar but slightly lighter. If the cartidge was more full than those in the links with powder then they could be using real gunpowder which is less powerful.87.102.23.18 (talk) 17:53, 5 July 2010 (UTC)

You might find the cutaway shells here [15] [16] interesting to look at.87.102.23.18 (talk) 18:05, 5 July 2010 (UTC)

(edit conflict)In that case, I'd guess smokeless gunpowder (smokeless gunpowder has so many different formulas and ingredients, probably one of them is dark green, or it could even be coloured like that on purpose). --The High Fin Sperm Whale 17:56, 5 July 2010 (UTC)

Yes, this is exactly what I found. The link seems to say that such shot is only meant for bird game, i.e. not in serious ammunition ? What about the standard shotgun ammo in the west ? And what about pistol and revolver etc. ?  Jon Ascton  (talk) 18:26, 5 July 2010 (UTC)

And, the other question, actually I already asked it in another post - talking of novel uses of 12 gauge shotgun cartridge, is there any machine which exploits the rush of its blast to dig a hole in ground when you have no time for a shovel ?

shotgun shell with slug

Erm - shotguns are mostly for bird game, (or firing at rats etc) - the small amount of powder is due to it's high power - shotgun cartridges were originally made for gunpowder - see Shotgun_shell#Construction_of_a_typical_shotshell " Modern smokeless powders are far more efficient than the original black powder used in shotgun shells, so very little space is actually taken by powder" - even the ones for bigger game have a lot of wadding eg see Shotgun slug (image right).

Were you thinking about military use? 94.72.242.84 (talk) 18:40, 5 July 2010 (UTC)

For rifle or pistol cartridges I can't do better than this [17] which has numerous cutaway diagrams of the 'shells'.94.72.242.84 (talk) 18:44, 5 July 2010 (UTC)

cordite in several colours

.--Stone (talk) 20:07, 5 July 2010 (UTC)

A Mythbusters episode tried to reproduce using a gun to cut a circular hole in a floor to escape from the bad guys. I recall that regular firearms were ineffective, and I imagine digging through dirt would be a lot harder. Besides, the dirt would be mostly pushed forward rather than to the side. Clarityfiend (talk) 02:06, 6 July 2010 (UTC)

Yeah - I think the most effective thing you could do to dig a hole with a shotgun shell would be to extract the explosive charge and push it as deep into the soil as you could and then set it off with a fuze. The resulting crater would be the best result you could possibly hope. The energy from the charge would mostly be directed upwards - towards pushing dirt out of the hole - although in soft soil, maybe half of the energy would go to compressing the soil beneath and making it harder to dig the next bit out. But the 'crater' made from a single shotgun shell would be unlikely to be more than a couple of handfuls of dirt...barely one shovel-full, I'd expect. There just isn't that much energy there. People do use small amounts of explosives to break up large rocks while excavating - but the solidity of a rock makes it much more amenable to breaking by explosives than soft, flexible, compressible soil. Furthermore, the goal is generally to break the rock into manageable chunks - not to physically remove it from the hole, which is energetically much more challenging. SteveBaker (talk) 13:30, 6 July 2010 (UTC)

Check out nuclear demolition. There were several plans to blast instant harbours and canals using atom bombs, and the Soviet Union is thought to have actually used bombs to stop gas well fires. Cod Lover Oil (talk) 16:34, 6 July 2010 (UTC)

Yes but there is a tad more energy involved with a nuclear bomb than a shotgun shell. Obviously you can do excavation with explosives - mines and quarries do exactly that all the time. But the puny amount of explosive in a shotgun shell just isn't enough to make much of a hole...even less so if you use that explosive to drive things into the ground rather than lifting stuff out of the hole. SteveBaker (talk) 19:46, 6 July 2010 (UTC)

Bunker buster#Missiles is kinda, sorta close to the concept, although it's job isn't to dig, either. The accompanying image does show some uplift, but I think almost all of it goes back down in roughly the same spot. Maybe Nuclear bunker buster is even closer. These would work better on asteroids, where the gravity is lower. -- JohnWBarber (talk) 00:01, 7 July 2010 (UTC)

Duty or pleasure?

I remember reading in my beginner level Biology books that living things should be able to reproduce. I used to think that all organisms had a sort of 'obligation' to reproduce and hence propagate their species. My current view is that almost all animals copulate due to attraction to the opposite sex and the resulting offspring are not on the parents minds during courtship. Is this natures way of ensuring that species propagate and do not go extinct by hiding duty behind pleasure? Thanks. --119.155.112.195 (talk) 20:08, 5 July 2010 (UTC)

Propagation of Life is the foremost project of evolution, nothing is more important, natural selection is a by-product of the process. Therefore the biological apparatus is designed in such a way that greatest pleasure (enjoyment of sex) can only be achieved by engaging in the process of intercourse. You see, it is a sort of bribe Evolution offers us so that can make the nature work ! By "we" I mean we all, that includes all other species. Interestingly Homo sapiens are the only organisms that can "cheat" mother nature i.e. family planning that is we can have sex for enjoyment alone, I think that Evolutionary Forces will now find some round about to overcome this danger ! Jon Ascton  (talk) 20:30, 5 July 2010 (UTC)

You'd love Idiocracy --mboverload@ 21:45, 5 July 2010 (UTC)

"Almost all animals" is probably too strong. There seems to be a neural "reward" system in all vertebrates, and I believe it is activated by sexual activity in all vertebrates that have been examined. There is also a reward system in insects, but I don't know if it is activated by sex. I don't think there is much information about other types of invertebrates such as molluscs and worms. In any case, there is no requirement for pleasure to be associated with reproduction -- it is theoretically possible for organisms to be hard-wired to give the necessary responses to the appropriate stimuli, without pleasure coming into the picture. Looie496 (talk) 21:30, 5 July 2010 (UTC)

Of course, that's what I meant in other words, in a scientists words (yours) it may sound a bit different, but believe me we are saying the same thing - "pleasure" is a human term," hard-wired to give the necessary responses to the appropriate stimuli" is what I meant if I were you. Perhaps I should not use the word "pleasure"... Jon Ascton  (talk)

I probably didn't explain clearly enough. To say that an animal does something for pleasure means that it is capable of anticipating the consequences of actions, estimating the pleasure expected from each, and choosing the one that is expected to yield the greatest pleasure. Those are sophisticated capabilities, probably beyond the reach of many species. The simplest alternative is to have direct, mechanical connections from certain sensory detectors to certain motor systems, so that detection of the presence of a possible mate leads automatically to performance of the response. I think it would be more reasonable to describe that sort of system as "duty" than as "pleasure" (although neither term is totally accurate). Looie496 (talk) 04:16, 6 July 2010 (UTC)

Both assign too much conscious agency. The essential point is that the evolutionary process leads to drives for reproduction—and if it didn't in some species, they would eventually die out. The mechanisms of those specific drives are not a conscious will to reproduce, even in human beings. Humans are sort of a special case because our understanding of reproductive biology and social consequences are probably much higher than any other animals, and we have rich cultures of reproduction. But our basic drive for sexual intercourse is not motivated by a conscious desire to have babies—it's the other way around. Our gonads say, "do stuff," and we do it, and end up with babies as a result. We see the connection. I'm not sure other animals do. "Pleasure" is probably a better term than "duty" for lots of animals, but "duty" is a bad term even for bugs, who do it not because they feel consciously required to, but because they are programmed to do it. --Mr.98 (talk) 13:03, 6 July 2010 (UTC)

I think you're looking at it backwards. I forget who originally said it - but the truth is that an animal is just a gene's way of making another gene.

From the perspective of a gene, it has to make an animal (or a plant or whatever) that has the highest probability of successfully preserving the gene into the future and spreading as many copies of it as possible. The whole business of animals and plants and mating and all of that stuff is just the way that the gene has found (by evolution) to be most effective in performing that task. The gene had to evolve to make sex pleasurable - or to provide some other 'drive' to do it because that's an effective strategy for ensuring that it'll get propagated.

But this strategy is complicated. If we (as the intermediaries 'caused' by the genes) don't feel an obligation to have kids - if we decide to use birth control through our fertile years...or to become homosexual or to simply abstain from sex altogether - then the gene has failed in it's task to provide that 'sex drive' and will (by definition) be eliminated from the next generation. The gene failed. Do we care that the gene failed? Maybe not. We only care if our genes gave us brains that made us care!

It goes beyond the actual act of creating a child too. If we were genetically 'wired' to have a baby and then abandon it - then those genes would also die out after just one generation. So having kids can be fun - it's definitely rewarding to see your son or daughter go to college and be a success. But the reason why that's a pleasure is because it was to the gene's advantage to have us continue to promote the continued existence of the next generation to the point where it too could propagate the next generation. Why are grandparents suddenly so fixated on their grandchildren? Guess what? Once their child has produced offspring, the most necessary thing for the continued propagation of those genes is to protect the grandchildren - and to hell with the intervening generation!

Hence, the process of evolution has molded the gene to the point where it produces human brains that have instinctual goals to produce offspring and to protect and educate them appropriately.

SteveBaker (talk) 13:20, 6 July 2010 (UTC)

I think your thinking of the Gene-centered view of evolution most notable expressed by Richard Dawkins in The Selfish Gene who I think was the one to coin the phrase ;a whatever is just a gene's way of making another gene'. Nil Einne (talk) 00:02, 7 July 2010 (UTC)

Aha! Thank-you! I was half-suspecting it was Dawkins, but I couldn't remember the precise wording of it so I couldn't search for it and I was side-tracked by wondering if it was Desmond Morris who first said it. Well, whatever. It's an interesting (and entirely valid) way to view the world. SteveBaker (talk) 05:24, 7 July 2010 (UTC)

'Hence, the process of evolution has molded the gene to the point where it produces human brains that have instinctual goals to produce offspring and to protect and educate them appropriately.'

Is it the genes that make a human parent want to send his child to school or the modern norm of good education = good job = respect that has changed the way people think about education? --119.155.11.103 (talk) 15:34, 6 July 2010 (UTC)

Yes, I think so. We have a general, instinctive (and therefore, genetic) drive to want the best for our children. That drive is filtered through intelligence, learned behavior, logic, economics, etc - but in the end, the reason we send our kids to school is that we are instinctually driven to have them perpetuate our genes. Of course, we're using our brains to actively consider how to turn that general 'drive' into actions: "Will sending my kid to school result in him/her getting a better job?"...and..."Will having a better job result in my kids not being hungry, having a roof over their heads?"...those are conscious thoughts - but deep down, it's probably genetics giving us the general drive to want our kids to do well. It's the same with sex, while we only have a rather general drive to reproduce, the precise details of whether to take the girl to the dance or to the movies and whether candy or flowers would make the best impression are coming from our intelligence because we haven't had time to evolve to believe that owning a red convertible with turbochargers and wide tyres will definitely get us laid. (Hmmm - I think I may have just disproven something! :-)

Without genetics, this is a crazily strong drive. Almost all parents would truthfully say that if it came down to it, they would happily lay down their lives to save their kids...and there are plenty of cases where parents have done exactly that. That's an amazing thing - there is really no other instinct that so completely overrides our goal of self-preservation. If not for the pressure of our genes trying to be perpetuated, why on earth would we do that?

The genes provide the "Why" and our intelligence provides the "How".

SteveBaker (talk) 19:34, 6 July 2010 (UTC)

Steve, I agree with all that you say, but I am also puzzled by the fact that some people - including myself and a number of my personal acquaintances (mostly male) - have never felt any desire whatever to have children (as opposed to just having recreational sex should the opportunity arise) and indeed have always actively disliked the idea. Do you think these are examples of individual 'nurture' overriding (genetic) 'nature', or do you know of a genetic explanation that might account for it. (NB: this is not merely a reflection of physical immaturity, as I'm in my 50s!) 87.81.230.195 (talk) 21:36, 6 July 2010 (UTC)

It's complicated (warning: I have an anthropology degree). In very general terms, evolutionary pressures have shaped our desires so as to best leave more of our DNA in the gene pool. It's important for us to eat, drink, pass waste, exercise, and have sex, so these things all make us feel good in various ways. By virtue of their biology, female mammals are much more likely to be responsible for raising the children than the males are - and women are no exception to that rule. But raising kids is no mean feat; it takes a lot of time and energy to carry a baby to term and it then takes even more time and energy to raise the kid enough for them to strike out on their own. That kind of drawn out task is hard to reward with a bribe (like an orgasm); it's rewarding, but nobody has kids so that twenty years later they can look back on the sweat and toil with satisfaction! What's needed instead is a brain that's hard-wired to want to raise kids; it's no accident that young girls play house and put diapers on their teddy bears; their brains are wired to make them instinctively want to do that. They therefore find that kind of play rewarding and are more likely to plunge into the sacrifices needed for the real thing. Men have different drives. We could, in theory, simply enjoy our orgasms with every woman in the area and not think at all about wanting to have a child. If you wanted to look at it cynically, I guess you could say it's because women are forced to care for the child anyway. However you equate it, men can get away with contributing less to the child-rearing, so their brains are not wired in the same way as to find such stuff enjoyable. I'm not saying it's fair, I'm just saying that's how the balance sheet works out. The first complication is that men who have children and stay with the family unit can form extremely strong bonds to their kids. Even men (such as myself) who had little desire for children before parenthood find that switch suddenly turned on. Which makes evolutionary sense as well, of course; children of parents who both care for them have a survival advantage against those that must make do with one parent. This is in contrast to the so-called Cinderella effect, which involves some grim aspects of children being raised by step-parents (see here as well). The most over-arching complication of course, is that people are all individuals and while it's easy to talk in generalizations, specifics will always be cloudy. When I first became a dad, I had no idea I would become a devoted parent, for example; there are an unfortunate number of parents (or either sex) who do not. And not every girl plays with dolls and not every woman has the strong basic urge to "listen to their biological clock". But in general they do; those stereotypes didn't just come out of the blue, they illustrate some of the ways natural selection has worked to shape our desires. Some of this comes from my understanding of parenthood as I've experienced it in, some of it comes from my degree, but it's not entirely OR. You may find the works of Helen Fisher and Desmond Morris to be of interest, for example. Our article on Sexual selection in human evolution also touches on some of this. Matt Deres (talk) 23:19, 6 July 2010 (UTC)

It's worth pointing out that everyone who decides not to have kids, and sticks with that decision, has essentially extinguished his particular genetics, while those who do want kids and are able to have them will pass their genetics along. And since there is no shortage of kids in the world, everybody's happy. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:36, 6 July 2010 (UTC)

Seen from the perspective of the gene, those people who choose not to (or fail to) have healthy, long-lived children have failed in the one single thing that the gene is "trying" to make them do (it's hard not to anthopomorphize!). And from the perspective of the genes of people who do have offspring, it's absolutely wonderful that some people choose not to do that because it leaves more potential mates and more resources for the offspring of those who do...which is a huge win! But I agree - why do we (as humans) have to satisfy our genes? If they fail to compel (or at least sufficiently encourage) us to do what they "need" then it's no big deal so long as we don't somehow suffer as a consequence. In times of old, not having children meant not having anyone to look after you in old age - which would have been a really serious problem for subsistance farmers and hunter-gatherers - but in modern society, we can avoid that issue fairly easily by saving money, buying into pension plans, creating systems of government with welfare, and so forth. In a very real sense, we've managed to cheat our genetics. Evolutionary theory says that sooner or later, the genes will catch on and adapt some clever new strategy...maybe in a few generations, some weird genetic flook will pop up and cause us to gradually suffer more and more horrible pain if we don't have a kid by age 30! Such a mutation would have significantly more breeding success than the lackadaisical gene that let's us get away without reproducing - and would therefore spread rapidly throughout the population, to the general detriment and misery of us humans. SteveBaker (talk) 05:13, 7 July 2010 (UTC)

Identifying a Howitzer

Greetings,

I am trying to identify a howitzer. The photographs were taken at the entrance of the fort of Ouvrage Saint-Gobain; I was told that it is a Skoda 105mm, but I have doubts.

• 
• 

Thank you very much in advance! Rama (talk) 21:23, 5 July 2010 (UTC)

Commons has a couple of pictures of a 105mm howitzer in the Skoda artillery category -- you can compare them. Looie496 (talk) 21:43, 5 July 2010 (UTC)

July 6

I don't get gravity

It's supposed to apply to everything right? Even light. And yet according to the article on helium it was light enough to fly away from the cloud that the early Earth condensed from. Is helium somehow exempt? —Preceding unsigned comment added by 68.69.73.195 (talk) 01:12, 6 July 2010 (UTC)

Helium floats, so it'll always be on top of the atmosphere. Maybe you know that the velocity of a atom of gas depends on its temperature? (See Kinetic theory if not.) The hotter it is the faster it moves. The thing is, this speed is only an average - in any given sample of gas, some are moving faster than this, and some slower. It can happen that the faster ones are moving fast enough to escape the gravity (like a rocket). Ariel. (talk) 01:16, 6 July 2010 (UTC)

I don't think this is actually a question of heat, per se. gravity decreases the farther you get away from the core of the earth. Helium will tend to rise above other gases (it is lighter and less dense), and in the upper reaches atmosphere it will be exposed to the solar wind, which can strip it away from earth's weakened gravity. --Ludwigs2 01:38, 6 July 2010 (UTC)

I'm not sure it could be that, according to the article at weightlessness gravity is only 6% less at 200 km up. Unless the solar winds are really strong anyway. 68.69.73.195 (talk) 01:40, 6 July 2010 (UTC)

No, Helium is not exempt. It's just light and flies away due to a combination of two factors mentioned above. 1.) High thermal speed. 2.) Solar wind. Same thing happens to hydrogen, by the way. Dauto (talk) 02:17, 6 July 2010 (UTC)

According to our atmospheric escape article, atmospheric loss due to solar wimd is not significant for the Earth, on account of our magnetic field. So the primary mechanism for the loss of hydrogen and helium from our atmosphere is thermal loss, also known as Jeans escape. Gandalf61 (talk) 09:57, 6 July 2010 (UTC)

The simple answer is that gravity is weak, weak, weak, as far as the forces go, but acts over a loooonnng distance. Consider that two magnets from your refrigerator can hold themselves together, suspended in the air, and doing so can contradict the gravitational pull of the ENTIRE planet. But their force diminishes quickly with distance, whereas gravity can have effects for hundreds of millions of miles distant. Gravity is good at doing things like keeping the planets aligned, and for roughly keeping heavy things like you and me on the face of the Earth (and even then, with just a little effort we can liberate ourselves from it, at least temporarily). But when you talk about things that are really light—like gases—other forces can be a lot stronger on a local scale, even if gravity rules at planetary and galactic scales. --Mr.98 (talk) 12:59, 6 July 2010 (UTC)

Gravity certainly applies to everything with mass. But lighter things float on denser things just as wood floats on water. Hydrogen and helium (being the least dense gasses) naturally float to the top of the earth's atmosphere. Hydrogen usually doesn't make it that far without running into some oxygen atoms and turning into water - but helium is a noble gas and doesn't react with anything, so it can reach the upper atmosphere fairly easily. The solar wind is a fairly significant force at those altitudes and is quite capable of defeating gravity and gradually blowing away the upper layers of the atmosphere out into space...which makes helium a fairly rare commodity.

The only helium we have is that which is created by radioactive processes and becomes locked underground in natural gas deposits. Ironically, although helium is one of the most abundant substances in the entire universe, it's quite rare down here on earth - and we're actually in danger of running out of the stuff sometime in the next 200 years. SteveBaker (talk) 13:02, 6 July 2010 (UTC)

We can also produce Helium using the Fusion_power#D-T_fuel_cycle. 157.193.175.207 (talk) 13:28, 6 July 2010 (UTC)

So we only need to make our helium stocks hold out for another 50 years? -- 58.147.52.199 (talk) 13:56, 7 July 2010 (UTC)

There's probably still some primordial helium left on Earth, but not much, as evidenced by sources that are rich in helium-3: see this review at p. 735 (p. 53 of the PDF file). Physchim62 (talk) 18:05, 6 July 2010 (UTC)

Yes - but both natural sources and fusion-reactor by-product are truly microscopic quantities compared to industrial needs. A 100MW fusion power plant, operating continuously, would take something like 300 years to produce a kilogram of helium (I hope I got my math right! But that sounds about right to me.) - all of the future fusion power in all of the world wouldn't make enough helium to keep the Goodyear blimp flying...let alone provide inert gasses for welding, birthday balloons for your kids, etc!

Primordial helium is certainly going to be rare because it leaks out into space. Naturally occurring helium is a by-product of natural breakdown of radioactive substances in the ground and ocean - which is indeed being slowly replenished. But think about that for a moment: If it takes just a few hundred years from first human exploitation of helium to when we are likely to run out of the stuff - then it would take a billion years for natural processes to replenish what we use in just a hundred years...and that assumes that the rate of radioactive decay is constant over all time - which it certainly isn't. It's safe to say that when we've used it all up...we're screwed. Other resources (such as copper) are becoming alarmingly scarce too - but at least pretty much every scrap of copper we've ever used is sitting in land-fill someplace waiting to become valuable enough to dig up and recycle. But when you 'consume' helium, it's gone. VERY gone! The nearest place to get more is the Sun...and there are considerable engineering difficulties associated with that! SteveBaker (talk) 19:22, 6 July 2010 (UTC)

The sun might be closer, but I bet it would be easier to harvest the helium from Jupiter, which also has plenty. Could we use tritium to produce light helium (He-3)? I mean tritium is expensive, but is it more expensive then strolling over to Jupiter to pick up some Helium? Googlemeister (talk) 19:43, 6 July 2010 (UTC)

Why does an acidic oxide reacts with water to form an acid?

And why does an acidic oxide reacts with a base to form a salt?

What is the principle behind them? Thank you. 114.247.10.148 (talk) 04:30, 6 July 2010 (UTC)

I assume you mean metal oxides and non-metal oxides. A non-metal oxide consists of a non-metal bonded via covalent bonds to oxygen. The addition of water generally created an oxyacid, which consists basically of hydroxide groups now being attached to the nonmetal. Since the covalent bond to the oxygen withdraws some of the electron density away from the H-O bond, it makes the H very easy to leave. The definition of an acid is something that loses H⁺ ions to solution. Thus, non-metal oxides produce acids. A metal oxide consists of metal ions and oxide (O^2-) ions held together by ionic bonds. When the O^2- ions come into contact with water, it tends to remove one of the hydrogen atoms from the water, producing two hydroxide (OH^1-) ions. The definition of a base is something that produces hydroxide ions in solution. Thus metal oxides produce bases Its basically the difference in reactivity between a covalently bonded oxygen atom, and the oxide ion. --Jayron32 05:07, 6 July 2010 (UTC)

The farther to the left on the periodic table you go, the more basic the oxides are. The farther to the right, the more acidic. Also, higher valence oxides such as chromium(VI) oxide tend to form acidic solution. Lower valence chromium(III) oxide is only very weakly acidic. --Chemicalinterest (talk) 10:51, 6 July 2010 (UTC)

Thank you. I see the reason why non-metal oxides produce acids, not something else. But I start to wonder why non-metal oxides react with water, rather than being nonreactive? Thanks. 114.247.10.133 (talk) 12:44, 6 July 2010 (UTC)

Not all non-metal oxides react with water: take carbon monoxide as an example of a neutral oxide. But that's really something which is in the "character" of each oxide. Physchim62 (talk) 12:55, 6 July 2010 (UTC)

Hmm... Maybe on some of them the bonds are strong between the nonmetal and oxygen so they wouldn't accept a hydrogen and as a result, wouldn't dissolve in water to form an acidic solution. Very few nonmetal oxides are unreactive toward water, though. --Chemicalinterest (talk) 13:10, 6 July 2010 (UTC)

It depends on what you mean by "react". Aldehydes such as formaldehyde for example, are hydrated by water (they form hemiacetals and acetals). It all comes down to whether your compound/element is an electrophile or a nucleophile. Lewis acids tend to be electron-withdrawing / electrophilic; Lewis bases, electron donating / nucleophilic. John Riemann Soong (talk) 16:04, 6 July 2010 (UTC)

You with your organic chemistry again... over my head --Chemicalinterest (talk) 16:31, 6 July 2010 (UTC)

Rifled slug?

"Shotgun slug" - rifled?

Copying the attached pic from a few threads above this one. Why would a shotgun slug be rifled? I didn't think shotguns were rifled. Comet Tuttle (talk) 05:56, 6 July 2010 (UTC)

There are rifled shotguns designed specifically for slugs. 68.69.73.195 (talk) 06:11, 6 July 2010 (UTC)

http://www.rogueturtle.com/articles/shotguns.php

Normal bullets are not rifled. The "rifled" slugs do not, to my understanding, cause significant spin. As mentioned above, modern slug guns often have rifled barrels, but are more often used with sabot slugs. The part of the article at Foster_slug#Foster_Slugs says that these fins on the slug were more about reducing friction than imparting spin. Friday (talk) 15:05, 6 July 2010 (UTC)

I'm no expert but might it be that the 'fins' cause spin due to aerodynamic forces AFTER the slug has left the barrel? That would make sense because you don't need fins to make a bullet spin when the bore is rifled and since so many shotguns are smoothbore, it would make perfect sense to put fins on a shotgun shell for the odd occasions where you want a more accurate shot and less scatter. SteveBaker (talk) 18:48, 6 July 2010 (UTC)

There is no scatter at all from a slug - it is a solid chunk of lead. Rmhermen (talk) 20:40, 6 July 2010 (UTC)

Statistical scatter perhaps (multiple shots?)

Actually the links above note that the centre of gravity of such a slug is close to the tip (like a shuttlecock) so there's (probably) not need for a spin to be imparted to prevent tumbling, since the drag does that. It also claims the fins are there to reduce friction in the barrel, and that there is no spin on the slug in flight. That's what it says, I'm just repeating it.77.86.6.186 (talk) 21:00, 6 July 2010 (UTC)

If the fins are just to reduce friction - surely you'd want them to be parallel to the direction of flight? SteveBaker (talk) 00:12, 7 July 2010 (UTC)

That would increase the edge length for potential abrasion though? from just the leading part of the fin (and to a lesser extent the shallowly angled sides) to multiple circular edges. (Assuming that edge abrasion is a larger contributor to gun fouling than side abrasion - since the sides are more solidly supported - 2pi steradians of support vs pi steradians of support). Spiraling the fins ensures even distribution of support around the circulat cross section of the barrel compared to hypothetical straight fins.

According to book mentioned here the fins impart a slight spin of 1 turn in 24feet - I'm fairly certain this is not effective cf spin distance mentioned in Rifling (though much mass is on the outside so the rotational inertia is greater than a similar solid bullet). There's also an video here [18] (might be a solid slug, not clear).. but it may not convince either way.

Ignoring all the above the reason to have non-circular fins is swaging - see Swage#Firearms_and_ammunition (not that link - doesn't mention gun chokes see [19] instead) - It would be great way to acchieve consistent jamming if the slug wasn't perfectly lined up. So longitudinal fins are needed, and non-twisted longitudinal fins are not the best solution for reasons explained above.

This gives rise to a bullet that looks like it's designed to rifle.. However if you think the description on the page Foster slug is wrong I'd take it up there. It's not difficult to find sources that say they are rifled to make them more stable if you look though; I'm suggesting it's just consequence of other design parameters.

It would be interesting if anyone can be bothered to calculate the stabilisation acchieved from the slow rate of spin that is imparted, and say whether or not it is actually significant.77.86.6.186 (talk) 00:39, 7 July 2010 (UTC)

I'm fairly certain the number one factor behind the design here is making a shell that goes through a Choke (firearms) with no problems.77.86.6.186 (talk) 01:55, 7 July 2010 (UTC)

It's possible to assemble a thousand opinions on this subject. Here's one

"Heavy external "rifling" was cast into these Foster type slugs, allegedly to allow the air they flew through to impart a slow spin that would help stabilize the slug. Like most something for nothing schemes, the rifling proved ineffective, but it did provide some space for some compression if the slug had to squeeze through a tight choke.."[20]

77.86.6.186 (talk) 02:01, 7 July 2010 (UTC)

Duh - just realised that's a Brenneke slug and not a Foster slug - the shuttlecock effect doesn't apply to the brenneke slug.77.86.6.186 (talk) 02:08, 7 July 2010 (UTC)

Seeing as it's a Brenneke slug I can link this:

"The Brenneke design with angular ribs is not to create spin, but to ensure problem-free choke passage" www.brenneke-munition.de

That's the company website, hopefully it can be taken as fact.77.86.6.186 (talk) 02:15, 7 July 2010 (UTC)

Hyrachyus

Hi, I wish to know where lived the Hyrachyus? Because from this two articles I can't find out, where lived this animal.

http://en.wikipedia.org/wiki/Hyrachyus

and

http://en.wikipedia.org/wiki/Rhinocerotidae#Evolution

One say Europe and the other North America. Please answer. —Preceding unsigned comment added by 89.123.165.191 (talk) 13:24, 6 July 2010 (UTC)

Here's a cite for Hyrachyus in North America: [21] (it's in there somewhere). There might also have been Hyrachyus in Europe, not sure. That 1883 report mentions Lophiodon, a European cousin. There's a document here: [22] which I think says "Lopliiodon is limited to Europe; Hyrachyus, Hyracotherium and Pliolophus to Europe and North America" ... but I'm not allowed to read it unless I want to pay thirty quid. I had to extract those words from google in two searches. I think it might be volume III of the same report, which is also available here [23], but I can't find those words in it. However, it does say "Hyrachyus is the American Lophiodon, the difference between them being but slight; both are found in France ; the former in the Lower Parisian, the latter in the Phosphorites." So that's pretty definite: the answer is both. (Not sure how that jives with Laurasia being already separated a million years previously?) 213.122.47.102 (talk) 14:28, 6 July 2010 (UTC)

Thanks anyway! —Preceding unsigned comment added by 92.86.247.63 (talk) 10:30, 7 July 2010 (UTC)

Hawaii

Would Hawaii's climate be considered tropical, or sub-tropical? Googlemeister (talk) 16:45, 6 July 2010 (UTC)

Probably tropical according to File:ClimateMap_World.PNG. --Chemicalinterest (talk) 17:51, 6 July 2010 (UTC)

The state as a whole is tropical, but the actual climate varies quite a bit. Indeterminate (talk) 20:25, 6 July 2010 (UTC)

Cooking Pizza

Not sure if this belongs here or the Misc. desk, but I think this is the best place to start. Buying a pizza from a supermarket and cooking it at home is drastically cheaper than buying one ready-cooked from a pizza takeaway, I don't know why, but it is. However, a pizza cooked at home is never as good as that from a shop - but why? I am sure that the method of cooking is the problem here. When I cook a pizza, the centre is lukewarm but the edges look they've been incinerated but pizzas cooked in a shop are crisp and even.

I know the pizza is slightly thinner towards the edges but we're talking less than a centimetre so I don't see why the difference should be that profound. I also follow the cooking instructions to the letter - put on the top shelf of an oven and cook for 12-14 minutes at 200C. After 12-14 minutes, the outside is just about right but the middle needs at least another 5 minutes (minimum) otherwise it is barely cooked at all. By this time, the edges are hard, dry and burnt. I don't think it's a problem with the oven as I have experienced this with other ovens. I know pizza shops have special ovens but that can't be that much of a factor.

Deep-pan pizzas are just about edible after this cooking ordeal (if you leave the crusts) but thin crust ones? Forget it, the crust is so hard it's virtually inedible.

Is there any way I can cook a pizza more evenly? I would assume I am doing something wrong, due to a. the fact that pizzas for home cooking are available and b. the picture on the box always looks perfectly cooked. Would cooking it more slowly on a lower heat help? Covering up the edges in foil? Anything else? —Preceding unsigned comment added by 94.196.158.83 (talk) 18:01, 6 July 2010 (UTC)

First, frozen pizzas have a number of constraints that make them less than edible (they are usually pre-baked and frozen, and they need to be very cheap. They are also much thicker than real pizza). Fresh pizza is made from unfrozen dough. Secondly, real pizza ovens are a lot hotter than 200°C, and the pizza is placed on a hot stone, cooking from the top and the bottom very quickly. Oh, and thirdly: Do you thaw the pizza before cooking? --Stephan Schulz (talk) 18:19, 6 July 2010 (UTC)

(EC) I must admit, I do not usually have these problems when cooking ready made pizzas except the crusts on the cheap frozen thin crust pizzas does become quite hard, but out of curiosity, are we talking about the frozen ones, or ones that you can find that are refrigerated, which I find are usually higher quality and more expensive? One other possibility, are you at a high elevation (1500+ m)? Googlemeister (talk) 18:22, 6 July 2010 (UTC)

Thanks for the reply. This seems to happen with the shrink-wrapped non-frozen pizzas from the supermarket (which they apparently make in-store from 'fresh' ingredients and yes, they are more expensive) as well as the boxed and shrink-wrapped frozen ones, although the frozen ones seem to cook slightly more evenly than the non-frozen ones for some reason (I usually cook these from frozen as that is usually what the instructions say). I am not at a high elevation, barely above sea level. I thought the frozen ones cooking better might have something to do with humidity (the burnt edges are very dry). 94.196.158.83 (talk) 18:27, 6 July 2010 (UTC)

Pizza stone says you can buy a small one, or use a bunch of unglazed tiles. 213.122.64.116 (talk) 18:33, 6 July 2010 (UTC)

I believe it has more to do with the mode of cooking than with the source of the pizza dough or whether it's frozen, thawed or fresh. My wife (who is an excellent cook) has made pizza completely from scratch and we've had the same disappointing problem of incinerated edges and half-cooked middles. Making 'thin-crust' style pizza seems a little easier than regular or 'deep-dish' styles - but still, it's tough to get it right 100% of the time even with thin-crust.

What seems to help is a proper pizza cooking tray. Ours is a round metal plate with a slightly concave surface and a few hundred half centimeter holes punched in the center third or so. The idea is (presumably) to let the radiant heat from the oven pass through the holes and thereby cook the center of the pizza a little faster than the edges. This seems to be partially succesful...but still, I have to say that it's hard to beat delivery pizza on crispness and crust texture (although it's entirely tivial to do vastly better on taste with fresh ingredients, proper spices and not spending an hour being kept lukewarm in a delivery pouch).

It's frustrating because a combination of the delivery crust style with homemade toppings would be a truly awesome thing to behold.

...and please stop asking these questions on days when I've just missed lunch!

SteveBaker (talk) 18:43, 6 July 2010 (UTC)

Surely the little holes are to allow steam to escape? --Sean 21:19, 6 July 2010 (UTC)

(1) It's a frozen pizza. It is contrary to all the Laws of Science that it will ever (a) taste as good as an average restaurant-made pizza, or (b) look as good as the picture on the box (that would also violate the Laws of Advertising); or (c) cook as good as the instructions lead you to expect -- IIRC, this is the Law of Frozen Pizza Imperfectability (where are those Wikipedia articles when you actually need them?); (2) The old "Red Baron" frozen pizzas had a metalic-like inside cover that customers were told to fold up and under the pizza before putting it in the oven (or maybe the microwave -- maybe it wasn't quite metalic, but it looked like it was -- it's been a long time since I reformed from my evil ways; as I recall, it cooked little mini-pizzas well enough); (3) the hallowed Frank Pepe's Pizzeria, when a new generation of the family ownership decided to do the unthinkable and expand to different locations than the one on Woooster Street in New Haven, Connecticut, arranged to have each new oven built to match the original, ancient oven at Pepe's on the unscientific understanding that you don't mess with success, so that's anecdotal, unscientific evidence that the oven must have something to do with it although this is pretty mystical for a science reference desk, I admit (see Frank Pepe Pizzeria Napoletana#The oven) (3) You do realize, I hope, that there's a special circle in hell reserved for frozen-pizza buyers and sellers? It's in Dante's Inferno somewhere, and I'm told modern translations just don't do it justice. Something about getting shoved inside a brick pizza oven. Or that might have been in The Godfather; (4) try a cookie sheet under the pizza. I encase it with aluminum foil first, shiny side up (at least it's easier to clean). Still doesn't work quite right. Usually, we buy it from a pizzeria; (5) Please realize that scientists all over the world have been working on this problem for decades, and while they haven't solved it yet, side benefits have included the microwave oven, the Tang drink and those diapers that look like blue jeans. Rest assured that while science sometimes progresses slowly (cure for the common cold, anyone?) it does progress. -- JohnWBarber (talk) 18:51, 6 July 2010 (UTC)

By the way, pizza ovens are sold to homeowners. -- JohnWBarber (talk) 18:57, 6 July 2010 (UTC)

The difference is basically in the oven: you can't cook proper pizzas in a domestic oven, and the one's you buy in shops are a compromise that lets you get something edible (and lets remember that pizza, like sex, is often disappointing but only very rarely is it truly bad) The official pizza napoletana is cooked at 485 °C for 60–90 seconds... Physchim62 (talk) 19:04, 6 July 2010 (UTC)

I've never tried his pizza, but Alton Brown has never let me down on anything else. The transcript and recipe from the Good Eats pizza show is here. He uses an unglazed quarry tile to bake on. There are a number of (to me) surprising bits (such as allowing a very long cooled rise for the dough), but he bases his recipes and instructions on food science and tests them empirically before airing them, so I would tend to believe him. As far as fresh/frozen, the problem is likely with the dough being 1) of poor quality and 2) blind baked for too long, making it difficult for the end user to balance the sogginess with the, um, burniness. Matt Deres (talk) 19:38, 6 July 2010 (UTC)

A very long cooled rise is generally the default for people who want to maximise the tastiness of their yeast product. In my experience, you can make good pizza with a short rising too: the only real difference is in the 'grain' taste of the crust, but I'm guessing you eat bread that hasn't been slow-risen, so I don't think you'll notice anything bad. A cool, slow rise also means you can start the dough and leave it overnight or while you go to work: if you don't want to do that, you can rise it in a cool (< 50 C, in a bowl covered with a towel, oven turned off) oven in less than an hour. 86.164.57.20 (talk) 21:00, 6 July 2010 (UTC)

A perpetual motion machine

Gravity is a force, and as such it does work. If I drop a stone from my window, the Earth's gravity will exert a force of roughly 10 newtons per second on it to bring it down.

But then, is gravity really ethernal, or does it eventually run out?

• If gravity lasts forever, that means any piece of rock can exert an infinite amount of force, and thus an infinite amount of energy. Wouldn't it be a perpetual motion machine then?
• If gravity does not last forever, when is it calculated to "run out"?

I'm just having a hard time reconciling the idea of an ethernal gravity with the second law of thermodynamics, and I'd like someone to explain this to me. 88.1.139.43 (talk) 19:18, 6 July 2010 (UTC)

It is not perpetual motion though. Eventually, the rock will stop when it hits something and you would need to exert energy to lift the rock back up to drop it again. Basically, the rock has already stored energy and it is using it up while falling, but it will not fall forever. Googlemeister (talk) 19:31, 6 July 2010 (UTC)

Phrased alternately, this is the distinction in physics between force and energy (or work). You say "...an infinite amount of force, and thus an infinite amount of energy", but this incorrectly equates "force" and "energy". Rather, energy is (roughly) force multiplied by distance. If you want to turn that 10N of force into 10J of energy, you have to move the rock one meter (as the joule is a unit of Newton-meters).

Gravity is a force, but a force in and of itself doesn't do anything. A PM machine, on the other hand, has to be able to do infinite work, that is, the transfer of energy. As Googlemeister has already noted, gravity can cause a rock to fall -- once. Then it reaches a point of equilibrium (the surface of the Earth, the center of the black hole, the center of mass of the combined rock-attractor system, whatever) and it ceases to do any more work until you spend external energy to move it. Gravitational force is still there once the rock stops doing work; it doesn't need to "run out" for useful work to end. — Lomn 19:49, 6 July 2010 (UTC)

Basically agreeing with the other responses but saying it a bit differently: gravity only does work on things that move downward. It can't do infinite work because there isn't an infinite amount of down. If you move the object back up to where it started, gravity does negative work while you're lifting it, so there is no accumulation. Looie496 (talk) 20:15, 6 July 2010 (UTC)

A stone lying on a window ledge has potential energy. While falling the potential energy of the stone is converted to kinetic energy. When the stone has fallen as far as it can go, it hits something and its kinetic energy suddenly converts into sound and heat. Gravity is still there but a stone with no place to fall has no energy at all. I think you mean "eternal". Cuddlyable3 (talk) 21:16, 6 July 2010 (UTC)

Actually, your stone can "fall" forever, achieving a stable orbit around a planet for example (we omit interplanetary dust and solar wind for simplicity). However, this will still not make it a perpetual motion machine: if you try to extract energy out of it, you will slow it down / change its orbit to a lower one.--131.188.3.21 (talk) 23:41, 6 July 2010 (UTC)

Moreover, no orbit is truly "stable", not in terms of thermodynamics. — Lomn 02:24, 7 July 2010 (UTC)

I'm sorry, we don't stock actual perpetual motion machines, but we do have a fine selection of these other models that might serve your purpose: Gravity can give you practically a perpetual motion machine in the form of a hydroelectric (or mill) dam, as long as the water doesn't run out (Mother Nature brings it uphill for you). Harnessing tidal forces might be more "perpetual". A geothermal pump is practically a perpetual heating/cooling machine (although you have to actually supply some motion for the water or liquid to circulate -- hook it up to your dam), and a solar panel (or other device dependent on energy from the sun) is practically a perpetual electricity-making or heating machine -- in space, anyay -- until you get an eclipse or the sun dies. (I think some utilities use solar and wind power to pump water back up into the reservoir of a hydroelectric dam and there's an idea out there about pumping air into a cave or mine and releasing it later, saving energy until the sun doesn't shine and the wind dies down, a more tenuated "perpetual motion" machine.) Of course, you're simply taking energy from some massive forces of nature (we always are) and these will all die out eventually, but eventually we'll all be dead. But you didn't actually want a machine? You just wanted an explanation of a concept? Sorry, we don't stock those either, but this week our models are all on sale ... -- JohnWBarber (talk) 23:42, 6 July 2010 (UTC)

All of your suggestions only work for very small values of "perpetual", of course! Unless you manage to answer The Last Question. --Stephan Schulz (talk) 08:31, 7 July 2010 (UTC)

The problem here is a confusion between a 'force' and 'energy'. This confusion is so common - and the question comes up here so frequently, that I've written an answer on my personal Wiki HERE so I don't have to keep repeating my answer every couple of weeks! I recommend you go and read that. SteveBaker (talk) 04:58, 7 July 2010 (UTC)

Stellar parallax and the movement of the sun

Stellar parallax is a way to measure distances to stars, comparing their position in the sky when earth is at opposite points in its orbit. This gives a distance of 2 AU. But the solar system is moving at 220 km/s around the galaxy, which is about 46 AU per year.[24] How does this square with the stellar parallax method? Are all nearby stars approximately at rest relative to our sun? EverGreg (talk) 19:38, 6 July 2010 (UTC)

For the most part yes. The fastest star near us, Kapteyn's Star, moves on the order of about 0.001 c relative to the sun and is 13 ly distant. Googlemeister (talk) 19:49, 6 July 2010 (UTC)

Put in perspective, if the solar system is moving with respect to a target star at 46 AU per year, that's less than .000001 c. However, I'm not sure that's really the point here. In the time it's taken the Earth to complete half an orbit (to allow the 2 AU parallax), the solar system has moved about 23 AU, which is to say, 0.00036 light years. On the other hand, we're using data from the Hipparcos satellite to measure stellar distances of up to 1600 light years, a factor difference of about five million. So the motion of the solar system is messing with parallax measurements by thousandths of a percent, which is probably well within the margin of error. Even Proxima Centauri, the closest star, would have its distance mis-measured by only one part in 12000 if we didn't account for the motion of the solar system. So it's not whether nearby stars are at rest relative to us as much as a few dozen AU being insignificant when you're talking light years. — Lomn 19:57, 6 July 2010 (UTC)

Note that it's pretty easy to check on this. All you have to do is repeat the measurement when the Earth comes back to its starting point. If the distant star is moving enough to cause a problem, you will get a substantially different result on the second measurement. Looie496 (talk) 21:07, 6 July 2010 (UTC)

Are all nearby stars approximately at rest relative to our sun? Yes, in fact this 220 km/s frame is called the local standard of rest. -- Coneslayer (talk) 11:25, 7 July 2010 (UTC)

Hmmm - interesting - so I have a followup question: At what time of year does the earth move in the same approximate direction as the sun's motion around the galactic center? SteveBaker (talk) 23:31, 6 July 2010 (UTC)

December. Physchim62 (talk) 23:51, 6 July 2010 (UTC)

So right now, we're moving slower than average relative to the galactic core? Cool! SteveBaker (talk) 04:40, 7 July 2010 (UTC)

Bass energy sucking

Is it possible for a bass drum to suck my bass energy from my bass cab and make it sound bad if they are too close? —Preceding unsigned comment added by 88.104.83.202 (talk) 20:29, 6 July 2010 (UTC)

It would definitely resonate or buzz when bass notes were played, and that sound would likely degrade the quality of sound you heard in the room. So it could easily make the bass cabinet sound different. I just knew that some good would come out of having a bass drum occupying space in the living room. I tried putting a large bass drum in front of a speaker and playing classical or rock music with the bass turned up. I could feel the drumhead vibrating, but couldn't detect much difference in the sound. "Bad" is in the ear of the listener, since musicians often want distortion in a guitar amplifier. Edison (talk) 20:41, 6 July 2010 (UTC)

Yeah, I'd expect that a bass drum could attenuate the sound energy, but not that it would significantly alter the characteristics of that energy. Beyond that, there's no "energy sucking". You might get lousy sound quality, though, if the bass guitar is playing a note that's near, but not at, the note played by the drum -- but that's just being out of tune, not anything specific to these two instruments. Similarly, the room might have lousy acoustics for that frequency range, but again, nothing particular to the instruments themselves. — Lomn 21:22, 6 July 2010 (UTC)

But doesn't a bass drum have a fairly low Q factor, so that it can absorb energy over a reasonably wide frequency range and thus distort the bass from my bass cab? —Preceding unsigned comment added by 88.104.83.202 (talk) 21:52, 6 July 2010 (UTC)

If the bass drum absorbed acoustic energy, should it convert the energy to heat, and either radiate heat or have its temperature rise? Edison (talk) 04:47, 7 July 2010 (UTC)

Sprite freezing

Have a question guys. I just took a sprite out of my mini fridge (pretty much a freezer) and when I opened it the liquid froze from top to bottom.

What is this magic?

I thought it might have something to do with the pressure being released, but doesn't high pressure raised a liquid's freezing point. 74.15.137.192 (talk) 22:04, 6 July 2010 (UTC)

Pressure tends to lower the freezing point of a liquid. However, what I think is more likely is that the Sprite was supercooled. In fact, our article notes that Sprite was, at one time, specifically sold in the UK in a supercooled state such that it turned to slush when opened. Supercooled liquids tend to freeze rapidly when they're disturbed, which is why opening the can/bottle caused the freezing -- both the physical shock of opening the can and the pressure change associated with it. A similar, and significantly more dangerous, phenomenon is superheating, which is relatively easy to accomplish with water in a microwave and can lead to severe scalding. As such, it's recommended that you not try to boil water in a microwave unless you've added some rough surface (such as a wooden spoon) to the container so that superheating won't occur. — Lomn 22:18, 6 July 2010 (UTC)

Pressure tends to raise the melting points of liquids – except for the essential exception that is water. Otherwise, I agree completely, the phenomenon seems to be supercooling. As you release the pressure, you get tiny bubbles of carbon dioxide forming, and these act as nuclei for the surrounding liquid to freeze around. Physchim62 (talk) 22:47, 6 July 2010 (UTC)

You're absolutely correct; I should have said "water-based liquid" (as I expect Sprite to be sufficiently water-like for water's general pressure-temperature relationship to hold). — Lomn 02:21, 7 July 2010 (UTC)

Is it possible that the unopened Sprite doesn't freeze because of the dissolved carbon dioxide, and the sudden loss of CO2 upon opening changes the freezing point? I would imagine that this has a small effect, given that there is a ton of sugar dissolved, and still considerable amounts of CO2 remaining in solution, but it may contribute. What if there was a tiny imperfection in one of these supercooled cans? If it was sitting in a store for weeks, wouldn't it be possible to turn to slush before opening?24.150.18.30 (talk) 00:57, 7 July 2010 (UTC)

I would expect that most stuff isn't stored supercooled, though -- generally any soft drinks I buy are at room temperature in the store, and I therefore see no reason that they'd be stored otherwise in the warehouse. In the case of the sold-supercooled link above, I expect that the Sprite was only supercooled once it was in the vending machine, protected from further disturbance. — Lomn 02:21, 7 July 2010 (UTC)

I've seen it happen with bottled water, so dissolved CO2 is not required. (It's a neat trick. Makes you feel like a super hero.) APL (talk) 03:22, 7 July 2010 (UTC)

Unless you didn't want it to go slushy, and just put it in the freezer to cool fast then forgot about it and thought you pulled it out in time but ooops maybe not Nil Einne (talk) 06:04, 7 July 2010 (UTC)

Thanks, everyone, for the quick replies. I've actually always been interested in questions like this. I don't know what the field this would be called (maybe physical chemistry?), but can anyone recommend some books on this subject? Or are there standard university courses that teach material like this? 74.15.137.192 (talk) 02:55, 7 July 2010 (UTC)

I don't understand quantum mechanics!

I was reading the article "uncertainty principle" in an attempt to find out why it is not possible to know a particle's position and its momentum at the same time. The probability that a particle is in a certain position is given by the amplitude of the wave packet at that point (correct?). It also says that to obtain an accurate reading of position, this wave packet must be 'compressed' as much as possible, meaning it must be made up of increasing numbers of sine waves added together (what does this mean?), and that the momentum is proportional to the wavelength of one of these waves, but it could be any of them. —Preceding unsigned comment added by 203.22.23.9 (talk) 22:38, 6 July 2010 (UTC)

First, don't be ashamed you don't understand it. It's genuinely hard and unintuitive when you start. Here is my attempt to break it into relatively easy concepts — it is extremely simplified but hopefully will get the gist across and is, I hope, not so wrong that it will be just laughed at by those who have deeper understandings of it. (Or, if it is as bad as that, hopefully they will write up similarly straightforward corrections!) I am not a physicist but I dabble in its history and concepts.

As for waves. If you combine two waves together, they can either destructively or constructively interfere. See the diagram here. That part is easy. If you add two waves together of different frequencies, you can get interesting results, like so. The bottom sine waves have been added together and constructively/destructively interfere to make the wave packet on top.

Now imagine that we wanted to make that wave packet one infinitely small wave—just a single high point with nothing around it. Well we could keep on adding more and more sines together and we'd get something that looked more and more like a single wave in the middle of calm. A nice illustration of that is figure 7.4 on this page.

Now in QM, the wave packet's amplitude is the probability of finding a particle in a particular region of space. So at the parts of the sine wave that are high, you have a high chance of finding the particular. Now the issue is that if you want that uncertainty to be zero—you want to know exactly where it is—that means you have to add up a very large number of waves, so that you have basically one giant spike in the middle of a bunch of nothing. The more you confine that wavepacket, trying to nail down the position perfectly, the more waves it has to be made up of. As you add more waves to it, you are adding more uncertainty about the momentum of the wavepacket as a whole. So you can get the position down perfectly—no problem. But what do you lose at the same time? You lose the ability to know the frequency of the wave, the more you nail down its amplitude into a tiny space. That's the momentum issue.

Does that help at all, at least with the understanding of what the waves are supposed to indicate? The take-away message of the wavepacket description of UP for me is that UP is not a problem of measurement in and of itself (it is not just that when you physically "touch" one property, it physically modifies the other); it's a fundamental property of the universe, if the various properties of the universe (like a particle's position and momentum) are correctly described as these sorts of wave forms. --Mr.98 (talk) 23:40, 6 July 2010 (UTC)

I struggle, too, but I like Mr 98's explanation. You might be interested in these Wikiquotes. Dbfirs 07:46, 7 July 2010 (UTC)

But how does one "add" waves, in practice, by taking measurements? —Preceding unsigned comment added by 203.22.23.9 (talk) 03:41, 7 July 2010 (UTC)

He refers to the Fourier decomposition of the wavefunction.

Anyway, say we have an electron. To measure its position we can bounce photons off it (i.e. shine a light). The accuracy of the measurement is roughly the photon's wavelength. Photons have energy and momentum, so the resulting collisions destroy information about the particle's momentum. As p = hc/λ for photons, we can only improve precision in position at the expense of momentum. MER-C 10:44, 7 July 2010 (UTC)

Right, but that's actually not a good explanation of UP, because it makes it look like it is just an artifact of the measurement system, whereas the wavepacket model makes it clear it is a fundamental property of the universe. It's not that we don't measure it well—it's that the information is simply not there. (At least in the Copenhagen interpretation.) UP as put forward by Heisenberg was originally a measurement issue; Bohr is the one who said, "ah ha! It's actually about the fundamental limits of physical knowledge." Which is a much more interesting and important interpretation, and the wave approach is the only way I really know how to illustrate that. (It also makes more clear why EPR and Bell's theorem are important, and why it is not just position/momentum that are at issue, but lots of other states as well.)--Mr.98 (talk) 13:17, 7 July 2010 (UTC)

July 7

magictrick banknote in a fruit

I'm looking for how this magic trick works. A magician takes a dollar bill from the audience and makes it appear inside a fruit (orange/lemon). This youtube video demonstrates how it works: http://www.youtube.com/watch?v=cQBrXCcfDyc But I've seen many magicians ask the audience to sign the bill and making it appear in the orange rather than just having a different bill appear inside. Do you know any information on how this one works? I can't seem to find the information anywhere. 69.230.55.21 (talk) 00:19, 7 July 2010 (UTC)

At a guess they probably switch the dollar bill they put in the fruit with the one the audience gave to them at some stage after opening it Nil Einne (talk) 01:04, 7 July 2010 (UTC)

Of course, it is Sleight of hand or हाथ की सफाई as they call it in India. It is an ancient art. Nowadays tricksters tell you that is nothing but art, in India now that is even required by law, though that is not strictly required by law but Rationalist Society people get you otherwise  Jon Ascton  (talk) 01:13, 7 July 2010 (UTC)

Or the "random person from the audience" is in cahoots with the performer. 218.25.32.210 (talk) 01:19, 7 July 2010 (UTC)

ok thanks for the info, but can you give me specific steps in performing this trick? I know that this can be performed with a "random person" not a confederate of the magician. 69.230.55.21 (talk) 01:21, 7 July 2010 (UTC)

Yes, there are always more than one way to do a trick. And a clever magician never repeats a trick before the same audience  Jon Ascton  (talk) 02:24, 7 July 2010 (UTC)

Hey! I just saw the video (after writting my above comments). The magician simply, honestly gives away the trick - that's amazingly easy ! Did'nt ya get it, man ? What happened is this : Before you do the trick you take a dollar, note its serial number on a paper. Then you roll up the note and stick it neatly in an orange, do it in such a way that it does'nt look as if someone's done something to the fruit. Now you are ready for the trick. You ask a guy to come up and give you a dollar, he will give his own dollar of course with it's different serial, but that does'nt bother you because you don't really write it anywhere, but only pretend you do so ! by moving a pen on the paper on which you have already written the number of the dollar which is readily stuffed in the orange. Now you roll the guy's dollar and put the handkerchief on it, this is where you slip the dollar in your other hand or somewhere else he can't see it. He thinks it is there in the handkerchief, but it is some piece of paper rolled in the handkerchief's hem ! Now you unfurl the handkerchief and the dollar "vanishes" (it is already soemwhere else, what the dumb bastard was feeling was the paper in the hem !) Now put the handkerchief out of picture and cut open the orange which has your original dollar whose serial number is safely recoreded on the paper ! Got it ? Cool !  Jon Ascton  (talk) 03:08, 7 July 2010 (UTC)

I used to dabble a lot with magic tricks when my kid was younger. My favorite fruit trick is to hand several members of the audience a banana pulled from an entire bunch I have brought with me. I ask them to inspect their bananas carefully - then without me being anywhere nearby, to peel them. They are all amazed to find that the inside of their banana is neatly sliced into a half dozen pieces - and even after that, they can look at the skin, eat the banana and not see how the trick was done. It's nice because there are so many bananas that I couldn't possibly have that many confederates - and everyone in even a moderately large audience is sitting close enough to one of the bananas to see the trick happen in front of their eyes.

I'll let you figure out how I do that one! SteveBaker (talk) 04:31, 7 July 2010 (UTC)

Hm...I think it's got to do with the art of slicing the inside without damaging the skin ! No ? That can be accomplished either with a sharp needle or a needle and a thread. Right ?  Jon Ascton  (talk) 04:58, 7 July 2010 (UTC)

I think you're somewhat missing the point. The OP is aware the video shows how it works for that case. What they want is to know how it works when the illusionist shows a bill that was signed by a member of the audience, so you can't just use a different bill. As you and I have said, it would likely be some sort of sleight of hand, swapping the bill inside the fruit for the signed one (alternativing signing the one inside, but this seems far less likely given that the signature could easily be recognised as a fake) but the OP wants more. Nil Einne (talk) 05:18, 7 July 2010 (UTC)

Oops, Nil, you are right, I missed that point ! How stupid of me ! Terribly sorry indeed ! Oh, the OP knows how he did that, my ! I was taking so much pains to tell what he already knew, perhaps better than me ! Sorry, OP man. I overlooked that part - the magician does not explain the whole trick - but leaves a vital loophole - the signature thing. Oh my ! So this is a new way to show magic. You do an easy trick. Explain it is such a way that an asshole like me thinks that all is explained, but when he thinks over he learns there is something he can not get through. Well, there should be a special term for this kind of thing, no ? Yup, Nil man, its good old slieght of hand of course... Jon Ascton  (talk) 07:15, 7 July 2010 (UTC)

...and what do ya think about the SteveBaker's banana ? Like my solution, eh ?

side mirror

what type of glue/ resin do they use to attach a side mirror on a car? i mean attach the actual glass part to the metal part (honda) —Preceding unsigned comment added by Alexsmith44 (talk • contribs) 00:27, 7 July 2010 (UTC)

Are you sure it's glued on? I would have assumed it was bolted on. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:35, 7 July 2010 (UTC)

i think its glued on. mine broke today and there were no bolts —Preceding unsigned comment added by Alexsmith44 (talk • contribs) 00:37, 7 July 2010 (UTC)

Glued onto what? The pivot mechanism? ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:40, 7 July 2010 (UTC)

only the glass part broke not the metal part —Preceding unsigned comment added by Alexsmith44 (talk • contribs) 00:50, 7 July 2010 (UTC)

I don't follow. The glass part has to be attached to a pivot mechanism of some kind so it can be adjusted. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:53, 7 July 2010 (UTC)

No need to interrogate the guy. His question is clear enough. The glass shattered and has to be re-glued to the piece that the mirror itself attaches to. If you don't understand the mechanism, you don't have to answer.

There was a discussion on this very topic here.

Check out Steve's post at the end. If he's right, It looks like what you want is some rubbery adhesive pads specially designed for this purpose. (These two allegedly educational YouTube videos back him up on this : [25][26]) APL (talk) 02:00, 7 July 2010 (UTC)

No need to get snippy. I was asking these questions out of curiosity. Believe it or not, I'm not an expert on everything. :) However, if it were me, I would take it to the dealership and let them figure it out and explain it to me. ←Baseball Bugs ^{What's up, Doc?} carrots→ 11:06, 7 July 2010 (UTC)

do they use a formaldehyde resin in the factory to attach it?--Alexsmith44 (talk) 02:08, 7 July 2010 (UTC)

I don't know - but in the previous discussion on this topic, the person said that when they bought the replacement mirror, it came with double-sided sticky foam pads to mount it with. He was skeptical that this would work and glued the mirror on instead - finding that it broke within a very short time later. I deduce that the pads have some role in isolating the mirror from vibration and that when you buy your replacement mirror, you should definitely use them if that's how they come from the auto-parts store.

I asked the previous person this too (but didn't get a response): Did you look to see whether there was signs of glue or sticky pads on the shards of broken mirror glass? That would be a clue at least. SteveBaker (talk) 04:18, 7 July 2010 (UTC)

To reiterate what I mentioned in the aforementioned previous discussion, FWIW, on the several occasions I've had to replace a broken side mirror glass, the replacement glass came with a strong adhesive layer over the whole of the rear surface (covered by a peel-off paper layer): separate glue or adhesive pads was/were unnnecessary. Major UK car component stockists such as Halfords stock a large range of replacement mirrors specific to individual car makes and models, which are generally considerably cheaper than replacements supplied directly by the manufacturers themselves. If Alexsmith44 is not in the UK this information may of course be useless 87.81.230.195 (talk) 09:06, 7 July 2010 (UTC)

Animal Eyes

Why do eyes of animal light up thus when photographed ? And why the same does not happen with humans ?  Jon Ascton  (talk) 01:07, 7 July 2010 (UTC)

They probably have much better night vision than we do. I'm sure there's an article about that phenomenon somewhere. Very noticeable with cats, for example. ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:10, 7 July 2010 (UTC)

I don't think that's got something to do with that. Cows are not known for their sight, they can't even discern colours  Jon Ascton  (talk) 01:16, 7 July 2010 (UTC)

Bingo. I looked up night vision and it led me to Tapetum lucidum, a layer of tissue in the eyes of many animals but not in humans. ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:13, 7 July 2010 (UTC)

Um...the same does happen with humans. See red-eye effect. Under the right conditions (low light, high sensor sensitivity, subject looking directly at lens), I've seen human eyes glow much brighter than those cow eyes. --Bowlhover (talk) 01:29, 7 July 2010 (UTC)

You don't get that "mirror" effect with humans that you do with animals that have that layer in their retinas. ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:32, 7 July 2010 (UTC)

It's of note that red-eye effect and eyeshine are actually two different effects. And notably, red-eye is just seen in photos. Eyeshine you can actually see in nature (shine a flashlight on your dog at night, for example). And note that the cow's photo is in daylight. --Mr.98 (talk) 11:17, 7 July 2010 (UTC)

Goa'uld? -- 58.147.52.199 (talk) 13:33, 7 July 2010 (UTC)

Dragonfly

?

Any idea what this is? --The High Fin Sperm Whale 03:59, 7 July 2010 (UTC)

Should we assume that you took that photo in British Columbia since your user page points out that you live there? Dismas|^(talk) 04:02, 7 July 2010 (UTC)

Yes, you can also assume that since the title says it's in Langley and it has Coordinates. --The High Fin Sperm Whale 04:08, 7 July 2010 (UTC)

If we can't figure it out, you may want to contact the person behind this site. For some reason, their "Gallery" link doesn't actually contain a gallery. Dismas|^(talk) 05:17, 7 July 2010 (UTC)

Looks like a skimmer to me, family Libellulidae. Possibly male Plathemis lydia, not sure at all. --Dr Dima (talk) 06:45, 7 July 2010 (UTC)

An open tube is placed into a container of water

An open tube is placed into a container of water and a vibrating tuning fork placed over the mouth of the tube. As the tube is raised so a greater length of the tube is out of the water, resonance is heard. This occurs when the the distance from the top of the tube to the water level is 12 cm, and again at 50 cm. Determine the frequency of the tuning fork.
The naïve approach would simply evaluate $f={\frac {v}{\lambda }}={\frac {v}{4l}}={\frac {344}{4\times 0.12}}=717Hz$ but this is not correct. hElp?--Alphador (talk) 07:01, 7 July 2010 (UTC)

λ=50cm−12cm? Bo Jacoby (talk) 09:49, 7 July 2010 (UTC).

No, that's λ/2 because the second harmonic occurs at l = 3λ/4 => λ = 0.78 m => f = 441 Hz (a likely correct answer). The reason why 717 Hz is wrong is because of end correction. MER-C 09:58, 7 July 2010 (UTC)

North American bugs

If I find some random strange looking bug in North America, what are the chances that this bug is unknown to biologists? I know that globally the majority of insects have not been cataloged. But is this true in North America as well? Ariel. (talk) 07:04, 7 July 2010 (UTC)

This will depend on where you find it. I know from my own country that bugs and plants are surveyed much better in the vicinity of a major university and in locations that are easily accessible. The chance of it being unknown is also not necessarily larger if it is "strange looking". It is the case with many plants and fungi that they are overlooked by biologists because they are very similar to other species. This could be the case with insects as well. 80.202.238.149 (talk) 10:01, 7 July 2010 (UTC)

Source of stream goes both ways?

The source of the Lawrence Brook is at 40°22′33″N 74°32′32″W. Google Maps shows this coordinate along a stream. One direction is the Lawrence Brook and the other direction is the Devils Brook. Is it a spring that goes both ways? Or are the streams interconnected? Or is the map mistaken and the streams aren't connected at all? Thank you. --Chemicalinterest (talk) 10:59, 7 July 2010 (UTC)