Wikipedia:Reference desk archive/Science/2006 October 16

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bananas[edit]

I have conducted a psudo-scientific experiment: I have left a bunch of bananas in my kitchen at room temperatue, and i have a single banana i wanted to eat in my bedroom. The skin of the one in my bedroom turned brown at an accellerated rate (and tasted jolly nice too). Why is this? The bananas were regular supermarket fayre and not organic or free range. I have been playing che séra séra on a number of occasions and have not mastrubated recently. Why would this be? --86.133.176.75 00:18, 16 October 2006 (UTC) -

Maybe they need to be kept in the dark.8-|--Light current 00:32, 16 October 2006 (UTC)
Well, gaseous emissions are known to affect the speed of fruit ripening and decomposition; is there something you're not telling us about your bedroom? Anchoress 02:03, 16 October 2006 (UTC)
What is a free range banana, and where do you get such things? :) --best, kevin [kzollman][talk] 02:12, 16 October 2006 (UTC)
My guess is that the banana wranglers have to chase them down. I understand they roam the same territory as the cat herds. Clarityfiend 02:36, 16 October 2006 (UTC)
Doods, you're all wrong! They're called 'free range bananas' because they give them out FREE at the DRIVING RANGE. Anchoress 02:38, 16 October 2006 (UTC)
Seriously, the wild ones are fast, shifty little beggars. One of their most effective defense mechanisms is to drop a peel, causing their pursuers to slip, and then they just...umm...split. Clarityfiend 02:49, 16 October 2006 (UTC)
Try this: Put a bunch of bananas in your bedroom and a single one in the kitchen and report what happens. I wonder wether removing a single banana from the bunch accelerates the ripening process. The needs of the many... ;) Maybe bananas don't decompose as quickly while still on the bunch?? Vespine 03:39, 16 October 2006 (UTC)
I have also heard that splitting a bunch can have an effect on accelerated ripening. (look at the singles in the shop) Also the way they are stored can have an effect. You are supposed to use a banana tree to hang them in your kitchen. Refrigeration does not help and is not recommended. Im sure light has more to do with it.--Light current 10:11, 16 October 2006 (UTC)
If your room is colder than the kitchen, that could have an effect. (I bought bananas once when the furnace was broken in the winter, and they were turning brown pretty fast.) - Rainwarrior 15:23, 16 October 2006 (UTC)
The ripening of bananas is affected by ethylene gas, a plant hormone, and by temperature. So you should look at differences between the temperature in the kitchen and the bedroom, and at ventilation, since less ventilation would allow ethylene to become more concentrated. (The best way to take advantage of ethylene, however, is to place the bananas in a paper bag. Ripe bananas can ripen other fruits using this method as well.) --Ginkgo100 talk · e@ 19:27, 16 October 2006 (UTC)

Critters in the stomach[edit]

Greetings. In 1954 this guy Palmer published a study where he looked at biopsies from over 1,000 patients' stomachs searching for organisms which had been reported off-and-on for the last 70 years. After extensive study he reported that he did not find any colonizing bacteria. We know now that he missed helicobacter pylori, but of course he didn't know that. From what I've read people seem to say he missed h. pylori because he didn't use the silver staining technique that apparently reveals h. pylori. Unfortunately, the stuff I'm reading is for specialists, and I'm kinda dumb. Can someone help me here? Why does silver staining reveal h. pylori when other staining techniques don't? (Is this even right?) Even better, can anybody say one way or the other if this was a serious error on Palmer's part, or in the 1950s, could he have reasonably decided this was not required? Thanks in advance. --best, kevin [kzollman][talk] 01:58, 16 October 2006 (UTC)

Well, staining only works if the thing that you're staining holds on to the chemicals used to stain it. Think about invisible ink squirted on a newly laundered white shirt. It should stain the shirt, right? However, as the ink evaporates, the shirt is left white. Microbiology has a lot of different stains used to look for bacteria. However, some bacteria (such as H. Pylori) do not take up some of the more common stains, just as a white shirt doesn't take up invisible ink. It took looking with the right kind of stain (silver) to actually see H. pylori. It wasn't a mistake - his report stated the stains that he used and so the true way to interpret his results is "there are no bacteria in the stomach which take up the stains tested by Palmer" rather than "there are no bacteria in the stomach." InvictaHOG 10:56, 16 October 2006 (UTC)
Thanks for the response. Could you be a little more specific about why H. pylori doesn't take up other stains and does take up silver? This is the bit that I don't understand. WRT proper conclusions, I understand that a more strict induction might have been better, but scientists use limited evidence to reach broader conclusions all the time. I'm wondering how out of line Palmer was in comparison to others at his time. In other cases would people looking for bacteria have used the silver stain? Would others have concluded there was no bacteria without using such stain? You see what I'm getting at? Thanks again! --best, kevin [kzollman][talk] 19:44, 16 October 2006 (UTC)
However, there was indeed a mistake, made by the medical community. Overlooking the fact that Palmer might have not seen all there is, the claim entered most textbooks that research had proven that stomach ulcers and gastritis are not due to bacteria and hence, prescription of antibiotics to treat them is not worth a try. Hence, it took quite a while until somebody looked again, and when they found something, Warren and Marshall had a really hard time convincing their colleagues that the textbook knowledge was wrong. Well, that's at least how I remember the story. Simon A. 14:06, 16 October 2006 (UTC)
Simon, thanks for the response. You indeed have the story right, and looking back it was indeed a mistake. At the same time, if Palmer had been right and people had followed your recommendation, we might now be looking back wondering why all those scientists wasted their time redoing what we already knew. :) Its a tough balance, and what I'm trying to figure out is if Palmer was out of line compared to other scientists at the time. Thanks again. --best, kevin [kzollman][talk] 19:44, 16 October 2006 (UTC)

Arm wrestling[edit]

I've heard that it's harder for long-armed people to win at arm-wrestling. How exactly are they disadvantaged? I would image it has something to do with torque, but I'm not sure how the muscles in the arm work. --Anon

Yes, it has to do with torque, but also leverage. Imagine holding a sledgehammer from the heavy end, horizontally, so that the handle was parallel with the ground. This would be quite easy, because the heavy part is closer to your body. But if you held it from the handle, with the heavy end far from your body, that would be much more difficult. So you can see that having long arms might make things difficult in a situation like arm wrestling. Also, the human arm is a third class lever. If you look at the diagram on the lever page, you can see that it is easiest to move the load on the far end of the lever when the load, fulcrum, and force are close together. This is analogous to a short-armed person. Gary 02:45, 16 October 2006 (UTC)

Can you identify this strange bug?[edit]

Hi, I live in New England and I have encountered this strange bug a few times. It can fly, and when you squish it smells strongly of apple. Here is picture of the bug, it is not my picture but the bug is exactly like this: http://i50.photobucket.com/albums/f305/Parkram412/bug.jpg If you can identify this bug, I would greatly appreciate it. Thank you.

You will need a clearer picture, I expect. BenC7 05:49, 16 October 2006 (UTC)

Yeah, it's a little hard to tell from the picture, but I think it might be some sort of kissing bug. In which case be very wary of it. I didn't think they got that far North, though. --Trovatore 06:32, 16 October 2006 (UTC)

  • What you seem to have there is some species of stinkbug. They're not too uncommon around the NE Unites States (as opposed to the kissing bug, which numbers exactly zero). From the image, however, I can't be much more specific. If it is such an insect, than it's not dangerous, unless you happen to be an apple, of which it is a serious pest species. – ClockworkSoul 15:41, 16 October 2006 (UTC)
I've seen this bug many times in extreme-southern Canada. From http://www.cirrusimage.com/bugs_stink.htm, is seems to be either a brown stink bug or a shield bug. --Bowlhover 00:42, 18 October 2006 (UTC)
I was under the impression that stink bugs are shield bugs. Actually I'm too lazy to read that article. – ClockworkSoul 01:58, 18 October 2006 (UTC)

Why is some skin immune from sunburn?[edit]

The palms of the hands and soles of the feet can be exposed to the sun all day at the beach, yet these areas do not show any sign of damage from solar radiation. Why is this so? And do these skin areas produce Vitamin D from sunlight? And in evolutionary terms, why isn't our whole body covered in this apparently better quality skin, wouldn't that save us a lot of trouble? I could do with that type on my bald patch a) for solar damage immunity and b) for those times when I scrape my head, painfully, against the underside of my car tailgate :0) --EdX20 05:33, 16 October 2006 (UTC)

Just as a small note, I can't answer the entire question. Skin cancers usually occur after someone has reproduced, and there is less evolutionary pressure to change that, for it would probably bring some disadvantage. Also note, that evolution has done something to combat burn, white people lost it when they went to cold regions. It is interesting though, as black people have white feet and hands. --liquidGhoul 06:14, 16 October 2006 (UTC)

Going out on a limb here, but I really kind of doubt it's true that palms and soles of feet don't burn, if overexposed. I think they just don't get much sun under ordinary circumstances. How often do you sunbathe palms up? But maybe someone who actually knows something about it will comment, so I'll shut up now. --Trovatore 06:59, 16 October 2006 (UTC)

I don't know if soles and palms get sunburnt, but they sure do suffer sun damage. I read an interview a few months ago with a dude who spent years in the tropics, and when he got some foot damage, I think rats chewed on his soles, he was advised to expose them to the sun for long periods to heal them and he later got skin cancer on the soles of his feet. Anchoress 07:10, 16 October 2006 (UTC)
Oh, it was John H. Groberg. Anchoress 07:11, 16 October 2006 (UTC)
They do burn. It's just they're not exposed to much sunlight. Feet are usually in shoes/socks. Even when tanning, feet sometimes end up in sandles, or you end up playing around with the sand with your feet. Plus, your soles lie verticle when you lie down, not horizontal. People rarely tan with their sole facing up directly to the sun. Same goes for palms. Natural position for a human hand is with the fingers slightly curled in. It takes conscience though to straighten your fingers to fully expose your palms, which people don't do when they are trying to get a tan.
If you stood on your head for long enough your bald-patch would probably cornify to some extent thereby giving you what you seek. As other suggest, you would still get sun burned though. From a theoretical point of view, palm and sole skin could be even more sensitive to sunburn. If memory serves, palms and soles of in dark skinned folks are often lighter than other parts of their body. This would mean less melanin to protect from UV and thus a greater risk of "burning". If you are A) Black or Asian, B) brave/stupid enough and C) have access to a UV transilluminator, you could (though really shouldn't) test this hypothesis out for yourself. Rockpocket 06:41, 17 October 2006 (UTC)

Diesel fuel life[edit]

How long does diesel fuel last? Say I want to bury a 50,000 gallon diesel tank and be set for life, will the fuel keep? Are there any hydrocarbon fuels that won't deteriorate over several decades?

We don't know for how long you aim to survive, but assuming it is less than 100 years I'd be more worried about the tank rusting through. And any engines you power with the diesel won't last that long and may be difficult to repair. Have you thought about medical emergencies? It's tough surviving on your own, without (naughty) nurses. Or should the fuel power a whole hospital?  --LambiamTalk 12:55, 16 October 2006 (UTC)
you liked the naughty nurse, huh ;)
I think he's betting on the idea that the price of fuel will only increase over time, so if he hordes it now he won't have to pay rising gas prices, or something like that. --Fastfission 15:03, 16 October 2006 (UTC)
Actually, the economics angle is where I'm comming from, for two reasons. First, I've heard that alternative energy (wind, solar and associated batteries and electronics) might cost more than just a big diesel tank for generators and vehicles. And second, the rising fuel costs, as you mention. It's a moot point however if diesel deteriorates too quickly. I would include a simple nitrogen purging system and want the fuel to last for 40 - 100 years.

The problem with your idea is that the tank will cost far more than the fuel, so there goes any savings. On the other hand, if you fill it and then sell it everytime the price goes up or down a dollar a gallon, then you could possibly make money. This is fuel speculation, and you don't have to store it yourself, there are companies that provide that service for you. StuRat 17:53, 16 October 2006 (UTC)

Appropopos another recent discussion here, see our pseudomonas aeruginosa article for another threat to your (hypothetical) diesel hoard.
Atlant 20:52, 16 October 2006 (UTC)
50000 gallons of diesel? Are you serious?? For that amount of outlay you could definitely fit yourself out with alternative power. Probably solar is the way to go I reckon, and an electic car, or at least a prius ;) .. I wouldn't even think about fossil fuels, in 40 years time you don't know what they'll invent, diesel could be obsolete, like leaded petrol.. I certinally wouldn't bet on it. Vespine 00:37, 17 October 2006 (UTC)

Most effective firearm[edit]

What is the most effective firearm for self protection? My application is for protecting a razorwire fenced compound from hoards of starving attackers after a nuclear holocaust or similar event. I'm only interested in weapons that are currnetly leagal in the US (I'm not one of thoes wackos that's into illegal weapons).

not at all a wacko... ;-) different strokes for different attacking folks, ain't nothing like a shotgun for close range, and a semi-automatic like the ruger M-14 with telescopic sights should take care of anything else. Xcomradex 06:52, 16 October 2006 (UTC)
(edit conflict)Is this North Korea thing bringing out the paranoia in everyone? --liquidGhoul 06:54, 16 October 2006 (UTC)
In stead, you're a wacko that's into legal guns? Oh, but you've got a great excuse. Of course, the starving hoards after a nuclear attack. Who wouldn't want to prepare for that? :) DirkvdM 07:26, 16 October 2006 (UTC)

If you have to ask this question, you probably are not the type of person who should be owning a weapon yet. Attend a firearms safety class offered by the NRA and ask your instructor about your specific needs.

If you have to ask this question, you probably are not the type of person who should be owning a weapon yet -- Why, what's wrong with asking a question? The US guarentees people the right to own weapons to protect thier liberty (in a well regulated militia). I know that in some civilized circles this is viewed as barbaric and is seen as not needed because civilization has (supposedly) moved past this point. However, a thousand years from now people will still be killing each other and taking their stuff. The most healthy societies will be the ones that still allow people to defend their liberty. I certianly am the type of person who should own a weapon - because I ask questions, have never intentionally harmed another human being, and I'm interested in self defense and liberty, not power. But this is a digression, and you haven't tried to answered the question.
Thats right, you shoot that nuke with your shotgun, that'll sort everyones problems. And your very wrong, the healthiest societies are the ones where people dont need to defend their liberites. Philc TECI 17:45, 16 October 2006 (UTC)
Thats right, you shoot that nuke with your shotgun, that'll sort everyones problems. -- Huh? what?
the healthiest societies are the ones where people dont need to defend their liberites -- Name one
The closest I can come to are countries like Germany, and scandanavian countries, where crime is so much less of a problem than in countries like america. Philc TECI 17:45, 17 October 2006 (UTC)
Oh Pleeese, this times a hundred. Germany and scandanavian countries have the exact same problems as we do, that of defending their liberty. And it has nothing to do with crime, I have no idea why you added that into the mix.

If you’re just using legal guns, you'll never survive an attack. Actually, if you’re just using guns period, you'll never survive. Get an electric fence that has enough voltage to electrocute those who touch it, and have a mine field for 50 yards outside of a 20 yard empty space outside of the fence. That is probably your best chance for survival, along with the ruger mentioned earlier for anyone who may get past. THL 13:45, 16 October 2006 (UTC)

good insight - thanks.
You might do better to just take Dr. Stephen Falken's strategy in WarGames: Move really close to a primary target so you don't have to worry about living in the aftermath. I'm pretty sure it works for me located where I am.
Atlant 16:42, 16 October 2006 (UTC)
Many people accet this for themselves, would you accept it for your children though, or would you try a little harder?

The better strategy is to build your bomb shelter underground and not tell anybody about it. That way, nobody knows you're there so nobody tries to steal your food. Hopefully, by the time your food is gone and you come back up, law and order has been restored. StuRat 17:48, 16 October 2006 (UTC)

Yes, I'm planning to use surplus shipping containiers and bury them, still need self defense mesures though.
How would law and order be restored if everyone just hides undergound in their shelters. Philc TECI 17:59, 16 October 2006 (UTC)
Not everybody would. Those surviving members of the military, for example. StuRat 19:58, 16 October 2006 (UTC)
Theyd rebuild cities in the hope that people would just emerge from the ground and leech off of them? You expect there would still be an organised military in nuclear holocaust, and that they'd have the charity to help people who do nothing but hide when it is a battle in itself to survive themselves? I must say I think you outlook on this sort of thing is very optimistic. Philc TECI 20:34, 16 October 2006 (UTC)
You appear to be trolling for a fight, Philc. I never said anything about them "rebuilding cities". I think it's a fair assumption that some members of the military would likely survive (as would some civilians), and would then set about re-establishing their authority, as much for their own protection as for that of everyone else. What do you expect they would do, just run about with their arms flailing wildly ? StuRat 02:57, 17 October 2006 (UTC)
Dont offend me please. Im not a troll, and I dont want a fight. But in nuclear holocaust, with a countries complete infrastructure destroyed, the military have no duty to help you. Not to mention a complete lack of resources. And the fact that they need to ensure their own survival ahead of yours. Philc TECI 17:43, 17 October 2006 (UTC)
It sure sounds like trolling for a fight when you claim I said the military would "help you" when all I actually said was that they would reestablish law and order. That would be done to help themselves, not you, but you might very well derive a benefit from it, anyway. StuRat 18:38, 17 October 2006 (UTC)
When did misenterpretation of someones point become trolling!? I'm sorry you felt it necesarry to bring this up, and interpret a difference in opinion as trolling, I think its really sad people cant just discuss things, accusing someone of trolling is somewhat an inflammatory move in itself, maybe so that you could claim any equally inflammatory action I may have taken as proof that I am a troll. Well I'm not. Its a shame, but I would like to end this discussion before you try further escalate it into an argument. Philc TECI 20:47, 17 October 2006 (UTC)
Theyd rebuild cities in the hope -- Oh sure, everything would be just Hunky Dory.
Yes, I was being sarcastic, you are infact, agreeing ith me. Philc TECI 17:45, 17 October 2006 (UTC)
If you are by yourself, I think you need much more in the way of passive defence. No gun on the face of the planet can save you if you can get flanked and over run and no amount of wire fence and barbed wire will stop a truck ploughing through it. I'd recommend an electric fence surrounded by a brick or concrete wall topped with barbed wire surrounded by a decent moat with a bunker on a steep elevated position in the middle.
To go to fantasy land and answer your question directly, I don't know what weapons are 'legal' where you live. In Australia, no automatic weapons are legal at all but I think that's where you should be looking. Also because of Hollywood, most people have a very distorted impression of how easy it is to shoot and kill people. Someone who is not experienced with weapons is not likely to hit anything man sized with a shot from a pistol over 20 meters maybe double that from a rifle, half it again in combat.. Truth is, anything that shoots will kill, I don't think the actual gun is as important as what you do with it. What I would look for is the cheapest, automatic rifle you can find and buy four, something like an AK-47 would be perfect, and MOUNT them in your bunker so their fields of fire overlap the entire 360 degrees around you. Mounting will greatly increase your chance of hitting something, also, make sure the optics are accurate and get red dot sights or laser sight!Vespine 00:24, 17 October 2006 (UTC)

He brings up a good point, trucks. Get trained in how to use firearms (and take anger management so you can resist any temptations), and take his advice on the accuracy accessories. I still like my minefield surrounding an electric fence, but you should probably have your entire perimeter surrounded by a modern Berlin Wall as well. Pray that you don't come up against a tank; I don't think that you would have a chance of stopping one of those; especially if they have ammunition for the big gun/rocket launcher. THL 04:22, 17 October 2006 (UTC)

If you upgrade a little you don't have to worry about any of those things, and you can keep your shelter above ground. Station automated Phalanx CIWS systems around your perimeter... but put a few Bushmasters in on top of the Berlin Wall for backup. Then, for yourself, you can use the Pancor Jackhammer automatic shotgun and the FN F2000 assualt rifle. — X [Mac Davis] (SUPERDESK|Help me improve)08:30, 17 October 2006 (UTC)

Humans driving a ship[edit]

If the people on a seaworthy ship would take turns driving the propeller with hometrainers (shifts of 8 hours per day), how fast would it go? I've got this vision of a gym in lieu of an engine room, with the slogan 'get a cheap passage, get a workout and save the environment'. Alternatively, how many people would it take to drive a cruise ship at a reasonable speed? DirkvdM 07:13, 16 October 2006 (UTC)

Ship diesels are huge and have thousands or even millions of horse powers. Given that you need several humans to replace a horse, you would need literally a whole city of people. On the other hand, galleys were quite fast, when powered by rowers with oars. So, that's an option. The reason why a galley can be propelled by humans and a cruise ship cannot indicates the main problem: you'd have to do without sun deck, swimmin pools, squash rooms and dining halls. Actually, galleys came out of fashion already during the early age of sail. It seems to me that you even have to do without heavy cannons. But the concept of a cruise on a galley has bee explored in an Asterix volume. Simon A. 07:39, 16 October 2006 (UTC)
And propellers are not very efficient at slow speeds. People-powered ships would get along better with oars or paddle-wheels.--Shantavira 11:34, 16 October 2006 (UTC)
Are you referring to the speed of the propellers or the vessel? If the former, couldn't you just use gears to speed it up? —Bromskloss 12:38, 16 October 2006 (UTC)
Then you would need to have even more people to drive it. For example, if you geared it up to 10 times the speed, you would need 10 times as many people, or perhaps more, due to increased drag. StuRat 17:39, 16 October 2006 (UTC)
Its not going to happen, it takes about an hour to get across a bay in a pedalo, you'd be better off conserving the energy and drifting, that way you might actually live long enough to get across. The power ships use is phenominal, in a series of excercises I think my horsepower was measured between 0.7~1.1 (I cant remeber) and if you need a million of these horsepowers.... Philc TECI 17:43, 16 October 2006 (UTC)
The US Navy converted an old sidewheeler steam paddle boat into a training aircraft carrier, the Wolverine in Lake Michigan during WWII, with 8,000 horsepower producing 28 mph claimed. Figure 1/8 HP per person, and it would take 64000 passengers, somewhat more than it could carry. The Confederate Civil War sub H. L. Hunley (submarine), powered by 7 men, could go 4 knots on the surface. A galley with 50 oarsmen could reportedly go 9 knots.A racing shell can probably go faster with a smaller crew, but it is hard to decode the abbreviations in Rowing World Records, and you were not discussing rowing a lightweight craft anyway.Edison 17:57, 16 October 2006 (UTC)
The world record for an 8-man racing shell (the fastest commonly used boat) equates to just over 12 knots over 2000 metres. But the difference being that the combined weight of the crew is massively more than the boat itself. For a crew made up of 90kg oarsmen and 55kg coxswain - a total of 775 kg, not far short of 8 times the weight of the boat. To achieve this with a cruise ship, e.g. the RMS Queen Mary 2 - you'd need nearly 7 million rowers on board (but you'd have far exceeded the power of the QM2's engines (and the QM2's load capacity).Richard B 23:56, 17 October 2006 (UTC)
A professional cyclist can churn out 300 to 350 watts (1 HP = 750 watts) over a period of several hours and much more over a shorter time frame, but he trains 20,000 miles a year to be able to do that. Your average, beer-bellied, sedentary cruise customer would be lucky to produce 100 watts for more than a few minutes, so I fear this idea is doomed. EdX20 03:31, 17 October 2006 (UTC)
Yep. Put it this way - you'd need about 7,500 cyclists at to move this 59-foot motor yacht at its top speed. You could probably get away with 1500 or so to move it at its hull speed of about 10 knots, though :)
If you want a boat that you can propel efficiently with cycling - this does the job nicely. --Robert Merkel 06:50, 17 October 2006 (UTC)
I was indeed thinking of galleys and figured that an oar is a much less efficient way to use a human body's enery than pedals are. But if propellers are less efficient at low speeds (never heard of that), that is a problem. Also, galleys went fast, but only for a short timespan, not quite enough to cross the Mediterranean or even an ocean. And the cruise ship idea was a bit wacko. But what I was thinking about is a lightweight boat that is big enough to be seaworthy and has only basic amenities - just the engine room and sleeping bunks. Of course sails make sense, but they shouldn't require a professional crew. The idea is to make a very cheap passage for budget travellers to places that are expensive to fly to. Robert's cadence link doesn't say how fast it goes, but something similar for 100 people would be much more efficient and therefore faster. There would be enough budget travellers interrested in this, priovided it doesn't take too long. Say one week to cross the Atlantic. West Africa to Brazil is about 3500 km, so that would require a speed of 20 km/h or 10 knots. Given the answers above, that would be pushing it. And more than one week of constant cycling would be too much even for most budget travellers. So the idea is pretty much out the window, I suppose.
A siling ship would still be an option, though. DirkvdM 09:27, 17 October 2006 (UTC)

Unknown Goat[edit]

Unknown Goat

I think this would add value to an article, but I am not sure what type of goat it is. Does anyone know? NauticaShades 12:17, 16 October 2006 (UTC)

Hehe, reminds me of a perhaps well-known story.

An astronomer, a physicist and a mathematician were holidaying in Scotland. Glancing from a train window, they observed a black sheep in the middle of a field. "How interesting," observed the astronomer, "all Scottish sheep are black!" To which the physicist responded, "No, no! Some Scottish sheep are black!" The mathematician gazed heavenward in supplication, and then intoned, "In Scotland there exists at least one field, containing at least one sheep, at least one side of which is black."

SourceBromskloss 12:31, 16 October 2006 (UTC)
If I had a coat like that, Id feel a bit sheepish(Or sue the dry cleaners) 8-)--Light current 13:55, 16 October 2006 (UTC)
It is a beautiful goat. I would say that it is an Angora goat, but it could also be a cashmere goat. I looked up some photos and it most resembles Angoras. We used to have a goat dairy, and had a few Angoras, but I don't know how to differentiate the breeds outside of dairy goats. I can even tell crosses to some degree. --liquidGhoul 14:37, 16 October 2006 (UTC)

The black and white colouration and pattern (black front, white back-end) is characteristic of the rare Bagot and Welsh Black-necked breeds of goat but in all the pictures I've seen these breeds never have such a luxurious coat as the one pictured. It may be a cross breed but it is certainly not pure angora = for starters the coat texture is wrong (angoras have slightly curled, dredlock apperence to their coat, every angora I have ever seen is an off-white colour (except for crosses) and the horns are different too. Cashmere goats are of no particular breed but are more a type. This goat may be a cashmere but that is not it's breed. I think it is a cross between another breed and a welsh black-neck or bagot becaue the colour pattern is so distinct. Hope this helps! It is a very cool looking goat anyway whatever it is!

pistons in combustion engines[edit]

i would like to know why all the pistons in combustion engines are cylindrical...why cant they be in some other solid shapes like cuboid, prism etc.... Siva sankar 12:45, 16 October 2006 (UTC)siva

Good question. Three tentative reasons. First, the piston has to fit the bore of a cylinder snugly, and if that's not cylindrical they'd have to come up with a new name for it (a six-cuboid engine?). No that's silly. Start over. First, it's easier (cheaper) to manufacture up to the required precision, especially the bore of the cylinder. Two, a chamber of cylindrical shape is best able to withstand the pressure. Three, if there is accidental torque on the piston – and there's always some – the cylindrical shape is the only one in which this does not result in potentially damaging friction between piston and cylinder. By the way, pistons are also cylindrical in steam engines and other heat engines, or, more generally piston engines.  --LambiamTalk 13:13, 16 October 2006 (UTC)
May be it is also easier to seal a cylindrical piston rather than the one with angular corners? The gases can escape from the improperly sealed corners... -- Wikicheng 13:17, 16 October 2006 (UTC)
Yes It would be difficult to make polygonal piston rings--Light current 14:00, 16 October 2006 (UTC)
I think you'll also find that a circular piston sweeps the largest cylinder volume with the least contact area around the skirt (and rings) of the piston. So from the point of view of minimising sliding friction, the circular piston is the winner. The same also holds true for the heat loss from the combusting gases. The cylindrical "cylinder" has the least amount of wall surface area to drain heat from the combusting gases. This also raises the efficiency of the engine. Heat loss from their narrow, high-surface-area combustion chambers is one of the reasons why Wankel engines aren't very efficient.
Atlant 14:04, 16 October 2006 (UTC)
But 'maybe' not so good for removing the heat from the hot piston. THis is one of the functions of piston rings apparently. 8-)--Light current 14:11, 16 October 2006 (UTC)
Engines often have specific oil jets aimed at the bottoms of the piston (in order to remove heat from the pistons). Me, I'm looking forward to ceramic adiabatic engines with no cooling systems. It ought to be very cool to see these glowing when you raise the bonnet.
Atlant 14:19, 16 October 2006 (UTC)
Oil jets. Sounds cool!--Light current 22:09, 16 October 2006 (UTC)
Stress is channelled into corners, and experience a lot more of it than smooth parts of the same shape, this is why a lot of high stress thnigs are round, spacecraft, missiles, gun barrels, etc.. Philc TECI 17:38, 16 October 2006 (UTC)
There have been some reciprocating engine designs that have used something other than a cylindrical cylinder. The Honda NR had an oval-shaped piston (and cylinder bore). However, this bike was designed to meet racing rules which restricted the number of cylinders the bike could have. The oval pistons allowed a larger valve area with this limited number. The Formula One rule makers promptly banned the technique from their competition, so it's never been seen in cars. Absent such regulations, the benefits are so marginal it's not worth doing, apparently. --Robert Merkel 23:48, 16 October 2006 (UTC)
I suspect it's also an easier shape to manufacture. RJFJR 20:09, 17 October 2006 (UTC)

Epidermis[edit]

What is the layer of the epidermis where the cells are considered protective but nonviable? Tyler Argue

Nonviable? You mean, dead? The stratum corneum. Simon A. 14:12, 16 October 2006 (UTC)

Fuel consumption[edit]

In an effort to improve fuel consumption, I have pumped up the tyres to the max recommented pressure, and am now experimenting with driving in high gears most of the time. I have 5 gears and the 5th is only supposed to be used above 34 mph. Will I damage the engine if I go slower that this in 5th?--Light current 13:57, 16 October 2006 (UTC)

With regard to tyre pressure: Take care that you don't wear out the center of the tyre tread abnormally early. Excess inflation (for a given weight load in the car) tends to make the center of the tread bulge, leading it to ear out first. For the same reason, over-inflation may reduce the tyre's traction (grip) somewhat. But it's certainly good for mileage.
With regard to engine RPM: Your owner's manual will probably specify a minium RPM for the engine, but that usually means "under load". You can certainly coast (or run very lightly loaded) below that minimum RPM with no risk of engine damage. Usually, cars are pretty good about letting you know when you're trying to load the engine too heavily at too low an RPM: That shuddering and bucking lets you know you're "lugging" the engine. (Is "lugging" an Americanism?)
Atlant 14:09, 16 October 2006 (UTC)
Seems like it is. Over here we use the term 'labouring' for running the engine in too high a gear. I always thought this practice used to hammer the big ends--Light current 01:55, 17 October 2006 (UTC)
Also, my handbook (Ford) doesnt seem to say much about RPM: just gives the recommended speed ranges in each gear. I know the engine idles about 900 RPM nad I can get it to go without acceleerating at 1500 RPM in top gear.

I had a friend ruin his transmission by letting it "lug" in 5th gear driving at slow rates of speed. You should be able to hear/feel your vehicle vibrate when you do this. It definately is hard on your tranny. Tyler

You've seen Fuel economy in automobiles and Fuel efficient driving, right?
Atlant 14:38, 16 October 2006 (UTC)

How you drive can also have a major effect on mileage, particularly in stop and go traffic. Here are some things I do:

  • Slowly accelerate, don't floor it.
  • Rather than braking, try to time traffic lights and let the car decelerate so you don't get to the light until it's green.
  • In heavy traffic, leave enough room in front so you don't have to brake every time the car in front brakes. This might result in people cutting in front of you, so you need to have an easygoing temper to do this.
  • Allow your speed to vary on hills, slowing down as you climb the hill and speeding up and you descend. This may be illegal in many areas with tight minimum and maximum speed requirements, however. Ironically, those maximum speeds put in place to save gas frequently cost gas instead (petrol to you Brits !). StuRat 17:05, 16 October 2006 (UTC)
Thanks Stu. I did most of that already. However, I have now started driving 'one gear up' on what I used to and get into 5th at about 30 - 35 mph. THis doesnt labour the engine if Im not accelerating. Below 30 I need to go into 4th at highest.--Light current 06:56, 17 October 2006 (UTC)
That certainly is a first - putting in a dialect note for UK people. Thanks for noting it, but it seems we're all so used to "that's gas for you Americans"'ing we're pretty fluent in the majority of US specific words, shame it's not the other way round, eh? Benbread 18:48, 16 October 2006 (UTC)


I can write an entire article about the mathematics of automobile fuel comsumption but my time is short. In short, it depends on what type of car you drive. Different techniques for different cars. You must always target the main cause of fuel consumption first. 202.168.50.40 23:21, 16 October 2006 (UTC)

For the majority of Americans, the main cause of fuel consumption is always MASS.

When you are accelerating your SUV the following occurs

1st Law: F = M * A
Energy: F * Dist = M * A * Dist
Power: F * d(Dist)/dt = M * A * d(Dist)/dt
Power = M * A * V

strictly speaking

Power(t) = M * A(t) * V(t)


In short, the main cause of high rate of fuel consumption is Mass times Acceleration times Speed.

  1. Reduce your mass
  2. Reduce your acceleration
  3. Reduce your maximum speed

Noticed that I did not talk about air resistance, that is because the main usage of engine power (in city driving) is overcoming MAV and not air resistance. Within a city, air resistance is not the main cause of fuel consumption. 202.168.50.40 23:36, 16 October 2006 (UTC)

Well apart from ripping some body panels out and swapping the engine for a lighter model, Im not sure what can be done about mass. I do carry round a 56 lb speaker cabinet, But I dont think that really makes much difference to the total mass of the car plus driver.--Light current 07:04, 17 October 2006 (UTC)

Bacterial Infection[edit]

What is the name of the bacterial infection that enters the blood and is often found in burn patients? 11:20 A.M. Tyler

Pseudomonas?
Atlant 14:28, 16 October 2006 (UTC)

Don't think its that, pseudomonas is most often acquired through the fecal-oral route when the infectee is near cows and causes intense GI issues. Do you know anything else about the bacteria you are asking about? pschemp | talk 14:38, 16 October 2006 (UTC)

No, just that it enters the blood and is often found fatal in burn patients. I don't think it's pseudomonas though, my text book says that occurs in the ear and can be treated with ear drops.

I suggested Pseudomonas because, in the book 365 Days (by Ronald J. Glasser, an American military doctor writing about his tour of duty in Viet Nam), that was the opportunistic infection that seemed to mark the end for many of the doctor's patient soldiers.
Atlant 14:46, 16 October 2006 (UTC)

Well it could be lots of things, especially the antibiotic resistant variants of bacteria like Staphylococcus aureus which is one of the causes of bacteremia. pschemp | talk 16:44, 16 October 2006 (UTC)

The correct answer would be Pseudomonas aeruginosa. Sepsis is a problem generally in hospitalized patients, and of course other bacteria, such as Staphylococcus aureus are very important concerns, but the organism associated specifically with burns in hospitalized patients is Pseudomonas. The list of organisms isolated from necrotic tissue in burn patients is, in order of frequency: methacillin resistant Staphylococcus aureus (MRSA), Pseudomonas aeruginosa, Enterococcus, Proteus species, Escherichia coli, Candida albicans and Acinetobacter calcoaceticus anitratus. Part of the reason for the affinity of Pseudomonas for burn patients is that it is often found colonizing accumulations of water (in hydrotherapy pools, ventilators, and other items found in burn units), and that the skin's ability to prevent infection has been effectively neutralized by the burns. Yes, when Pseudomonas infects the external ear canal it is called swimmer's ear and is treated with topical antibiotics, but that doesn't mean it doesn't cause other, serious or fatal, infections. - Nunh-huh 18:54, 16 October 2006 (UTC)
Yes, that's it, and it smells like rotten fruit. You can smell a pseudomonas infection when you walk in the room. -THB 03:23, 17 October 2006 (UTC)

Compact Bone[edit]

Is compact bone replaced faster than spongy bone? Susan 11:23 Am October 16/06

Hormones[edit]

Several hormones control the remodeling of bones. Which two respond to the changing of calcium levels? Tyler 12:07 16 October 2006

Tyler: I answered your question on the epidermis by simply searching for it, and I'm sure, you can do the same yourself. So, please read the top of this page: Do your own homework!. So please look through the articles on bones and hormones, check your textbook, and if you then still cannot figure out your homework question, come back and ask again. Simon A. 16:06, 16 October 2006 (UTC)

Science Fair Proposal[edit]

In my science class we are expected to creat a proposal for a science project with a controll and 2 variables. I already have that but on the paper it asks for Research in the following way which I do not understand.

Research: At least two preliminary sources of information, correct MLA format

WHAT IS MLA? Devol4 15:09, 16 October 2006 (UTC)

MLA is just the style or format that you write your paper in.

But what style is it?? Devol4 15:13, 16 October 2006 (UTC)

Have you searched for the term? If you enter MLA into the search box, you'll find the disambiguation page MLA which points you to MLA Style Manual. I suppose the latter article should help. It seems your teacher wants you to list your two sources in the format recommended in MLA Style Manual#Citation. Simon A. 16:07, 16 October 2006 (UTC)

MLA is just a way to write a citation for a book or newspaper article or magazine article. If I had a book called "A Great Book About Books" which was written by A.J. Funkhouse and published by the New York-based Alimony Press in 1923, my MLA citation would be: Funkhouse, A.J. A Great Book About Books. New York: Alimony Press, 1923. It is just a way of writing out a citation, one way of many. --Fastfission 01:47, 17 October 2006 (UTC)

Dodgy Bike Chain/Sprockets[edit]

My bike has hub gears but the sound that the chain makes as it goes round, sounds like deraillier where the gears aren't calibrated properly and the chain wants to change gear. It also feels "grindy" and occasionally jumps (my foot jerks forward while pedalling). I've taken the chain guard off but I can see what could be causing it. Any ideas? --Username132 (talk) 16:58, 16 October 2006 (UTC)

The obvious answer is a mis-calibrated deraillier. Check to see if the sound is coming from the metal which actually shifts the gear. If there is not sufficient clearance (it should not be rubbing at all), that is likely you problem. Other than that, one commonly overlooked cause of difficulty is a frozen link in the chain (which in my experience causes jumping, but rarely grinding sounds). If the cogs are new and chain is old (or vice versus, or potentially both old), there may just be slipping due to wear on the cogs or a stretched chain. My experience is this is most common when one of the elements was recently replaced. --TeaDrinker 19:20, 16 October 2006 (UTC)
Once in a while, you'll also find that the chain has gotten "stiff" and so individual links don't "hinge" smoothly. This produces symptoms that partially mimic a badly adjusted derailler but it's easy to confirm or deny this diagnosis: Just see if the entire chain flexes smoothly.
Atlant 20:18, 16 October 2006 (UTC)
i fixed a similar probelm a few days ago with this site[1]. click the "Derailleur, rear tuneup" option and it'll show you whats needed. Xcomradex 20:54, 16 October 2006 (UTC)
The question is about a hub gear, not a derailleur. Not sure if hub gears are user-serviceable, if indeed the problem lies within it. EdX20 04:36, 17 October 2006 (UTC)

Hey, I just discovered Hub gear! Looking at the diagram, they are impossible to service! You have to replace them. --Zeizmic 12:08, 17 October 2006 (UTC)

Check this out: Hub assembly video (MP4)
Atlant 23:18, 17 October 2006 (UTC)

To reiterate, the problem exists on a hub gear bike BUT this problem applies to the FIXED gears that exist between the pedals and the rear wheel. Next time the chain falls off (whenever the bike falls over), I'll check each hinge in the chain. --Username132 (talk) 13:54, 17 October 2006 (UTC)

frequency / function generator? Buying one?[edit]

how much and where?--64.12.116.74 17:47, 16 October 2006 (UTC)

You can buy new from any number of suppliers on the internet. Price can go as high as you can afford, depending on features and specifications. Ebay.com has lots of test equipment. Search "function generator" and there are over 100. Ones which say 'as is' are about $25. They might or might not work. Ones which claim to be in good working order seem to be $75 and up. Register, and you can check completed sales to see what the actual sale price was for a given model. I would avoid old tube type clunkers and kits unless you want to tinker. Older ones may not go to as high a frequency. You must determine what functions and specifications you might want. Check the seller's feedback ratings to see they have lots of sales of equipment such as you are buying, with close to 100% satisfaction. Check for shipping charges and don't be afraid to ask questions before bidding, and for the lowest price, "snipe" by bidding your maximum price in the last minute before the sale closes. Good luck. Edison 18:11, 16 October 2006 (UTC)
Be sure you understand what specs are important to you as well. If you're going to be doing testing and repair of audio equipment, you don't need frequencies above (say) 100 KHz, but you probably do want a nice, clean low distortion sine wave and you may very well want a square-wave output as well. If you're going to be doing radio frequency work, you'll want whatever carrier frequencies and modulation schemes you'll be using. With either, you'll also need to decide if you need swept-frequency capability, remote (computer) control, and all the fancier bells-and-whistles.
Once you've settled on what you really need, your choices will be more-clear.
Atlant 20:25, 16 October 2006 (UTC)
Ever considered building one? THe MAX 038 [2] function generator chip goes to 20MHz, sine, sqaure, trangle etc and you dont need many external components (just some timing Cs a nd a pot or two). A real gem of a chip from MAXIM. (or so it appears from the data sheet). Googling will get you the data sheet plus application notes.

Pickled food and tooth decay[edit]

Does pickled food contribute significantly to dental caries? I know some pickles contain sugar, but I am asking only about the acid they contain directly, assuming a low sugar food such as pickled onions without added sugar. According to the article, "Enamel begins to demineralize at a pH of 5.5", while the vinegar article gives the pH of pure vinegar as typically around 2.4. But I don't know about the pickled food after draining. Thanks. Arbitrary username 21:34, 16 October 2006 (UTC)

That doesn't sound right, 7 is Neutral, 5.5 is only slightly acidic, 2 to 3 is lemon juice and vinegar.. I'm no dentist but I'm guessing here that the "demineralisation" you are talking about isn't actually a problem, it probably only occurs in very very tiny amounts and your saliva would act to neutralise the acidity almost straight away, I'm guessing the effect is negilegable. The REAL thing you have to worry about on your enamel is Dental_plaque which is caused by bacteria and has a constant effect on your teeth if allowed to build up. Vespine 23:03, 16 October 2006 (UTC)
Finally, a question I can understand! :-P The pH is important because with a pH lower than 5.5 the minerals in enamel are not resistant to the bacterial advancement found in plaque. Further, when you eat something like sugar which result in bacteria producing lactic acid, the mouth maintains an acidic pH for about 30 minutes. During this time, tooth structures like enamel and cementum remain vulnerable. Now, if you increase the time throughout the day in which the teeth remain vulnerable, such as snacking on candy throughout work, your teeth will be more likely to develop dental caries. On a similar topic, sucking on lemon juice often also demineralizes tooth structures because of the low pH, and over a long period of time it can cause the loss of tooth structures. This would be known as erosion. - Dozenist talk 23:57, 16 October 2006 (UTC)
Thanks for the further info, but I'm afraid I haven't quite followed what conclusion I should draw from it about pickled food. Please could you clarify. Many thanks. Arbitrary username 12:23, 17 October 2006 (UTC)
Although acid foods are thought to contribute to enamel erosion, the consensus is that they do not play a role in the initiation or propagation of dental caries. Admittedly, this may be counter-intuitive, since the initiation phase of caries involves a demineralization of tooth enamel from the acidic metabolic by-products of bacteria.--Mark Bornfeld DDS 17:56, 17 October 2006 (UTC)
Thank you very much for the explanation. Would you say that, all things considered, the enamel erosion directly from acid foods is itself a significant enough consideration to make it worth limiting one's consumption, given what you say regarding lack of link with caries? I ask because in other respects unsugared pickled vegetables would appear to be a healthy snack. Thanks. Arbitrary username 12:13, 18 October 2006 (UTC)
I would say all things considered, saliva would act to neutralise the acidity of pickled food faster then it could effect your teeth significantly. Acidity of those levels is only detrimental when it is allowed to build up under a layer of plaque and affect the enamel for significant periods of time. Interestingly, I also found out that the acidity of a certain female bodily fluid is also below the tooth enamel threshold, the intake of which I would not like to limit my consumption of in fear of tooth erosion. ahem;) ... Vespine 01:50, 19 October 2006 (UTC)
Actually, saliva is quite limited in its buffering capacity. If it were all that effective in that regard, one would not see dental erosion. In fact, erosion is quite common, and may be due to various acidic foods (citrus juices, acidified soft drinks or vinegar, for example) or refluxed gastric contents-- most visibly in cases of bulimia. Short of outright erosion, acid exposure of cemental root surfaces that have become uncovered due to gum recession can exacerbate dental hypersensitivity. So, although it would be churlish for dentists to ask their patients to forgo all pickled vegetables (giardiniera is one of my favorites), it would not be wrong to place some limits on the frequency of their consumption. Remember-- anything in excess is--- excessive.--Mark Bornfeld DDS 02:55, 19 October 2006 (UTC)
Oh yeah, I apologize for not answering the question directly about pickled foods, but alas I do not claim any knowledge about pickled foods in particular! :-P And I agree with Bornfeld, saliva is not usually enough to protect teeth all the time from acidic foods. - Dozenist talk 03:01, 19 October 2006 (UTC)
Many thanks to you all for your help. Arbitrary username 09:15, 20 October 2006 (UTC)

Water[edit]

Does anybody know if water can change directly from a gas to a solid, without first becoming a liquid? I saw a diagram of the changes of states of water in my textbook, and it doesn't show gas to solid, and I'm curious. Thanks. Яussiaп F 22:27, 16 October 2006 (UTC)

Phase transition calls it deposition. Err, that's all I know, but the article does mention that it happens with snow and whatnot. -- Consumed Crustacean (talk) 22:36, 16 October 2006 (UTC)
If we assume that the phase diagram here[3] is correct, then at pressures rather lower than we're used to you can find water ice subliming directly into water vapor and vice-versa.
Atlant 22:41, 16 October 2006 (UTC), revised 13:54, 17 October 2006 (UTC)
He was asking about water vapour -> ice ;). -- Consumed Crustacean (talk) 22:46, 16 October 2006 (UTC)
The reaction is reversible ;-). Atlant 13:47, 17 October 2006 (UTC)
Once upon a time refrigerators were not frost-free. Every month or so the owner had to turn it off and scrape off or let melt the buildup of ice in the freezer compartment, which could get an inch thick if the defrosting was neglected. This was an example of water vapor (from the air, from open liquids, veggies, ice cream, meat, etc) freezing direcly from vapor phase (gas) to ice (solid) on the cooling coils, which were kept around -18 °C (0 °F). In a modern frost-free unit, I have seen the opposite happen: I fill the ice trays, go back a month later and there is only a fraction of the thickness of ice, because the automatic defrosting has melted the frost on the cooling coils and drained it away, but the solid ice has sublimated, and become vapor in the arid freezer (thence deposited on the cooling coils) without going through a liquid phase.Edison 23:35, 16 October 2006 (UTC)
This is an odd question, because I'm just now learning about this in school. Basically, this is how frost and snow forms(frozen dew is not frost). AMP'd 02:46, 17 October 2006 (UTC)

question about microwaves [EM really] and momentum transfer[edit]

I one could create a machine to focuss micro-waves to cause large scale momentum and energy transfer in air molecules, would there be and reaction on the emitter itself, i would think not right? because the momentum transfers after the emisions, this would make the EM drive unworkable as a flight system wouldnt it, have read the section but just checking if this was the problem, even if it hasnt got stuff to do with please answer question anyways curious

There's a reaction on an antenna when it produces EM radiation, and on a mirror or lens when it redirects such waves. (For the antenna, consider the system of the antenna and (eventually) the radiation from its center of gravity; it has 0 momentum to start, so it must have 0 afterwards, but the radiation has some in one direction so the antenna must have some in the opposite direction.) So you would in fact get the drive you want before the interaction with the air, and the air isn't even necessary: you can strap some lasers on the back of your spaceship and literally ride light around (well, push off of it anyway). However, photons carry very little momentum for their energy, so such engines tend to have efficiency problems. --Tardis 23:36, 16 October 2006 (UTC)
I actually think light propulsion systems (eg. using a laser) would have extremely high efficiencies - the problem would be the maximum power - if you tried to pump a gigawatt through a pen laser, it would just explode - however, öthe milliwatt that pen lasers do shoot out the end is probably transferred very efficiently into kinetic energy. You just need a super-high-capacity laser. --18.214.1.158 05:09, 17 October 2006 (UTC)
See nuclear photonic rocket, which I recently rewrote to address some of these questions. Besides, the energy of the photons that you eject does not correspond to the kinetic energy that you get in any direct fashion; energy is not a vector quantity, so there are no third law considerations. --Tardis 20:02, 18 October 2006 (UTC)

I can't understand your question (learn to use sentences, they are your friend !). However, maybe you are talking about a high power ground station sending microwaves to the underside of a spaceship, as a way to launch the spaceship. I believe this is a viable method, but does have quite a few drawbacks, like potentially cooking any occupants. StuRat 23:50, 16 October 2006 (UTC)

StuRa: He asked whether there is a reaction (physics) force on an emitter of electromagnetic waves. The answer is Yes, as Tardis already pointed out. EM waves have a momentum, but in most cases the momentum transfer is to tiny to notice. Simon A. 12:55, 17 October 2006 (UTC)
Additional remarks: For an example where the momentum of EM rardiation becomes significant, see the Mössbauer effect and its application, Mössbauer spectroscopy. For the idea to use the momentum transfer due to the reflection of light off a mirror to drive a spacecraft, see solar sail. There, it seems we are really talking about using the momentum of light in the visible frequency range. For the idea to use the momentum of photons emitted by the spacecraft, this has also been discussed, but as the power required would blow apart a laser device, they envisioned a nuclear reactor: see nuclear photonic rocket. (I just read about this for the first time in the article, so no idea whether it is accurate.) Whithin an athmosphere, there is also the idea to let a spacecraft absorb a collimated microwave beam (maybe from a ground-based maser) which heats up the air below the aborption plate. The air then pushes the aircraft further on. I think they did somewhere some not too succesful experiments but I forgot how they called it. Simon A. 13:07, 17 October 2006 (UTC)

Angle of reflection for light.[edit]

This is a homework problem I'd like to help a student with (I don't want the answer, but how to do it):

A beam of light is emitted in a pool of water from a depth of 65.0cm. How far away, relative to the spot directly above it, must it strike the air-water interface in order that the light does not exit the water?
I am really confused on this problem. I've started it by using the equation nsin(theta) = nsin(r).
I know that the index of refraction for water is 1.33 and air is about 1. But I don't know where to go from there? I'd appreciate any help.

How do we do this problem ? StuRat 23:29, 16 October 2006 (UTC)

See total internal reflection. In short, you want to find the angle of refraction to be 90 degrees (or worse, to be undefined), so the light never really makes it out into the other medium. You also want to make sure you have Snell's law correct; I don't understand your notation, although the indices are right. --Tardis 23:39, 16 October 2006 (UTC)

Thanks ! StuRat 00:22, 17 October 2006 (UTC)

The answer is 73.70cm :-). I suppose that you'll still need to show the calculations to the teacher. So this shouldn't hurt.--Wikicheng 06:50, 17 October 2006 (UTC)
Then you replaced 1.33 by 4/3. If you use 1.33 the answer is more like 74.13 cm. Our page List of indices of refraction gives 1.333, which is closer to 4/3 and produces the answer 73.75 cm. You can round all to 74 cm.  --LambiamTalk 14:37, 17 October 2006 (UTC)
Your expression for the angle of refraction will contain an arcsin. You want the argument of that arcsin function to exceed 1. Arbitrary username 12:28, 17 October 2006 (UTC)

When I do the math, I get 57 cm:

theta = arcsin(n2/n1)

theta = arcsin(1.00/1.33)

theta = arsin(0.752)

theta = 48.75 degrees

     x
  +-----+ <-water
  |  49/    surf
  |   /
65|  / <-light
cm| /    beam
  |/
  + <-light
      source

tan(48.75) = 65cm/x

1.14 = 65cm/x

x = 57cm

Did I make a mistake ? StuRat 15:00, 18 October 2006 (UTC)

Just a trivial one: the angles in Snell's law, etc., are measured from the normal to the surface, not the surface itself. So the angle in your triangle should be the complementary angle: 41.25°. Hope this helps. --Tardis 17:34, 18 October 2006 (UTC)
Thanks, that clears it up. StuRat 20:52, 18 October 2006 (UTC)