Wikipedia:Reference desk/Archives/Science/2007 May 16

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May 16

One Ounce

I was curious thinking about railguns and coilguns, if an object weighing one ounce was accelerated to 99% the speed of light how much damage could it do smacking into Earth? Or even would about an atom traveling at 99% the speed of light how much blow would that pack? —The preceding unsigned comment was added by 67.127.166.135 (talk) 00:25, 16 May 2007 (UTC).

You can calculate this using Newton's equation F=MA, where in this case, F is the force of the impact (in Newtons), M is the mass of the object, in Kilograms, and A is the deceleration of the object- in this case, 99% of the speed of light, divided by the time it takes for the object to come to a complete stop. For ease of calculation, I'll assume that a 100g object traveling at 99% of the speed of light comes to a complete stop in one second. In this case, the object would exert 29679453.342 Newtons of force on the earth. Ninja! 00:38, 16 May 2007 (UTC)

For ease of comprehension, this is roughly 6,672,238 pounds of force. Ninja! 00:41, 16 May 2007 (UTC)

I think a 1 sec decel time is too high, since it would travel 150,000 km in that much time. How about a 0.001 decel time, so it will stop in 150 km, instead ? In that case, multiply your answer by 1000. StuRat 00:56, 16 May 2007 (UTC)

More relevant than the force is the amount of energy dissipated in the impact. According to the Special relativity page, the kinetic energy (total energy minus rest energy) is $(\gamma -1)mc^{2}$ where $\gamma ={\frac {1}{\sqrt {1-(v/c)^{2}}}}$. Plugging in v/c=0.99 and a mass of 0.03 kg (about one ounce) gives a kinetic energy of 1.6x10¹⁶ J, which our Orders of magnitude (energy) page claims is comparable to the amount of energy dissipated by the meteorite impact that created Meteor Crater in Arizona. --mglg_(talk) 01:06, 16 May 2007 (UTC)

F=MA is not valid in relativity, see Special_relativity#Force. Icek 12:15, 16 May 2007 (UTC)

Wow, I am kind of disapointed that it doesn't plow through the Earth but hey Meteor Crater is a pretty massive hole. Thank you for the quick responce.67.127.166.135 01:23, 16 May 2007 (UTC)

As for the atom, it would just crash into an air molecule in the upper atmosphere and create a little shower of elementary particles. Most of those particles would decay on the way down, and only a few muons would reach the Earth's surface. Hydrogen and helium atoms (or at least their nuclei) traveling at highly relativistic speeds hit Earth in exactly this way all the time – see cosmic ray and Ultra-high-energy cosmic ray. --mglg_(talk) 01:33, 16 May 2007 (UTC)

Maybe we would get lucky and produce some antimatter!!!!!!!!!!!!!!67.127.166.135 05:18, 16 May 2007 (UTC)

Quite possibly - but that would immediately combine with some of the handy normal matter lying about the place and turn into a bunch of photons and probably, yet more exotic particles. Antimatter appears and is annihilated all the time, everywhere - it's not that odd. SteveBaker 11:34, 16 May 2007 (UTC)

Steve, you must be mistaken, look at the number of exclamation points. 213.48.15.234 13:57, 16 May 2007 (UTC)

I think it probable that antimatter would be created - there's a huge amount of energy involved, and a matter-antimatter pair can be made from a pair of high-energy photons (photons - which are the particles associated with light, x-rays, radio waves, etc - would very likely be created and be of high energy during high speed particle collisions, like in a supercollider). This might create, for example, an electron and a positron (anti-electron). Of course, as soon as these antimatter particles interact with normal matter, they will annihilate each other and will produce another pair of high-energy photons (gamma rays). The antiparticles will be very short-lived, probably disappearing in less than a second. --h2g2bob (talk) 14:44, 16 May 2007 (UTC)

It seems like it would resemble an H bomb explosion closer to the ground, except for the gamma rays and fallout. TNT equivalent says that a megaton of TNT is equivalent to 4.184 x 10¹⁵ J, so the energy stated above of 1.6 x10¹⁶ J would be equivalent to the detonation of a 3.8 megaton H-bomb. The Tunguska event is thought to be the result of a much larger asteroid travelling much slower, about 1/29700th as fast. The kinetic energy varies as the square of velocity and directly with mass, so the little projectile packs quite a punch for its size. The Tunguska object is estimated as tens of meters across, depending on composition. The Tunguska object is said to have been the equivalent of a 10-20 megaton nuke. There are eyewitness accounts of the Tunguska object from a survivor 40 miles away, who said it appeared as a fire which split the sky in two creating a wave of heat followed by a blast wave and huge noise. Of course no naturally occurring object in the universe is expected to be travelling at .99c relative to the earth (is it?), and we certainly have no present technology to impart that much energy and velocity to a 1 oz object. If you had a 1000 megawatt stationary nuclear plant working full time powering lasers to drive a space sail,and ignoring inefficiencies and losses and the object likely being vaporized by the focussed energy, it would take about 6 months to accelerate the object to .99c, if my math is right (no guarantees). That much solar energy would fall on a 1 kilometer square area of the moon in direct sunlight. Edison 17:10, 16 May 2007 (UTC)

Edison, re your statement/question Of course no naturally occurring object in the universe is expected to be travelling at .99c relative to the earth (is it?): If you count really small objects, there certainly are ones traveling very much faster than that relative to Earth. According to the article Ultra-high-energy cosmic ray, there have been at least 15 recorded instances of particles (protons?) hitting the Earth at speeds on the order of 0.999999999999999999999995 c. --mglg_(talk) 17:25, 16 May 2007 (UTC)

By "object" I was thinking of the 1 ounce hypothetical projectile. I would certainly extend the concept down to a BB, a paperclip, a Mustard seed, a flea, or a gossamer wing. I was not thinking of photons, protons, gamma rays, etc, which fall a bit short of my notion of "object". Edison 20:57, 16 May 2007 (UTC)

How much antimatter could be produced with the one ounce object, I wonder?67.126.131.253 05:06, 17 May 2007 (UTC)

1.6 x 10¹⁶ J/c² = 0.18 kg. That's the maximal amount of matter and antimatter created, and since the amounts must be equal, the maximal amount of antimatter would be 0.09 kg. That's only an upper limit, the amount created would probably be considerably less. Icek 08:35, 17 May 2007 (UTC)

What if, instead of a one-ounce object, you used a 1-cubic-centimeter section of a neutron star's core? Methinks that would be some serious carnage. - 2-16 14:03, 17 May 2007 (UTC)

Taking a middle value for density from the article, it looks like it's $3.5\times 10^{12}$ oz/cm³, so just multiply all the numbers by that. Of course, at that density and being so hard to stop, it might very well just pass through the Earth (in about 45 milliseconds) and leave a hole, rather than "explode" so much. --Tardis 15:06, 17 May 2007 (UTC)

Except that your chunk of neutron star would of course have exploded back to normal matter as soon as you took it out from the extreme pressure inside the neutron star. --mglg_(talk) 17:21, 18 May 2007 (UTC)

You're right, of course; I was just thinking about that substance, not its possibility. But there's still a solution; just move it so fast that in our reference frame it doesn't have time to explode! --Tardis 17:25, 18 May 2007 (UTC)

True, though at a final speed of only 0.99c it would have plenty of time to explode once inside Earth. Now that could be ugly... but it seems like cheating to consider non-kinetic energy here. If we assume that the chunk is somehow stable at its neutron-star density, it does seem reasonable that it would pass through Earth without stopping: it would weigh 30000 times more than the material it would directly hit while going through Earth. But it would certainly not pass through unnoticed. Wouldn't it deposit something like the same amount of energy per unit length all the way through the Earth as the meteorite did in the short distance it took it to stop a 1 cm³, 0.99c meteorite would in the short distance it would take to stop? --mglg_(talk) 19:00, 18 May 2007 (UTC)

Macadamia nuts - inedible to everything except for man and parrot...

As I understand it (I was reading about this today), the only other animal besides man capable of cracking open a macadamia nut is the Hyacinth macaw. Considering that bird and tree live on different continents and never encounter each other in the wild, how is it then that the macadamia tree disperses its seeds? --Kurt Shaped Box 00:57, 16 May 2007 (UTC)

I would speculate that they fall off the tree and grow new trees right there. Lucky nuts might get washed down into a river and deposited in a nice place to grow. StuRat 01:14, 16 May 2007 (UTC)

I always thought that it was a Bad Thing for a tree's offspring to grow beside the parent - because of competition for nutrients and risk of pollination incest. I dunno, do these trees tend to grow on riverbanks? I've never actually seen one. --Kurt Shaped Box 01:25, 16 May 2007 (UTC)

It doesn't need to be on a river bank, a flood plain will do nicely. StuRat 03:08, 16 May 2007 (UTC)

Notice how you said "only other animal". Natural decomposition of the nut is not out of the question.--Kirby♥time 03:00, 16 May 2007 (UTC)

A HYACINTH MACAW CAN CRACK A MACADAMIA?!?!?! Are you serious??? I used a hammer and an industrial vice to crack those suckers after we broke our proper nut cracker on the 1st one. Vespine 07:07, 16 May 2007 (UTC)

Sure thing - video I found here. They're apparently a hyacinth macaw's favourite food. --Kurt Shaped Box 10:52, 16 May 2007 (UTC)

Just to be sure, I threw "macadamia seed dispersal" (without quotes), in Google (TM), and the first thing that came up was a JStor article about an Australian bird that disperses seeds: "However, some plant species, e.g., those of Macadamia, Dendrocnide and Aleurites, seem to be dependent on small indigenous rodents (mainly Rattus, Melomys, and Uromys spp.)..." The article is was called "Seed Dispersal by Cassowaries." I assume, like most seed dispersing animals, the birds and rodents (the article is a little unclear, it seems both may disperse the seeds) eat the flesh of the fruit and let the seed pass through (or discard it in the case of the rodent). Remember, if a bird actually eats the nut than there is nothing to germinate. Mystery solved thanks to Google (TM) once again. --Cody.Pope 08:19, 16 May 2007 (UTC)

Oh, of course! Thanks. I was thinking wrong - the macaws (and humans!) go against the plant's 'wishes' by simply destroying the entire fruit. Presumably the macadamia evolved such a tough coating to avoid this very situation with the local parrots and cockatoos, which are less powerful in the beak department. --Kurt Shaped Box 13:38, 16 May 2007 (UTC)

Have you seen "Clever Crows" on Google video? It may well be that these crows would be able to eat Macadamia nuts as well.

Atlant 11:44, 17 May 2007 (UTC)

Intelligent Design, my wagon!

Please explain to me the evolutionary history of hair on the behind... what purpose does it serve other than to stick to the exit crowd and cause a lot of pain?--Kirby♥time 01:31, 16 May 2007 (UTC)

http://www.skidmore.edu/scope/spring2004/features/hair.html David D. (Talk) 02:44, 16 May 2007 (UTC)

Potentially offensive comment moved here: [1]. StuRat 03:12, 16 May 2007 (UTC)

Well, we used to be hairy all over, for warmth, I assume. We've lost most of our hair, actually, so it's not that hair on the behind serves a specific purpose, but instead it hasn't been detrimental enough for it to be selected against. Wait a few hundred thousand years, and maybe butt hair will be gone. ;-) zafiroblue05 | Talk 02:46, 16 May 2007 (UTC)

They actually have a scientific name, you know: perianal hair (which we don't have an article on, unsurprisingly). Rockpocket 05:09, 16 May 2007 (UTC)

Potentially offensive comment #2 moved here: [2]. StuRat 03:12, 16 May 2007 (UTC)

Kinda like eyelashes for the butt, to keep detritus and pathogens from entering the rectum? Or, pubic hair informs me that perianal hair is androgenic, and does not show up until the pubarche stage of puberty; perhaps, then, its appearance is due to some quirk of biological similarity between the anal region and the places where androgenic hair actually does/did serve some purpose? Butt all this is just shooting in the dark. Eldereft 06:29, 16 May 2007 (UTC)

Didn't read your comment before I posted below, but I agree on the significance of the location/age of perianal hair growth vis-a-vis pubic regions. --Cody.Pope 08:48, 16 May 2007 (UTC)

Just to add-in here: humans didn't "lose their hair" per se, we still have the same amount of hair follicles as you'd expect on a big mammal, it's just that our hair isn't as long/thick. So the real question is, why did we keep long hair there? And the best answer that I can some up with is the old pheromonal argument for underarm/pubic hair -- that region is just close enough to retain some hair (perhaps even controlled by the same region of genes, in so much as losing perianal hair might mean losing pubic hair as well, but that's just speculation). I've also heard Nurse Sue of Talk Sex claim that pubic hair helps reduce friction during copulation, but not sure how that would apply here. --Cody.Pope 08:00, 16 May 2007 (UTC)

Gotta say this is the first time I've seen "wagon" as euphemism. —Tamfang 19:22, 19 May 2007 (UTC)

Proof of Potential Difference and Resistance Formula

Hello. Can anybody prove the potential difference formula, V = E/Q and the electrical resistance formula, V = I * R? Thanks. --Mayfare 02:48, 16 May 2007 (UTC)

I'm not sure the former needs to be proven since it's just the basic definition of a certain physical quantity we call electrical potential. The latter is also just a definition; resistance is defined as the amount of electrical potential drop per unit current. In terms of the linear behavior implied (a constant R), it becomes more of an observation that somewhat holds on the macro scale, it being the definition of a certain type of current-voltage characteristic observed in many circumstances (so-called "ohmic" behavior). -- mattb 03:48, 16 May 2007 (UTC)

See Maxwell equations. The divergence of the electric field equals the charge enclosed. From that 1st principle and the magnetic field equaiton (del cross H), Ohm's law can be derived. --Tbeatty 04:00, 16 May 2007 (UTC)

Spiders of Oregon

We were working in the yard tonight and saw a mean looking spider. It looked like a black widow, but less intensely black, more of a dark chocolate brown. It did not appear to have the red hourglass either. We thought maybe brown recluse, but apparently these are not endemic to Oregon. We live just outise of Portland. Any other thoughts on what the species of spider this might have been? thanks! —Gaff ^ταλκ 02:59, 16 May 2007 (UTC)

Maybe a brown widow? Wiwaxia 04:56, 16 May 2007 (UTC)

hmm...at first I thought this was a joke reply. But then I looked it up. According to WP, the brown widow, Latrodectus geometricus, does not live in Oreogn. Thanks though.—Gaff ^ταλκ 05:40, 16 May 2007 (UTC)

False black widow? --24.147.86.187 13:04, 16 May 2007 (UTC)

I am in the same area as OP, I bet it is Neoscona arabesca or a similar species of orb weaver. They are found just about everywhere in North and Central America. Google that name for some pics, they look a lot like the ones I have in my yard. 161.222.160.8 23:35, 17 May 2007 (UTC)

Evaporation of Raid

How long does it take Raid to completely evaporate after it is sprayed? Wiwaxia 04:56, 16 May 2007 (UTC)

I don't think insecticide really evaporates, if you mean how long before you can't smell it? or how long before it won't kill insects any more? Those questions are highly subjective and they would depend on many things like the size of the room, if a window or door is opened, what kind of insects you are killing, whether you are spraying it on a surface. I'm sure the company that makes it will make all sorts of claims about useful lethal duration and stuff but I'm sure you'll find their claims are just high estimates. Vespine 07:01, 16 May 2007 (UTC)

My mother told me that Raid actually evaporates rather than just losing its potency, and that it is highly "volatile". Wiwaxia 06:22, 17 May 2007 (UTC)

If you follow the links from the article you can ask them yourself.--Shantavira 08:13, 16 May 2007 (UTC)

It's likely that the water content evaporates quickly, while oily residues and other complex chemicals which were in solution may stay behind "indefinitely" (at least, on time-scales compared to water evaporation). Nimur 08:41, 16 May 2007 (UTC)

Bitten cat

what to do if a house cat gets bitten by a black widow?—Preceding unsigned comment added by 72.193.165.80 (talk • contribs)

Take it to the vet immediately. --Cody.Pope 09:30, 16 May 2007 (UTC)

Cobras

How can we get rid of Asian cobras under our house in Krabi, Thailand. We have seen several large skins. —The preceding unsigned comment was added by 222.123.98.98 (talk) 09:11, 16 May 2007 (UTC).

Dunno about Cobras and I've never been to Thailand - but here in Texas, the usual cause of large poisonous snakes taking up residence is because you also have a rodent infestation. According to our article on cobras, King cobras eat other snakes - it doesn't say what other cobras eat - but they are carnivores - so I'd bet that rodents are a large fraction of their diet. Get rid of the rats and mice - and the snakes will leave to look for food elsewhere. So I'd definitely try rat and mouse poison. But if you havn't seen any actual snakes, it's possible that the cobras just like the space under your house for shedding their skins - and that they leave to hunt elsewhere between shedding. If all else fails, I guess you could get a Mongoose. SteveBaker 11:25, 16 May 2007 (UTC)

Put them on a one-way flight to Los Angeles. - 2-16 12:42, 16 May 2007 (UTC)

lol!! Rangermike 14:48, 16 May 2007 (UTC)

It would kind of have to be a one-way trip, wouldn't it ? Who would want to take the return trip on that airline ? ("...well, they did have good peanuts"). StuRat 23:14, 16 May 2007 (UTC)

put them on a plane with Samuel Jackson "will someone get these damn snakes off the plane!" lol!--Lerdthenerd 08:23, 17 May 2007 (UTC)

Use of flowers

I kindly request you to tell me uses of the rose and the jasmine(mogra) flower. Are these flowers used for pollination? If yes then do these plants produce seeds?

-I will be very grateful to you for answering this question? _____________________________Thanking you________________________ 202.71.137.235 12:11, 16 May 2007 (UTC)Hemchandra

AFAIK all flowers are used for pollination, then produce seeds, that's their entire purpose. StuRat 23:09, 16 May 2007 (UTC)

(After edit conflict) I don't understand your question. Especially, the question if these flowers are used for pollination looks somewhat odd. Flowers do pollinate each other if they are genetically close to each other (that's to say, of the same kind). The actual carrying of the pollen is done mostly by insects, or by wind.

Could you please rephrase your question? 84.160.208.134 23:29, 16 May 2007 (UTC)

Flowers can also be useful in human "pollination".

Atlant 11:48, 17 May 2007 (UTC)

Atlant, you're a pistil. StuRat 23:24, 18 May 2007 (UTC)

calotropis gigantia

why do snakes love this plant. 202.71.137.235 12:13, 16 May 2007 (UTC)Thx Raaaaaaajukumaaaar

do they? I could find no reference to that fact. If you have any reference or link to give, by all means please do. BTW the correct spelling is Calotropis gigantea, not gigantia. The plant is used in traditional medicine to treat snakebite [3], but I found nothing on snakes being attracted to it. Cheers, Dr_Dima.

Physics

Why doesn't the frequency change from one medium to another while wavelength does? —The preceding unsigned comment was added by 210.212.10.130 (talk) 14:30, 16 May 2007 (UTC).

The frequency of a wave (how many oscillations it makes per second) must everywhere stay the same as the frequency of the source that produced the wave. If you think of a loudspeaker membrane making a sound wave, for instance, it does so by vibrating in and out with some period T. Since the motion is periodic, there is no difference between the 1000000^th time the membrane reaches its maximum-out position and the 1000001^st time it does so, a time T later. Since there is no difference at the source between those times, there should be no difference anywhere else in the propagating wave either between those times. Therefore the whole wave is periodic in time with period T, or, in other words, it has a frequency f=1/T everywhere. The wavelength, on the other hand, is the distance the wave propagates in the period time T, which is T times the propagation speed. If the wave enters a medium with a lower propagation speed, it will travel a shorter distance in the period time T, so its wavelength in that medium will decrease. --mglg_(talk) 15:58, 16 May 2007 (UTC)

Your answer is of course true, but the very notion that "no difference at the source" implies "no difference anywhere else" is a non-trivial result of linearity. All linear operations on a sinusoid, which is a pure frequency, yield other sinusoids (albeit perhaps with different amplitudes and phases) with the same frequency. Non-linear systems can and do change the frequencies of waves interacting with them; see frequency doubling for example. --Tardis 16:43, 16 May 2007 (UTC)

Well, it is a trivial result that periodicity with period T at the source implies periodicity with period T throughout the wave (assuming that there are no other time-varying things that could cause Doppler shifts etc.): this is just a statement of invariance under time translation by T. As you point out, nonlinearities can distort an originally sinusoidal wave so that it comes to contain harmonics of the source frequency, but the distorted wave is still periodic with period T, and has the same fundamental frequency 1/T as the source (except for the special case when the nonlinearity exactly eliminates the fundamental). --mglg_(talk) 19:17, 16 May 2007 (UTC)

For a (general) non-linear system I don't think you can claim that just because the source is invariant the effect has the same invariance: the medium can have memory and hysteresis and all sorts of ugly effects that just don't settle out even over infinite driving time. Do correct me if I'm wrong; is it in fact always the case that such effects are transient? --Tardis 22:42, 16 May 2007 (UTC)

You can also look at the math: Velocity = Frequency x Wavelength. So when the speed of the wave changes because it passed into a different medium, the wavelength changes by the same amount (eg if the speed doubles, the wavelength doubles) hence, the frequency has to stay the same in order to keep the equation balanced. SteveBaker 16:43, 16 May 2007 (UTC)

This is a very good question. As long as oscillations are small, indeed, the frequency does not change as mglg explained above. However, in case when the oscillations become rather large (such that there is a substantial difference in properties of the medium at different phases of the wave) the frequency may change: harmonics may be produced. Imagine a microphone. The air vibrations are picked up by the microphone membrane. Assuming the sound wave has only a single frequency w, the air pressure in the sound wave is proportional to sin(wt). As long as the sound volume is not too high, the membrane position will be proportional to sin(wt+f₀) where f₀ is a constant phase shift. The frequency w is the same. However, when sound becomes very loud, the membrane vibration amplitude becomes quite large and affects the membrane elasticity. Such a motion is no longer sinusoidal, and contains other frequencies besides w (usually 2w, 3w, ...). In acoustics and electronics that effect is generally called distortion; you can hear the reproduced sound becoming "dirty". For electromagnetic wave you may look at frequency doubling or at nonlinear optics in general. So, the frequency does not change in a weak ("linear") oscillation regime; but it can change (or, more accurately, new frequencies may appear while the original frequency still remains, as well) if oscillations are strong ("nonlinear"). If you have any further questions, please do not hesitate to ask. Best wishes, Dr_Dima.

Yes indeed this is a very interesting question, and one with which I have been struggling for quite a few years now. It seems that in wave propagation, frequency is the only invariant. Wavelength and velocity are related by v = f * lambda. Apart from the doppler phenomenon (that requires a moving source or reciever), it would appear that frequency cannot be changed 'on the fly'. It may, however, be possible to change the frequency of a short duration signal by somehow altering the velocity of propagation of a 'captured' wave.

As I understand it, the original wave induces oscillations at the same frequency in nearby electrons, and the new wavelength comes about by adding the original wave to the dipole radiation thus produced. Summing sinusoidals of the same frequency can produce changes in wavelength and amplitude, but not frequency. Nonlinearities enter naturally when the dipole part of the oscillating electrons is no longer a sufficient description. Eldereft 04:01, 17 May 2007 (UTC)

A simple way to look at it is to think about what happens at the boundary between two materials. The frequency determines how many wavefronts are both entering and leaving the boundary. What happen's if frequency changes? Somehow the boundary would have to create or store wavefronts, to adjust for the change in frequency, I think you'll see this is a strange idea. Cyta 07:28, 17 May 2007 (UTC)

A simple question but a really good thinking point!

Propagation of waves is all determined by local continuity conditions - adjacent oscillations in time and space have to link up with their neighbours in a well-defined way, as expressed by the wave equation.

The most familiar situation with light is where a wave passes from one medium into another with a different refractive index, where its speed is different. Here we have a system in which the properties of the medium are constant in time, but not uniform in space. To accommodate the change in wave speed, the spatial part of the wave has to adapt - so the wavelength changes. This gives us a constant frequency, with varying wavelength, and this case is well explained above. Non-linearities can introduce extra frequencies, but they are still harmonically related to the original.

But what happens if you have the reverse situation: a medium which is uniform through space, but whose properties change, everywhere at once, as a function of time? The initial conditions would dictate that the wavelength must remain unaltered, but the frequency would have to change to adapt. We would then have a constant wavelength, but varying frequency. (The maths is practically the same; we are just interchanging the time and the space coordinates).

A familiar one-dimensional example is altering the tension on a violin string while it is vibrating - wavelength remains constant, but the pitch varies. It would happen with a travelling wave as well as with a standing wave. The corresponding phenomenon with light is not so common, because it's difficult to capture a wave for long, but not impossible. You could do it, as Eldereft hints, by using an electro-optic medium inside a Fabry-Perot resonator, and changing its refractive index in a time short compared to the storage time of the resonator. (I know systems like this are studied quite extensively, but unfortunately that's all I know about them.)

If we allow the properties of the medium to oscillate with time (rather than just undergoing a step change), then there are several examples of changing-frequency effects. The common acousto-optic modulator shifts light frequency by a continuously controllable amount, using the diffraction of light from a moving sound wave: Doppler effect from one point of view, photon-phonon scattering from another. Raman scattering changes the frequency of light by reradiation from vibrating molecules. So, in a spatially homogeneous medium, time variations can lead to frequency variations.

In real life, we have three space coordinates to play with and run backwards and forwards in, and only one time coordinate that happens to be fairly inflexible. This may be why the frequency change effects are less familiar. --Prophys 11:24, 17 May 2007 (UTC)

acyldipeptide

I'd like to know the general structure of an acyldipeptide. Yes, I've googled it. Yes, I know what a dipeptide is. What I am unsure of is exactly what modification the acyl refers to. Where is it attached? ike9898 15:03, 16 May 2007 (UTC)

I assume we're talking about N-acyldipeptides (i.e., the ones listed by googling for "acyldipeptide"). That "N" tells you that it is a nitrogen that is acylated. A simple N-acyldipeptide would be N-acetyldiglycine: CH₃-CO-NH-CH₂-CO-NH-CH₂-COOH DMacks 15:36, 16 May 2007 (UTC)

Thanks! ike9898 15:57, 16 May 2007 (UTC)

Basal Metabolic Rate Question

Good morning,

I'm hoping that someone can clarify the Basal Metabolic Rate article for me, specifically BMR's effect on weight loss. I found the BMR article a tad confusing. To illustrate my question, I calculated the BMR for the "average male" at the Discovery Health's website BMR Calculator. Assuming a 25 year old, 175 lb, 5'10" male, the BMR is about 1,875 calories. So, this means that, at minimum, this male would need to consume roughly 1,875 calories per day just to sustain life? Additionally, assuming that the man consumed less than 1,875 calories, there would be a "calorie deficit" resulting in weight loss (with everything else being equal)?

I guess I'm not seeing the BMR's connection to weight loss. Maybe I'm overanalyzing.

Thanks! Rangermike 15:33, 16 May 2007 (UTC)

It seems to me that you have explained things very well, so I'm not sure what you don't understand. The BMR is essentially how much energy gets used up with no exercise counted. If you consume food containing less energy than that, then your body needs to get the additional energy it needs from somewhere. The usual place is the body's fat stores. When fat goes away, you weigh less. Thus, weight loss. --Tugbug 17:55, 16 May 2007 (UTC)

Your caloric intake doesn't need to be lower than your BMR to lose weight. BMR assumes that the person is in rest. Most of us move around if only a little. So to be clear, to lose weight, you need to consume less calories each day than you spend in that day . Of course in a sense your body IS consuming those calories, if by consume you mean taking fat/protein that is available in the body and burning that off.

so assume I'm a somewhat active person. I use about 2500 calories each day. I eat about 2000 calories worth of food. I'm left with 500 calories worth of deficit. Those calories get taken out of your fat stores and available protein. And you then lose weight PvT 20:18, 16 May 2007 (UTC)

It's not quite that simple. Your body will interpret insufficient calories as meaning you are starving to death, and will take action to save your life. It will make you think about nothing but food and shut down or reduce all optional systems to save energy. In this starvation mode you will then burn far fewer calories, and you will be listless, extremely hungry, unable to think clearly, stop growing (if a child), uninterested in sex, sleep more, need to keep your temperature up or shiver uncontrollably, etc. StuRat 22:10, 16 May 2007 (UTC)

Very interesting discussion, I apprecaite the reply! Rangermike 12:34, 17 May 2007 (UTC)

Lightning Strike

Any idea how lightning does not create an EMP wave unlike an EMP weapon or a nuke? 59.92.247.166

Well, lightning and nuclear weapons are entirely different things, much as how Quizno's doesn't create SUVs like General Motors. Is there some further point of clarification that needs to be addressed? — Lomn 15:58, 16 May 2007 (UTC)

Lightning does create EMP. --mglg_(talk) 16:03, 16 May 2007 (UTC)

An EMP (an Electromagnetic Pulse) is just a burst of electromagnetic waves - technically, turning a flashlight on and off again creates an EMP. I suspect that our questioner wishes to know whether the EMP is large enough to do massive destruction to nearby electronics and such. Clearly the EMP from a flashlight isn't - and the EMP from a nuclear explosion is. Somewhere between those two extremes lives the lightning strike. It's just a matter of degree. Since the intensity of omnidirectional electromagnetic waves drops off as the square of the range - a lot depends on how close you are to the source. If you are really close to a lightning bolt, I would expect the size of the electric field would be plenty big enough to cause a damaging voltage difference across the pins of a delicate silicon chip - but probably not enough to destroy the ignition circuitry of a car...it's a matter of range and intensity though - there isn't a black-and-white answer here. SteveBaker 16:37, 16 May 2007 (UTC)

Minor nitpicker's correction: A bolt of lightening is a linearly extended source of EM radiation, and hence, the field falls of linearly with the distance, not with its square. And yes, your electronics will get damaged if your neighbors house is struck by lightening, unless you use surge protectors or are lucky. Simon A. 19:09, 16 May 2007 (UTC)

Geez, last night we had a monster storm, and then one of those instant lightning strokes without thunder (too close!). I could hear all the surge protectors pop, as the power went out, almost like an explosion! Lost one surge protector, and one old ethernet switch. --Zeizmic 20:39, 16 May 2007 (UTC)

Have you done something to anger God ? Just feel lucky that His aim isn't better. :-) StuRat 23:04, 16 May 2007 (UTC)

We got a melted TV due to a close bolt. I love that smell! 213.48.15.234 07:59, 17 May 2007 (UTC)

Stability

Any idea on how do we model stability and how do we decide modes of resonance on systems such as rockets, planes, and other control systems??59.92.247.166

That's a homework question if ever I saw one. Flight dynamics might give you a starting point. --YFB ¿ 16:43, 16 May 2007 (UTC)

A static model wouldn't work, so you'd need a complex dynamic model, perhaps using fluid dynamics software to model turbulent flow. StuRat 22:57, 16 May 2007 (UTC)

To do it practically you can use a wind tunnel, either with the real thing or a scaled down model. As fluid dynamic systems are often chaotic and difficult to model mathematically, I suspect an experimental approach is used in conjunction with theoretical models. Cyta 07:31, 17 May 2007 (UTC)

Fuse

We know that a fuse wire melts due to overheating.But the practical resistance of a fuse is low. So how does that become equivalent to heat in melting the fuse, because P=i^2r? Also would it not cause a voltage drop?? How does the voltage drop suddenly increase when the fuse is about to blow and its negligible when the fuse is operated under max current rating??Also any idea how do same value and different wattage restistace work??59.92.247.166 15:52, 16 May 2007 (UTC)

It is unclear what you mean by "same value and different resistance wattage." If you solve some problems in basic Ohm's Law it may become clearer to you, or clarify the question. A Fuse (electrical) is made of a material with a lower melting point and a higher resistance per unit length than the conductor it protects. It was noted in the mid-1800's that if you connect a piece of iron wire and a piece of copper wire of the same size in series and pass (obviously the same) current through them, the iron wire will get red hot and even incandescent while the copper wire is not. A fuse wire does not get red hot because it melts first. So a 1/3 inch piece of small diameter low melting point alloy, like fine solder, will carry an amount of current without melting which an insulated copper conductor can carry without exceeding the rating of the copper (say 15 amps for number 14 copper). If the current goes a certain amount over 15 amps, the fuseheats up (the power dissipated in it is given by the current squared times the resistance) and softens and melts, interrupting the current and preventing the copper wire from starting a fire. This is of course contingent onthe fuse device design, to be adequate for the voltage level and the available fault current. The copper need not get nearly as hot as the fuse wire, which has a much lower melting point anyway. As for voltage drop, remember that the fusewire is very short. It has a higher resistance per unit length than the copper, but it is very short, so the total resistance it adds to the circuit is very slight. The voltage drop equals the current times the total resistance of the fusewire, and that resistance is proportional to the the very short length, hence a very small voltage drop in the fuse under normal operation. The first fuses were literally a solder-like alloy, and were connected between 2 screws or clamps. I have seen an ancient spool of 15 amp fuse wire. Today they are fancier, in cylindrical cartridges, screw-in plug fuses, in oil filled cylinders or sand filled cylinders for high voltage. Their melting and clearing time-current curves are studied by engineers so that they coordinate, meaning that the downstream fuse closest to the fault (short circuit) should clear before the upstream (backup) fuse, to minimize the circuitry deenergized. Edison 17:33, 16 May 2007 (UTC)

Yes. Also to answer the other Q, since metals usually have a positive temperature coefficient of resistance,(ie resistance goes up with temp), then under fault conditions, the fuse gets warm, increases its resistance and therefore gets even warmer (assuming the current is constant). This positive feedback phenomenon should cuase the fuse to blow more quickly on large overloads. It is worth noting though, that a fuse will take an infinite time to clear at its 'rated' current and still a long time to blow even if slightly overloaded.

This exact question was asked (and answered) about a week ago on one of the Reference Desks.

Atlant 11:55, 17 May 2007 (UTC)

platelets

platelet structure,its surface receptors?what is aggregometry —The preceding unsigned comment was added by 59.93.204.58 (talk) 16:26, 16 May 2007 (UTC).

Wikipedia's article platelet appears not to cover aggregometry, but according to Google it refers to the measuring of aggregation of platelets. You put some blood or platelet-rich-plasma in a test tube, add something that ought to activate the platelets, and measure how fast they clump together. Algebraist 16:58, 16 May 2007 (UTC)

In the U.S., at least, these would generally be referred to as platelet function tests, rather than the highfalutin' aggregometry. We don't have articles, but you can get a good overview here. - Nunh-huh 11:27, 17 May 2007 (UTC)

math

If I had an object weighting 1kg, and applyed an upward force of 9.8065...N on it, would it stay exactly where it is and appear to weight nothing, or would it move upward at 9.8065... ms-2?

If you do this experiment on Earth's surface, the object would stay where it is. In fact, if you just let the object sit on the ground, the ground will push it upwards with exactly that force, which is why the object does stay put even though gravity is pulling it downward. If instead you did the same experiment in space, far away from any sources of gravity, your object would accelerate at the rate you mentioned. --mglg_(talk) 18:42, 16 May 2007 (UTC)

So, if I applyed a stronger force it would move upward, then if I switched back to 9.8065...N it would stay were it was? A related question-is there any relationship between force, pressure and torque? And what is it?

F = ma (apologies if the cases are incorrect). F, in this case, is net force (gravity and your imposed force). When F is non-zero (when you apply the stronger force), the object accelerates. When F returns to zero (you apply the 9.8...N force), acceleration returns to zero. Velocity stays where it currently is (assuming no air resistance, change in gravitational field, etc). — Lomn 20:01, 16 May 2007 (UTC)

(After edit conflict) Yes, it would accelerate upward if you applied a stronger force. When you switched back, it would stop accelerating, but would keep going upward at a constant speed. Eventually it would get high enough that Earth's gravity would start to weaken noticeably, at which point the object would be slightly accelerating because your force was no longer being completely canceled by gravity. As for the relationships, see force, pressure and torque. Basically, pressure=force/area and torque=force*(lever arm length). --mglg_(talk) 20:03, 16 May 2007 (UTC)

hmm So Pressure is like a force density, and Torque is a force multiplier...? -Czmtzc 20:38, 16 May 2007 (UTC)

Say rather torque is a rotational force. Algebraist 22:32, 16 May 2007 (UTC)

Psychokinesis/Psychic Abilities

Are there any hard evidence of anyone possessing these abilities that were scientically proven? Is there a way for one to train oneself for these? --Juliet 18:49, 16 May 2007 (UTC)

No, and no. --mglg_(talk) 18:43, 16 May 2007 (UTC)

How do you know, mglg?--Juliet 18:55, 16 May 2007 (UTC)

It seems as though the Psychokinesis article has lots of information on studies; have you checked it/them thoroughly? Anchoress 18:51, 16 May 2007 (UTC)

I was hoping that someone in the AMA (American Medical Association) had proven it somehow....--Juliet 18:53, 16 May 2007 (UTC)

actually no and yes. I'm learning, but very slowly :( Ask again in a few months and it might be yes and yes :] HS7 19:20, 16 May 2007 (UTC)

Nobody has claimed the million dollars James Randi offers to anyone who can demonstrate psychic abilities. Clarityfiend 19:53, 16 May 2007 (UTC)

No for both questions. they're many reasons why people claim they have such powers... but there's just zero scientific evidence of either. in the case of the second one, its largely a matter of commercialy exploiting people who will buy it. in other words, scammers. the con artists are sometimes a good judge of character/situation and thats why they sometimes seem to have some substance to their services but there's nothing supernatural about that.  Adam2288  T  C  21:45, 16 May 2007 (UTC)

Of course there is hard evidence. I can make the head of any attractive girl turn the other way. Um, does that not somehow match the definition of psychokinesis? Perhaps more psycho than kinesis? ;-) 84.160.208.134 22:45, 16 May 2007 (UTC)

I wonder if those with psychokinesis find it any easier to drag themselves out of bed in the morning. StuRat 22:54, 16 May 2007 (UTC)

The second most horrifying dream I ever had was that I got up in the morning, labourously and exhausting as ever, and then to awake to real reality, just to find all that toil is still ahead. The uttermost horrifying dream, frequently as a child but long gone now, was that the curtains were moving and I found out that it was me, moving the curtains by psychokinesis. Sure I didn't know the word then, but sure as hell I knew perfectly well the idea of it. Never found out why it was so horrifying, though. That it was uncontrolable doesn't account for all the horror I felt. 84.160.208.134 23:57, 16 May 2007 (UTC)

Speaking of dreams, you might try Lucid Dreaming, it's probably the closest you can come to actual psipowers. -- Phoeba Wright^OBJECTION! 02:16, 17 May 2007 (UTC)

Whoever believes in psychokinesis, raise my hand! — Kieff | Talk 02:20, 17 May 2007 (UTC)

I think the art of conning is actually a much more interesting phenomenon than psychokinesis. It is amazing how somebody of intelligence can be persuaded of almost anything. Conning is a real and provable phenomenon; it happens every day amongst members of every socioeconomic stratus. Psychokinesis is a form of conning, but it is only a sub-set of a larger techniques. People are able to convince others of virtually anything, whether it involves magic, mental powers, or beach-front time-shares. Nimur 04:45, 17 May 2007 (UTC)

Why does everyone dissagree with me here :( Just because I can't proove it yet, doesn't mean it isn't real :) HS7 18:32, 17 May 2007 (UTC)

And you have to be famous to claim the $1,000,000 prize :( HS7 18:33, 17 May 2007 (UTC)

No, you simply have to demonstrate, to scientific standards, that it works. If you do that, I assure you that you would instantly become famous! --169.230.94.28 20:59, 17 May 2007 (UTC)

Actually, you do need to be famous to try for the James Randi Award. "Starting on April 1, 2007 only those with an already existing media profile and the backing of a reputable academic would be allowed to apply for the challenge." - thats from James Randi article, and comes with a sauce. BUT, there are several other (most likely for somewhat smaller amounts) that also reward any kind of scientifically proven manifestation of psychic powers. Shinhan 17:35, 19 May 2007 (UTC)

Finally dreams can be inexplicably horrible for no reason :( The scariest dream I ever had was of a small black cloud floating across the room :@ HS7 18:35, 17 May 2007 (UTC)

Sorry about the OT comment, I just wanted to use inexplicable somewhere :@ HS7 16:04, 20 May 2007 (UTC)

Floating Shoes

Assuming someone wanted to make 'shoes' that floated, (taking the rollerblade/heelies idea to a whole new level) what would be the most viable and feasible technology to do so? I'm thinking something that works akin to the Segway, where leaning changes direction...and also, for balance, the feet positioning would be one foot forward, one back, bike-style. Which tech would accomplish this? fans? anti-grav? magnets? 140.180.11.227 21:56, 16 May 2007 (UTC)

There's 2 options i can think of, both of which wouldnt be very practical. 1: true floating shoes with air cavities would have to be large enough do displace the rider's weight worth of water, roughly 20-50 liters (5-13 gallons) on each foot.. thats gonna make them huge.. it would be hard to qualify them as shoes, more like big pods under your feet. option 2: power floating you can have some kind of thrusters under it, so its essentially some kind of jet pack that makes u stand in water. again, the size is gonna be a big problem, unless you carry the engine part on your back and have tubes connecting to the shoes. using pulling water into the backpack and thrusting it back out through a tube system connecting to your feet... thats deviating quite a bit from "shoes" now isnt it? makes for an interesting invention nonetheless. if you ever get it to work, i might even buy one myself.  Adam2288  T  C  22:05, 16 May 2007 (UTC)

Perhaps floating shoes on mercury would work a bit better. StuRat 22:48, 16 May 2007 (UTC)

How about a heavy keel as a counterweight below the shoes for stability? --User:bmk

Certainly people have made these many times in the past - there are even contests for them. Mythbusters tried to make some and failed - but they made a few critical boo-boos.

1. The "shoes" have to be big boxes - the length of ski's - 20cms wide - 10cms deep - two meters long. Probably made of foam polystyrene stiffened with somelightweight material. They need to displace enough water to equal the weight of the person wearing them. 0.2x0.1x2.0m is 40liters - or 40kg of water per shoe. Two of 'em will support 80kg...so if you weigh more than that - think bigger!
2. It's almost impossible to keep the shoes (think 'skis') parallel in a zero friction environment. This means that you need something like a pair of short bungee cords strung between the fore and aft tips of the ski's to keep them more or less parallel.
3. In order to get some kind of forward propulsion as you walk, there need to be soft rear-facing pockets on the undersides of the shoes that open up and catch water as the shoe tries to slide backwards in the water - whilst collapsing shut when the shoe slides forward. Thus, as you slide your feet back and forth, the backward-going foot has lots of 'drag' so it doesn't slip back so far - whilst the forward-going foot slips easily through the water. Without that you'll just be shuffling your feet back and forth and going nowhere.
4. It's quite hard to get started with the shoes - you'll probably need to enter the water from a gently shelving beach.

SteveBaker 23:37, 16 May 2007 (UTC)

How about you improve on their design? If I could find some in my size (fsking 8EE) that WEREN'T A PAIN TO USE, I'd be right on it. -- Phoeba Wright^OBJECTION! 02:14, 17 May 2007 (UTC)

The most simple, if not the most feasible, answer is in the thread directly above. Check in with the 'PK apprentices' in a few months, they may be able to get something going for you. Anchoress 02:22, 17 May 2007 (UTC)

I read about a 19th century inventor who built float shoes. Each foot had a shoe which looked like a little canoe with a top(airtight). There were flaps on the bottom, I believe, which folded flat on the forward stroke and popped down to resist the water on the backward stroke. The feet may have been in hinged shoes to make it easier to move the feet back and forth. He claimed he was walking the length of the Mississippi River or some such, and people would turn out to see him walk on water past each town. A reporter said he made his way very slowly and shakily past the town. The reporter suspected that he had assistance getting between towns. I agree about the need to keep the shoes from moving apart. A sliding linkage betwen the shoes, like a drawer glide, would keep them parallel. Then you might need a rudder or just the ability to keep one stationary and propel with the other to make turns. A reverse feature might involve changing the flap so it latched for forward or reverse operation, so you could turn on a dime. It should work after a fashion, but not making use of the upper body would limit speed, since the legs would pretty much be doing all the work. Taking it a step further, if it really doesn't have to look like a pair of shoes, consider a lightweight version of the Nordic Trak wherein the arms and legs are contributing to the propulsion. There could be a rudder on each float controlled by a handgrip. Edison 14:07, 17 May 2007 (UTC)

On further research: see [4] about a modern inventor of modern shoe-kayaks and the aforementioned H.R. Rowlands who was the 1858 inventor at [5]. Edison 20:52, 17 May 2007 (UTC)

Weird question

ok, here's a poser - I'm writing a bit of prose set in Hell - it's in a stone chamber so ancient that the screams of the damned have eroded the walls. The question is: would the erosion adopt particular shapes - would there be peaks and troughs - nodes at the points where the soundwaves focused to gouge deeper hollows in the stone?

Ta

Adambrowne666 23:13, 16 May 2007 (UTC)

Creepy! Interesting idea - I think there would be patterns on the wall determined by the dominant sound modes in the room from the spectra of the screams of the damned. However, I think in reality other factors would erode the walls much faster than screams. Like the flailing of the damned. Or the moisture erosion from the breath of the damned. Anywho, it's Hell, so I guess your guess is as good as mine. -User:bmk

If it helps the story - we can cook up a plausible sounding reason! The loudest sounds would be at the resonant frequency of the chamber - so we could certainly imagine an effect whereby the wall eroded much like a washboard road. Over the millenia, the washboarding might even add interesting overtones to the sound - perhaps making it sound ever more blood curdling as time goes on? Yeah - I could buy that. Probably it would end up looking like the ripples in sand on a beach. SteveBaker 23:25, 16 May 2007 (UTC)

Of course, assuming the physics of Hell is the same as up top, then the mechanism is dynamic spalling [6] caused by sonic waves, or impact of energetic ectoplasm. This would tend to follow the structure of the rocks (joints, faults, etc), so you could write 'the torture of the rock made so plain, etc'. --Zeizmic 00:20, 17 May 2007 (UTC)

Thanks so much, guys, for taking the question in the spirit in which it was intended - your answers are perfect, extremely helpful -exactly the kind of stuff I was hoping for Adambrowne666 10:50, 17 May 2007 (UTC)

A PDS (Poor Damned Soul) might just sit/stand/lie in one place and shriek in eternal torment (there would doubtless be no icewater to drink, thus no bathroom breaks needed). In that case he would generate sound energies which would doubtless cause over eternity erosion of the interior surface at the locations where the sound waves of his shrieks cause the greatest amplitude of vibration, perhaps looking on the walls floor and ceiling something like the appearance of an egg carton. On the other hand he might not be restrained, so the erosion might be more diffused if he followed my motto: "When in trouble, when in doubt, run in circles and scream and shout." In that case the erosion pattern might be annular. Edison 13:56, 17 May 2007 (UTC)

Assuming most screams are in the falsetto or whistle registers, screams could easily reach 1000 hz, so their wavelength would be ${\frac {c}{f}}$, where c = 330 ms^-1 (speed of sound) and f = 1000 hz. So the wavelength of the screams would be about 34 centimetres (about 1 foot), and the length of each peak or trough would be 17 cm. It would look cool, but you probably wouldn't notice until you were fairly close to the wall (of course, men can scream in much lower registers, so if this happens to be close to the Hell mens room, the wavelength would be closer to 1 metre; much more dramatic). Of course, this all changes if your Hell is really hot or really cold; c is greatest in really hot conditions, so the wavelength would be much greater and more dramatic (and as a bonus, you'd get some very formidable looking Lavacicles forming). Laïka 09:08, 18 May 2007 (UTC)