Wikipedia:Reference desk/Archives/Science/2007 April 17

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April 17

Whites of eyes

Is it true that humans are the only animals to have whites eyes (outside of the iris)? zafiroblue05 | Talk 00:04, 17 April 2007 (UTC)

Sclera, the white part of the eye, does not say! I think other animals definitely have a sclera, but I don't know if it's white. I can't get a good image of many animal eyes to check... Nimur 00:27, 17 April 2007 (UTC)

Nope - first pic I found. --Kurt Shaped Box 00:26, 17 April 2007 (UTC)

While many other animals have sclera, the sclera in humans typically take up a much larger portion of the visible eye than in other animals. At least one biologist has suggested that having larger sclera was evolutionarily advantagious to humans as it made emotions easier to read (pretty speculative though). Someguy1221 00:33, 17 April 2007 (UTC)

I don't know about reading emotion - but it does make it a lot easier to see what direction other people are looking. SteveBaker 01:35, 17 April 2007 (UTC)

Every dog I've seen has a white sclera. However the iris and pupil take up almost all the space in the visible eye socket, so you can't see it unless the animal diverts its eyes. [Mαc Δαvιs] ❖ 02:11, 17 April 2007 (UTC)

I have seen dogs, etc. move their eyes just enough so that you can see a little bit like [Mαc Δαvιs] said. Teak the Kiwi 03:09, 17 April 2007 (UTC)

You can see it in this image Image:Staffordshire Bull Terrier - Labrador Cross.JPG. SteveBaker 03:36, 17 April 2007 (UTC)

cute !

What horrible bloodshot eyes, obviously from too much drinking and paranoia. [Mαc Δαvιs] ❖ 17:57, 18 April 2007 (UTC)

Prostate health

How many times per week/day should a man ejaculate to maintain optimum prostate health? —The preceding unsigned comment was added by 88.109.222.205 (talk) 00:09, 17 April 2007 (UTC).

Too much can decrease your overall sperm count. But in all seriousness, you don't need to ejaculate weekly or daily to maintain your prostate. bibliomaniac15 00:36, 17 April 2007 (UTC)

According to a study done by the Cancer Council Victoria in Melbourne, men who ejaculated 5 or more times per week were 30% less likely to develop prostate cancer[1], as it also states at prostate. Somebody said something mean to me several days ago when I said it is unhealthy to never ejaculate in a question about abstaining. Check that one too for some quote and references. [Mαc Δαvιs] ❖ 02:06, 17 April 2007 (UTC)

Inverted glass with water holds a card

If you fill a glass with water, place a card over it and turn it upside-down, you can remove your hand from the card and the card will remain in place. I understand that this has to do with the air pressure below, but

• Is the pressure of the water above the card really less that the pressure of the air below it?
• Is the density less? (obviously not)
• Why does the glass need to be filled with water? Wouldn't it be even easier with air? (obviously not)

Clearly I haven't precisely understood the experiment (you can view a video of it here). Can someone explain it to me a little better (or is there an article on it?). Thanks!

-- Josh, 02:03, 17 April 2007 (UTC)

Uh! I think you may be misunderstanding what is happening. The glass is actually like a gigantic straw. You can drink from the straw because when you remove air from the top, a vacuum is created, and the fluid rushes up to fill that space, right into your mouth. In the glass, if the water (and card) tries to fall out, there is a vacuum of empty space at the top, and that sucks the water back up in again. [Mαc Δαvιs] ❖ 02:10, 17 April 2007 (UTC)

Also need to consider the surface tension of the water. To make it work, may need to have the glass to be within certain limits on the ratio of diameter, water depth, and empty space above the water. DMacks 02:12, 17 April 2007 (UTC)

Can this sort of thing be generalized for all fluids? Or is this a water specialty?--Ķĩřβȳ♥♥♥ŤįɱéØ 02:20, 17 April 2007 (UTC)

Atmoshperic pressure will support a column of 760mm of mercury as in a barometer, or a column of water about 30ft high. The card just allows the air pressure to act evenly over the body of the water with which it is in contact. Otherwise any light disturbance on the water surface would cause the water to pour out.

Fill a tumbler under water and then raise it above the surface. The same effect is seen, but a bit of water sloshing in and out in this case is unimportant-- its not going to destabilise the whole thing.

I presume the narrower the glass tumbler the easier this is to perform as the water cannot move about so much sideways.

Of course in very narrow tubes, the capillary effect takes over.

In regard to vacuums sucking water up a straw, isn't it more accurate to say the pressure of the surrounding water pushes water up the straw because when the person sucks (ie. inhales the air in the straw), there's suddenly a space for the water to go into? JackofOz 03:38, 17 April 2007 (UTC)

Correct. Suction is not an actual force, but more accurately, it's not that the water has someplace to go, but rather that there is no longer anything pushing it back (minor distinction). However, the water is actually pulled up slightly by the walls of the straw, although this effect is primarily observable in narrow tubes (as the unsigned comment above states. Someguy1221 03:48, 17 April 2007 (UTC)

The water serves two functions in the experiment:

• It makes a nominal seal so that the fluid inside the glass can't exchange with the fluid (gas) outside the glass, and

• It makes the experiment much more dramatic.

The glass could be filled with anything that allows the formation of a seal and has a denisty low enough that the atmospheric pressure pressing up on the card bottom can overcome the weight of the stuff inside the glass.

Atlant 12:58, 17 April 2007 (UTC)

If you believe the effect depends only on the sealing you are completely wrong. The pressure of the water is the same as the pressure of the air when you fill it in the glass. If you turn the glass around, the original pressure plus the weight of the water is pushing from above and the same atmospheric pressure is pushing from below and the card will move downward. However: The capillary effect glues the edges of the card to the glass, and the card will bend instead of moving downward. Now the crucial effect comes into play: Water is essentially incompressible. The water will lose its entire internal pressure while extending only by a tiny fraction of its volume. The air pressure continues to push from below and keeps the card from falling. The experiment cannot work with air in the glass, even if you seal the edges with water, because the air will simply have the same pressure as the air outside (It does work with light cards, because of capillary force alone, but you can put small weights on the card in both cases to see a difference).

Did I say that it depends only on sealing? Someone asked why water; I answered that. One of the reasons "why sater" is that water forms an adequate seal (for several reasons) to keep the water from escaping the glass in small runnels acted-on by gravity. If you think that doesn't matter, try doing the experiment with Helium II superfluid instead of water and let me know what happens.

Atlant 17:14, 17 April 2007 (UTC)

You said anything that allows the formation of a seal and that is not to heavy. I don't want to be offensive, but you wrote that just two paragraphs higher. As for Helium, my guess is that the card will fall down. However, my kitchen sink seems to be out of helium today. —The preceding unsigned comment was added by 84.187.32.191 (talk) 20:31, 17 April 2007 (UTC).

And I stand by that statement ("allows the formation of a seal and is not too heavy") and haven't seen anyone disclaim it. I think I'm missing your point. Rather than debate what I said, why don't state your point (again?).

Atlant 13:28, 18 April 2007 (UTC)

Here's how I think about it. (And it's a fun experiment, which you should all try, but don't make the same mistake I did just now, namely holding the inverted glass+card too high above the sink, assuming it'll work the first time, because when the seal fails and the water comes splooshing out all at once, it splashes out of the sink all over your shoes. But I digress.)

If you fill a long, thin tube or pipe with water, and seal it at the top, you will discover that the water trickles out very slowly, if at all. You can describe this in terms of suction or in terms of pressure, but it's also just a question of mass transfer. If the water is going to trickle out of the bottom, it has to be replaced by air. (And, indeed, if the long, thin tube is transparent, and the water is managing to trickle out, you will see bubbles slowly rising, moving opposite the water flow, collecting at the top as the water level lowers.)

There's no magic involved here; we don't have to posit a special force which somehow defies gravity and keeps the water in the tube (or glass). If the tube or glass were filled with clay, or cement, or silly putty, or jell-o, or blackstrap molasses, or tapioca pudding, or ice, or some other solid or very viscous liquid material, we wouldn't be at all surprised if it didn't come out. Is it defying gravity? What's holding it up? Why, we're holding it up: we're holding the tube or glass, and the stuff inside is stuck to the tube or glass, so it doesn't fall down. Simple as that.

Well, almost. Clearly there's a difference between solid materials like clay and cement and jell-o and ice, and viscous materials like silly putty and molasses, and liquids like water. Also clearly there's some kind of a difference between a thin tube or pipe, versus a wider-mouthed glass or jar. But where's the crossover point between a thin tube or a thick liquid where the effect obviously works, versus a thinner liquid or a wider vessel where we would expect the liquid to run out? Clearly there's a continuum.

And there's one more factor to consider, which several others have noted: surface tension. Water tends to stick to itself, and to the walls of the vessel it's in. Among other things, this makes it even more difficult for (say) those bubbles to rise, moving opposite the water flow, in that long, thin, transparent tube I was hypothesizing. And the surface tension also plays a role at the bottom of the column of water, at the bottom end of the tube or glass you're expecting it to flow down out of. Obviously, you would think, all the dumb water has to do is pick a side, with the water flowing down out of one side, and the air rising up the other. But the water is too stupid to make this choice. Every spot on the bottom surface of the water wants to flow down and out just as much, and because all of it is trying, none of it can. It's a lot like commuters trying to exit a subway car at a crowded station, when there are so many people waiting on the platform impatient to board that the disembarking passengers can't get past them.

So where does the card come in? We may believe that water can stay "upside down" in a thin tube that's closed at the top, as long as the tube is sufficiently long and thin (whatever that means). But clearly a glass is not long and thin enough, as we can easily prove by holding one upside down while it's full of water and noticing that the water does come rushing out.

As I think of it, the card acts as a surface tension enhancer. Since the card is relatively inflexible (even when wet), it reinforces and enables that simultaneous impatience of every point along the bottom surface of the water to be falling out at once. The water can't bulge downward (beginning to flow) at one point, while simultaneously bulging upward (about to form a rising bubble) at another, because the dumb inflexible nonporous card is in the way.

So what holds the card up? Well, it doesn't weigh very much, so the surface tension of the water does the trick. We don't normally think of water as an adhesive, but surface tension definitely makes it a weak one, as you can realize if you think about one of the ways you pick up a tiny object, too little to grasp between your fingers: just lick one finger and touch, and the object sticks to your finger.

(Disclaimer: this has been a speculative, armchair explanation, with no actual scientific sources cited or even consulted. Apologies if anyone is offended by my lack of rigor.) —Steve Summit (talk) 02:47, 18 April 2007 (UTC)

emfs and internal resistance

If I had a circuit consisting of a battery with internal resistance that is hooked up to a variable resistor, and I know the terminal voltage of the battery and the current in the wire at a set resistance, would it be possible to determine the emf of the battery?

Thanks --K=.5mv^2 02:08, 17 April 2007 (UTC)

The emf of the battery is just the open circuit voltage of the battery as measured with a voltmeter. You dont need any other stuff. However, if you are not allowed to open circuit the battery, see Internal resistance #Batteries. (replacing the apparently useful comment of a banned user Rockpocket 20:59, 17 April 2007 (UTC))

Just to put a finer point on Rockpocket's answer, the reason you can (usually) accurately measure the EMF with just an ordinary voltmeter is that voltmeters normally have an input resistance that is many orders of magnitude larger than the internal resistance of the battery. If that isn't the case for the specific voltage source you're trying to measure, you'll either need to calculate (as you suggested in the question) or use a different instrument such as an electrometer.

Atlant 13:01, 17 April 2007 (UTC)

Knife into head

I was wondering how easy or hard it is to stab somebody with a sharp knife, and penetrating their head. If you wanted to kill somebody and you had snuck up behind them, could you easily just smash the knife blade through their skull or not? I don't think the bone could be strong enough, because of the force of impact and concentration (knife point) is fairly unoptimal for the target. [Mαc Δαvιs] ❖ 03:09, 17 April 2007 (UTC)

I would expect it would depend on where you hit the skull. If you hit at a weak spot, straight on, it would go in. If you hit at a strong point, at a slight angle, it would be deflected (but still cut the scalp up). Also, I expect a human cadaver would be needed for the experiment, as animal skulls of the same size tend to be much tougher. StuRat 04:25, 17 April 2007 (UTC)

If they were standing up, I don't think the knife will go through, but you'll rip their scalp and push their head forward. --Ķĩřβȳ♥♥♥ŤįɱéØ 04:28, 17 April 2007 (UTC)

Ah, so then it would be better if you hit the rest of their body? Where would be best anyway? [Mαc Δαvιs] ❖ 05:33, 17 April 2007 (UTC)

---

Wikipedia does not provide Criminal Advice. We do not provide advice on how to kill a person most effectively, how to infiltrate your apartment while being surrounded by SWAT teams, or how to make meth in your kitchen. Ask a professional criminal instead.

---

All jokes aside though, I guess somewhere near major arteries would be a good spot. --antilived^{T | C | G} 05:53, 17 April 2007 (UTC)

Some martial arts defend against attacks of the knife held with the ice-pick grip and attacks to the head (or other parts) with said knife. BTW, take a look at where the Frontal bone and Parietal bone join... it is a spot of weakness. I suspect it is possible, if done at the right angle with the right instrument. If I tell you any more they would send someone to kill me ;-) Root⁴(one) 06:38, 17 April 2007 (UTC)

I was told by a police officer that cutting the groin is probably the easiest way to kill someone with a single knife cut - obviously due to outbleeding. 213.48.15.234 07:33, 17 April 2007 (UTC)

"If in doubt, aim for the groin." - effective on most men --antilived^{T | C | G} 09:37, 17 April 2007 (UTC)

Oh yes, you can't underestimate the "OH MY GOD YOU JUST STABBED MY GROIN" effect. Why isn't there an article on that? 213.48.15.234 11:18, 17 April 2007 (UTC)

Probably women. [Mαc Δαvιs] ❖ 18:36, 17 April 2007 (UTC)

Professionals in this business have been called "cutthroats" for centuries, I am inclined to believe that this is no coincidence. —The preceding unsigned comment was added by 84.187.17.11 (talk) 12:49, 17 April 2007 (UTC).

I suggest you Nuke them... using micro waves or something. might not be as fast but it looks alot cooler then stabbing... and how can we all forget the neck?? slashing the neck the only one that mentioned it was mr 84 ^^^. User:Maverick423 If It Looks Good Nuke It 13:25, 17 April 2007 (UTC)

No, actually it wouldn't look that cool. They would feel burns underneath their clothing. Have you checked microwave gun? [Mαc Δαvιs] ❖ 18:36, 17 April 2007 (UTC)

Guess not, because that's not an article yet. http://powerlabs.org/uwavexp.htm is the first thing that comes to mind. He's starting a page on the new one soon. [Mαc Δαvιs] ❖ 18:38, 17 April 2007 (UTC)

Done!!!! still working on it though. microwave gun User:Maverick423 If It Looks Good Nuke It 16:24, 18 April 2007 (UTC)

Yep its been talked about before maybe it deserves its own article =P if you recall last time i tried to burn stuff outside using a microwave which didnt work =( the chips and all that stuff burnt up before it did any damage. anyways a knife to the head is kinda primitive dont you think i mean with the tech we got now adays we can imaging some pretty fun (for the person using it) ideas on how to umm end a life. User:Maverick423 If It Looks Good Nuke It 21:34, 17 April 2007 (UTC)

I think the best thing for the Microwave Gun is a redirect to Active Denial System. I have mentioned this at the Talk Page. Nimur 17:10, 18 April 2007 (UTC)

well i found a place where a guy was talking about other potental uses for this kind of gun. it included this being used as a faster way of melting glue like a glue gun. im not sure its a relyable source but its a pretty good idea. who knows it might be a future device one day. well enough chit chat heres the link http://www.whynot.net/ideas/1085 u tell me what u think its on my talk page ok. User:Maverick423 If It Looks Good Nuke It 20:37, 18 April 2007 (UTC)

I've hacked up a lot of chickens in my day, and even their puny bones take a good whack with a cleaver to get through. A human skull is probably enough trouble that you should probably just let go of your anger, and remember that the best revenge is living well. --TotoBaggins 13:34, 18 April 2007 (UTC)

Prevalence of freezing temperatures on Earth

Hi

I'm looking for some numbers for an introduction about how common temperatures below OC are on Earth. I'd love to be able to say "90% of the Earth's terrestrial environment experiences temperatures lower than 0C each year" or something like that. Does anyone have any insights and/or references about this? Thanks heaps for your help!

Aaadddaaammm 03:10, 17 April 2007 (UTC)

That's an interesting question. I've never seen that information presented that way. But if you look up information about climate zones, such as the Köppen climate classification, you can find maps showing how much of the Earth's land area has each major type of climate, and you can find how the climate types are defined. (And a Google search on things like "climate" and "map" will take you to more pages.) That'll take you part way toward an answer, although getting from a map to an accurate percentage of areas won't be easy. (Ocean maps with climatic information are harder to find; you may have to settle for land only.) --Anonymous, April 17, 2007, 03:45 (UTC).

Thanks for your reply - I did consider this kind of approach, but decided it was a bit too rough for my report. I really just want a throw-away statistic to get the paragraph started, and don't want to have to justify my calculations too much. Does anyone else have anything to offer on this question? Aaadddaaammm 01:17, 18 April 2007 (UTC)

CMOS and CCD image sensor comparison

If CCD is better than CMOS sensor, then why almost all professional SLR digital camera use CMOS as their sensor? roscoe_x 06:10, 17 April 2007 (UTC)

From Charge-coupled device, "Since a very-high-resolution CCD chip is very expensive as of 2005, a 3CCD high-resolution still camera would be beyond the price range even of many professional photographers." --Wirbelwindヴィルヴェルヴィント (talk) 06:28, 17 April 2007 (UTC)

Just backing up what Wirbel said, CCDs are not cheap! 213.48.15.234 07:34, 17 April 2007 (UTC)

Then would it mean Canon Ixus is better than Canon EOS? If not, what make the EOS better? roscoe_x 08:21, 17 April 2007 (UTC)

Because the sensor is much, much, much, much bigger and hence able to receive more light and hence higher SNR ratio and hence less noise and hence less detail diminishing noise reduction. Also, not almost all professional DSLR's are CMOS based, you forgot all the Nikon ones (among others). --antilived^{T | C | G} 09:35, 17 April 2007 (UTC)

spring !

im searching for a way to figure out the optimal proportion for a helical spring to propulse a person (lets say 150 lb)kind of like a trampoline and how many of those i would need

the thing is i want to figure out what the smalest amount i could use and still have a smooth unwinding and not take more than about 1 1/2" for the spring to be able to push back the weight

i also need to find out wich material would be good

clockwork fromage —The preceding unsigned comment was added by 216.113.96.143 (talk) 06:20, 17 April 2007 (UTC).

The only thing that immediately springs (sorry NPI) to mind is that of a recoil spring in a (very) large field gun

/cannon. —The preceding unsigned comment was added by 88.111.93.83 (talk) 22:21, 17 April 2007 (UTC).

There are equations that can help predict the behavior of a spring, including Hooke's Law. There are also calculators that can help you play what-if. As you can see, there are more variables than you have specified, so it's hard to give a definite answer without knowing more about the application you have in mind. How big around can the spring be? With what force are you trying to propel this person? Does the person's weight compress the spring, or does something else? Are you going for distance? How does balance factor in?

Also, before you go propelling any human beings, please think about safety. Human beings who are propelled can fall on their faces, twist their ankles, and plenty of other things. --Dvortygirl 03:32, 18 April 2007 (UTC)

See also Spring Shoes – b_jonas 20:59, 18 April 2007 (UTC)

Genetics of ear-wiggling?

Some time after I wrote this article, others added these unreferenced claims:

1. "The general consensus is that the ability to wiggle your ears is thanks to one gene, which has been turned off for some people."
2. "Approximately twice as many men than women can wiggle their ears."

While I can attempt to describe the learning process from experience, I have no idea where I might verify or discredit these claims. Neither Auricularis nor Ear discusses these details of human ear motion. In the name of accuracy, thanks in advance for any direction you can offer. --Dvortygirl 07:19, 17 April 2007 (UTC)

Many of the facial muscles not normally accessible as voluntary can be learned. Ears are the same way, but not sure about the gene thing. [Mαc Δαvιs] ❖ 18:34, 17 April 2007 (UTC)

Effectiveness of 'low blow' against bear attack?

I reading one of those 'SAS survival guide' books a few years ago in which it was stated that as a last resort when being attacked by a bear, one should attempt to strike it repeatedly in the groin/testicular area. Just as a matter of interest - has anyone ever heard of anyone surviving a bear attack by kicking their assailant in the nads? This is the only reference I could find to someone attempting a groin attack on a bear - it didn't end well. --Kurt Shaped Box 11:39, 17 April 2007 (UTC)

Different strokes for different bears. It's not something with a high success rate. --Zeizmic 12:04, 17 April 2007 (UTC)

Have you seen the John West salmon commercial? - that seems to back it up quite well. Capuchin 12:39, 17 April 2007 (UTC)

LOL! I've been trying to remember what that advert was selling for ages now. Great to see it again. Thanks. :) It was a memory of that ad that made me start the original 'man vs. bear' thread some time back (which spawned several 'x vs. y in a fight to the death' imitators). --Kurt Shaped Box 18:39, 17 April 2007 (UTC)

Lets not forget that that commercial had a human wearing a bear suit. User:Maverick423 If It Looks Good Nuke It 13:36, 17 April 2007 (UTC)

But what if the bear likes it ? StuRat 17:05, 17 April 2007 (UTC)

You'll probably die in an even more horrible, lingering way than if the bear had just crushed your skull and gotten it over with? --Kurt Shaped Box 18:40, 17 April 2007 (UTC)

For some reason I don't think that precision strikes against large beasts would be effective, because when a 500 lb. behemoth of muscle and teeth is bearing down on you, you're only going to feel terror. Vranak

Bigger beast, bigger testicles to kick? --Kurt Shaped Box 22:44, 17 April 2007 (UTC)

I don't think testicles on animals are usually in a place you can readily kick, so it hardly matters. Vranak

Does everything sink when it goes below a certain depth in the ocean?

A log floats. If I were to drag it deep enough under water, would the water pressure from above force the log down to the bottom of the ocean, or would the fact that the cellulose molecules would always be slightly less dense than the surrounding water molecules mean that the log would always rise to the surface? Is there some point of neutrality at a certain depth that would hold the log stationary until Brownian motion bumped it either up or down enough? — Jonathan Kovaciny (talk|contribs) 13:42, 17 April 2007 (UTC)

The pressure at depth would have no directional vector (so would not force a log down/up or any direction) although it might cause the cells in the log to become more compressed and thus the log to be more dense. If the log became more dense than the surrounding water it would sink. Objects less dense than water will always float.David D. (Talk) 14:54, 17 April 2007 (UTC)

The same would happen to the water though, as the pressure (and depth) increase, the density will as well, so the water may become more dense than the object. In this situation, there would be a depth where the object becomes neutrally buoyant, and it would remain there. anonymous6494 16:35, 17 April 2007 (UTC)

Water will not compress as easily as wood (which has a lot of air space in the dead cell chambers) so the density of water will not increase as much as wood due to depth, if at all. Changes in water density are primarily due to temp and composition (salt and nutrients). There is a good figure here that confirms the density does not change significantly with depth. http://www.windows.ucar.edu/tour/link=/earth/Water/density.html&edu=high David D. (Talk) 20:09, 17 April 2007 (UTC)

No, not everything sinks when it gets to a certain depth. Some things always float (Styrofoam), some things always sink (lead cannonballs), some things will find a natural level within the water (some plants), and some will sink once they reach a certain depth (a submarine that implodes at that depth). StuRat 17:02, 17 April 2007 (UTC)

In the case of the submarine, it doesn't necessarily sink. At surface, the submarine consists of the metal hull and the air inside. When it implodes, the air part quickly rises and the metal part quickly sinks. So, to be annoyingly technical, the submarine separates into the parts that float and the parts that sink. --Kainaw ^(talk) 18:50, 17 April 2007 (UTC)

Or, if the submarine is at sufficient depth and the air content of the water is low enough, the air might dissolve into the water before reaching the surface. StuRat 19:21, 17 April 2007 (UTC)

I dont think things usually find a "natural level". If something is even a little compressable, and especially if it has gas pockets, then neutral buoyancy is an unstable equilibriam. Nearly everything ends up either floating or sinking - unconscious divers for example.Polypipe Wrangler 21:56, 17 April 2007 (UTC)

I'll take this opportunity to recommend again the excellent book, "The Flying Circus of Physics", which addresses questions like this in spades. One item is on the topic of why a crate will sometimes float face-up vs. corner-up in the water. --TotoBaggins 00:01, 18 April 2007 (UTC)

Most buoyant materials are relatively compressible. Water is relatively incompressible. So, yes, most things lose buoyancy as they sink, due to compression. Scuba divers are intimately familiar with this: as you descend, your wetsuit compresses and becomes less buoyant, and you have to continually add air to your buoyancy compensator to, er, compensate. (The compressed wetsuit also doesn't insulate as well, which sucks, because the water generally gets colder as you get deeper, too.)

Deep-rated submersible vehicles use special, noncompressible buoyancy materials, such as syntactic foam.

In answer to the original question, I suspect that most materials would sink past a certain depth, not because the extreme pressure "forces them down", but because the extreme pressure would eventually compress virtually any material below its buoyancy point. (There are exceptions, though they are few: deep-rated submarine pressure vessels and syntactic foam are two.) —Steve Summit (talk) 02:09, 18 April 2007 (UTC)

The question is not whether the material is compressible, but whether it's more or less compressible than water. If you have a chunk of something that's slightly more dense than water at atmospheric pressure, but less compressible than water, then it is possible that it would be negative buoyant at the surface, but that when it sinks to a certain depth, the water would have become dense enough to make it neutrally buoyant, and there it would stop. I think there are lots of solids that are less compressible than water. The rare property is going to be that the substance is more dense than water at 1 atmosphere, but by such a slight margin that it's less dense than water at the bottom of the ocean. --Trovatore 02:20, 18 April 2007 (UTC)

Good point. And this reminds me of Galileo's thermometer -- I wonder how applicable that is to this discussion? —Steve Summit (talk) 03:27, 18 April 2007 (UTC)

Collegues, pleaselook at the bathyscape article. A bathyscape consists of a tank of (relatively) incompressible oil tank, controllable ballast, and a heaviar-than water payload. If a substance is (relatively) incompressible, its tendancy to sink will not increase with depth. -Arch dude 03:29, 18 April 2007 (UTC)

No one seems to have mentioned Archimedes principle yet. —The preceding unsigned comment was added by 88.109.207.222 (talk) 10:08, 18 April 2007 (UTC).

At what depth will the pressure cause a bubble of air to sink instead of rise? How would i go about calculating this? User:andbir

microbiology

Why is it important to differentiate glucose nonfermenters from Enterobacteriaceae? —The preceding unsigned comment was added by 209.213.220.109 (talk) 16:08, 17 April 2007 (UTC).

My faith in science would be seriously shaken if I thought the future microbiologists of the world were getting their homework done by a bunch of Internet dweebs. --TotoBaggins 23:57, 17 April 2007 (UTC)

do fish like rain

I am curious to know if fish like rain, or how they react to rain. —The preceding unsigned comment was added by 69.236.22.85 (talk) 17:22, 17 April 2007 (UTC).

Well, it is rather obvious that fish don't mind getting wet. However, the raid has different effects on them. It creates a darker atmosphere when it couds over - making it harder to see (though they can smell just fine). If the water is warm, a thin cool layer of water floats on top of the warm water for a bit - most fish don't care for that. They stay in the warmer water. If it is cold, a thin warm layer of water floats on top of the cold water for a bit, which draws fish closer to the surface. Heavy rains will cause water levels to rise. Fish tend to rise with a rise in water levels. Then, if it is a river, the current picks up. The fish have to swim against it or get swept downstream. That's a just a few things to think about. There are surely a lot more - such as the effects of acid rain on fish. --Kainaw ^(talk) 18:47, 17 April 2007 (UTC)

I could also imagine the the action of raindrops impacting on water would increase the oxygen content a little bit.-Czmtzc 18:54, 17 April 2007 (UTC)

In brackish waters, as in the Florida Everglades, rain tends to decrease the salt content in the water (with high tides increasing it again). StuRat 19:17, 17 April 2007 (UTC)

Rain would make lots of noise too, making it harder to hear other things.Polypipe Wrangler 21:59, 17 April 2007 (UTC)

Hypoxic tents

I searched a bit on these tents but they are pretty much too expensive for a poor student like myself, are there any good low budget methods to have similar results?Bastard Soap 17:47, 17 April 2007 (UTC)

You could put together the component parts and it would be cheaper, but still expensive. You need an Oxygen tank, a Nitrogen tank, an air quality regulation system, and a semi air tight tent and constant refills for the gas tanks. Personally I would not want to use an amateur Hypoxic tent because I would not trust the oxygen concentration. What if your Oxygen tank ran out before your Nitrogen tank? You may never wake up. Czmtzc 18:02, 17 April 2007 (UTC)

Also, in the presence of pure oxygen, many normally nonflammable things become flammable, like human flesh. So, only those with proper training should handle oxygen tanks. StuRat 19:13, 17 April 2007 (UTC)

Tapping on the fishtank...

I used to have a terrible habit of tapping on the glass of a fishtank whenever I happened to be passing one (I guess to make the fish look at me - which never worked anyway). I've been told off on more than one occasion for doing so and told to stop as "you'll kill the fish". Why does tapping on the glass harm the fish? --Kurt Shaped Box 18:55, 17 April 2007 (UTC)

The thundering booms from your taps cause stress which reduce immunity levels which leads to disease and eventually death. --Kainaw ^(talk) 18:58, 17 April 2007 (UTC)

Can you give a source for that? It sounds like an urban legend to me. I know that there have been stress related diseases with pigs in very bad living conditions, but fish? Just from hearing sudden loud sounds?

(And pigs eat their own young if the conditions are stressful)! Nimur 17:18, 18 April 2007 (UTC)

They can sense that you're a reincarnated seagull? Clarityfiend 23:06, 17 April 2007 (UTC)

Sometimes I do feel like I was a seagull in a past life. Did I ever mention that I can copy gull noises so well that the gulls will sometimes respond to them? ;) btw, the response you replied to wasn't mine... --Kurt Shaped Box 23:10, 17 April 2007 (UTC)

Asymptomatic carriers

Hello, me again. I'd like a bit more information on disease carriers; specifically, whether an offspring of someone with a recessive disorder will 'carry on' being a carrier of the disorder through three more generations. Lady BlahDeBlah 20:28, 17 April 2007 (UTC)

If this is a disease that's inherited through simple mendellian genetics, the child of a carrier will have a one half chance of becoming a carrier himself. This can continue without end through any number of generations. Someguy1221 20:02, 17 April 2007 (UTC)

So...a carrier who marries a normal person and has a child, that child will still/could be a carrier himself, and so on. Right? The disease I'm particularly thinking of, if it'll help, is what I got here last time I asked a question: Leber's congenital amaurosis. Lady BlahDeBlah 20:28, 17 April 2007 (UTC)

Precisely. The child of a carrier and a noncarrier has a 50% chance of being a carrier, 50% chance of being a noncarrier. And any children who are carriers will have their own children follow the same probability. If a carrier were to marry another carrier, the children have a 50% chance of being carriers, 25% chance of being normal, and a 25% chance of inheritting the full blown disease. How long it's been since someone actually had the disease is irrelevent, and this is how you get carriers who have no idea that they even might be carriers. Leber's congenital amaurosis appears to be inherited in this fashion. Someguy1221 20:42, 17 April 2007 (UTC)

Just to clarify, there are actually as many as six genetically distinct forms of Leber's congenital amaurosis (caused by mutation in different genes, including retinal guanylate cyclase and the RPE65 gene). This leads to the slightly odd situation where two parents who appear to suffer from the same genetic disease, can produce completely unaffected children. See PMID 12015276 for details. Rockpocket 21:14, 17 April 2007 (UTC)

Colleagues, we are conflating Genetic disease with communicatable disease. A genetic disease is communicated via sperm or egg. The offspring will inherit the disease genetically from the father or mother via nuclear DNA, if the disease is "(nuclear) genetic", and from the mother, if the disease is mitochondlear. Under extremely rare conditions, the offspring might inherit a mitochondlear disease from the male parent. The offspring can become infected with a bacterial,viral, or fungal disease from the mother, completely independently from the act of conception: the mother and offspring are in intimate contact, permitting a pathogen to move from mother to offspring. In species where father and offspriong are in intimate contact, a pathogen can move from father to offspring. -Arch dude 02:34, 18 April 2007 (UTC)

I don't believe the person posing the question was referring of a communicable disease, due both to their use to the term recessive disorder and their clarification they are interested in Leber's congenital amaurosis. Rockpocket 03:19, 18 April 2007 (UTC)

Uh, yeah, I don't think my genetics Babelfish is working, Arch dude. Rockpocket's right, I was only referring to a specific disease. Lady BlahDeBlah 18:57, 18 April 2007 (UTC)

And genetic diseases are not "communicable". "Communicable" has a specific meaning in medicine, and it doesn't cover genetic transmission. - Nunh-huh 02:22, 19 April 2007 (UTC)

v = frequncy  ?

I was taught that "v" stands for frequency, measured in hertz. How come it's not in the article "v" ? --24.76.228.161 22:19, 17 April 2007 (UTC)

No its not 'v', its the greek letter 'ν' (nu) which admittedly looks a bit like a 'v' but isnt! For instance E=hν. The photoelectric equation. —The preceding unsigned comment was added by 88.111.93.83 (talk) 22:26, 17 April 2007 (UTC).

The greek letter nu (ν) is often confused for vee (v), so many texts choose to represent frequency by the letter "f" to avoid mistaking it for velocity or voltage (as velocity and voltage are typically represented by v and V respectively). Someguy1221 22:29, 17 April 2007 (UTC)

In Arial the characters are identical, unfortunately. The relevant link is Nu (letter). --24.147.86.187 23:38, 17 April 2007 (UTC)

I was wondering why it looked different when I was editing it but then it looked identical here...Someguy1221 00:00, 18 April 2007 (UTC)

Oh, that explains it all...thanks. --24.76.228.161 00:53, 18 April 2007 (UTC)

This is why I prefer to write E = ℏw ... er... make that E = ℏω. -- mattb 06:02, 18 April 2007 (UTC)

Dust Settles... Or Does It?

If you were to completely isolate your bedroom (airtight, no vibration, no light etc.), would the initial airborne dust particles eventually all settled down on something (bed, desk, chairs, floor), or would some remain airborne infinitely?--JLdesAlpins 23:25, 17 April 2007 (UTC)

If they are heavier than air, they should settle, yes. --24.147.86.187 23:39, 17 April 2007 (UTC)

Perhaps Brownian motion would play a role? --TotoBaggins 23:58, 17 April 2007 (UTC)

I suppose it's theoretically possible for brownian motion to keep particles airborne indefinitely - but it's really unlikely. We know that typical house-dust settles because clean, horizontal surfaces end up covered in dust - which must have settled out of the air. So 'normal' house dust would definitely settle out eventually. SteveBaker 23:59, 17 April 2007 (UTC)

I guess it depends on your definition of "dust". Presumably a particle of a similar size to the air molecules would easily stay airborne forever, and dust bunnies fall quickly, and somewhere in between something might stay aloft for a good while. --TotoBaggins 00:06, 18 April 2007 (UTC)

My sense is no, that microfine dust particles would stay aloft indefinitely, as minor variations in heat distribution in the structure of the house itself, from the sun and winds moving around outside, will keep the air moving, just a little. Vranak

Interestingly, your body heat is easily enough to stir up dust and pollution, so if you have the right imaging techniques, you can 'see' your own plume of circulating air stirring up the dust, even if you are sitting still. Carcharoth 10:41, 18 April 2007 (UTC)

An airtight room or chamber could still have temperature differentials, which would cause convestion. The outside temperature might vary 20 degrees F (11 deg C) each day. The current of air along the walls and especially along the window panes could probably stir up smaller dust particles. Edison 13:46, 18 April 2007 (UTC)

Furthermore wouldn't dust particles -- those that come from skin and hair anyway -- eventually decay down to such a small level that they would become a part of the air itself? That's what it smells like anyway, when I enter a house that's been sitting vacant for a while. The air inside definitely isn't clean and fresh, with all the dust settled out. Vranak