Wikipedia:Reference desk/Archives/Science/2010 September 19

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

September 19

Videogaming vs Watching TV - which is worse? any conclusive data?

I'm a gamer, my wife is a TV watcher. I hate watching TV. I consider it the same as turning your brain off. My wife hates games, considering them trivial wastes of time. I counter that a player is much more actively engaged than someone sitting on the couch staring passively at a boxy appliance. She counters that there is plenty of educational programming but few if any educational games. I counter that no one watches educational programming... What I'd like to see is a scientific analysis of the comparative "value" of playing videogames and watching TV. Which is better for the mind? Is either activity a worthy pursuit or are both equally useless, etc. Real science only please, we're perfectly able to argue opinions ourselves. Thank you. The Masked Booby (talk) 00:48, 19 September 2010 (UTC)

Does not deal with video games directly, but you would be very interested in reading the works of Marshall MacLuhan whose life works looked at the way in which various forms of media and entertainment affected people psychologically and sociologically. His book Understanding Media explores the different forms of media and attempts to divide them into "hot" and "cold" varieties based on how people interact with them. He lived and worked well before the internet and video games, but his work is still very relevent to your question. --Jayron32 03:10, 19 September 2010 (UTC)

See this http://www.bbc.co.uk/news/technology-11295257 and the linked articles at the bottom of the page. Personally I think reading non-fiction books and a quality newspaper would be better for you, as success in life or in business depends not only on intellect, but even more on knowledge. And even more on taking the initiative, risk-taking, thinking for yourself, calculating your chances. 92.15.17.68 (talk) 16:49, 19 September 2010 (UTC)

Consider what you mean by the "'value' of video games and watching TV." Neither is known for providing high educational value (and I doubt all your wife watches is educational TV, or any TV watcher for that matter). They're mostly known for entertainment value, and everyone has their own likes and dislikes on what they find entertaining. I remember a while ago there was some study (or speculation) that playing videogames might help future doctors perform remote surgery, but aside from odds and ends like this, I don't see either as being particularly enlightening for the mind.--el Aprel (^facta-_facienda) 03:43, 20 September 2010 (UTC)

The philosophical issues raised in Sid Meier's Alpha Centauri were deep enough (for a previously-uninformed 13-year-old) to be quite educational. I read some major Descartes and social contract theory and whatnot after that. John Riemann Soong (talk) 04:46, 23 September 2010 (UTC)

TMB, you're asking a very relative question. Both activities require use of the brain to process information; how and what your brains are processing is vastly different. Television, as you are indicating, is relatively passive. However, it may be teaching cultural norms and, without question, the argument could be made that many daily cultural transactions (e.g., socialization at the workplace) engage mass media as their topic. Furthermore, some television shows are intended to be, and research indicates are, explicitly educational (e.g., sesame street) in the context of measurable results. Video games are also educational, albeit differently. Depending on the game and the social interaction, the player could be part of a social support group (see http://www.bitculture.org/storage/ANTH_1699_Lucky_v2.pdf ), learning structured mathematics, or simply becoming more aggressive ( http://www.psychology.iastate.edu/faculty/caa/abstracts/2000-2004/01ab.pdf ), all of which are known effects of gaming.

So, ultimately, the question is one of value relative to your end goals. Both experiences can "teach" something different that can, at times, be useful or could, in some cases, be harmful. Certainly, for instance, you're both learning something of value in certain cultural and social situations; but clearly, your bodies isn't getting much exercise throughout - which, I'm afraid to say, could be harming you both ( http://well.blogs.nytimes.com/2010/09/15/phys-ed-can-exercise-make-kids-smarter/ ) ! akiraplt / plml.org (talk) 19:46, 23 September 2010 (UTC)

Maternal death across mammal species... do the rates vary much?

I find maternal death an interesting topic, and was wondering if there are maternal death statistics available for many other mammal species? I can't remember ever hearing about a cat dying while giving birth to kittens, for example. Do the rates across mammal species fluctuate much? The Masked Booby (talk) 00:50, 19 September 2010 (UTC)

Human babies have enlarged heads. Human babies are usually grown to a quite matured phase of development. Most other mammalian animals are not born so big and so highly developed ... -- Toytoy (talk) 01:12, 19 September 2010 (UTC)

Human babies are fare *less* developed than other mammal species. Our infants are helpless, but we have a high level of parental care. Other species have babies that are much more developed. For instance, baby deer can run withing a day. Maybe Toytoy is intending to make some sort of argument based on head size of infant and pelvis size of mother, but that is not a simple matter of how `developed' the offspring is at birth. SemanticMantis (talk) 13:39, 19 September 2010 (UTC)

From what I understand, human babys are born so helpless because our species has such giant brains at birth. If the pregnancy lasted any longer babies wouldn't be able to fit through the birth canal. As it is, human babies are already born with a soft spot in their skull so that their head can compress during birth. —Arctic Gnome (talk • contribs) 18:34, 19 September 2010 (UTC)

Right, humans babies have (proportionately) much larger heads and smaller bodies than adults. This is called allometric growth, see figure 1.18 here: http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=dbio&part=A79

If All Creatures Great and Small is to be believed, vets often have to intervene to turn calves and foals around in the uterus, to prevent breech births, which could otherwise kill the mother. Rojomoke (talk) 06:52, 19 September 2010 (UTC)

It seems that evolution of bipedalism may be implicated in increasing birth complications in humans. See http://johnhawks.net/weblog/topics/bipedalism/bipedal_pelvis.html You are probably correct that wild (non-domestic) mammals have lower rates of maternal death than humans. SemanticMantis (talk) 13:45, 19 September 2010 (UTC)

The making of Thai fish stomach sauce

• http://www.youtube.com/watch?v=xc3XsOBtkk0

How could they use closed small glass bottles to ferment fish intestines? Aren't they going to explode? -- Toytoy (talk) 01:10, 19 September 2010 (UTC)

They could vent them periodically. --Jayron32 03:04, 19 September 2010 (UTC)

BTW, the ancient romans used a similar condiment. See garum. That article links to other similar fermented fish sauces. --Jayron32 03:06, 19 September 2010 (UTC)

From the way the question is worded, it sounds as if yeast fermentation is assumed instead of enzymatic. Off the top of my head, I think the main gas evolved in this case would likely be hydrogen, and this would immediately oxidise to water. Such a nice anaerobic environment will also discourage oxygen loving bacteria that would spoil the sauce. Also notice: all the bottles have about a third of free space at the top and this could be to allow for a build up of any gas or a reduction in pressure due to oxidization. Anticipating the next question of ”When did she add those enzymes? I didn't see her adding anything other than salt” The woman did not have to. The Gastric chief cell start to digest the stomach post mortem. When left in situ, they start to work their way through the rest of the abdominal cavity. Some more enzymes will also come from bacteria of course. Gosh! Why do I always end up feeling so hungry when reading the Reference desk? --Aspro (talk) 11:22, 19 September 2010 (UTC)

Angular momentum of very long wavelength light

A fact I should have learned many, many years ago but didn't: All photons have the exact same spin angular momentum: $\pm \hbar$. I wish they'd just called it "angular momentum of the photon", it would have been a lot easier to keep straight than "hbar"! A great blogger tuned me in to this [1] by citing an old experiment in which circularly polarized light actually turns a target disk.

But here's the thing that gets me: Very low wavelength photons take very little energy to make... but they still have angular momentum. It seems like, in theory, you could make a sort of sub-sub-radio that beams out nearly pure angular momentum, with no energy attached, allowing a satellite to twist around any which way without annoying conservation issues. And the hapless recipient of the beam likewise would be twisted around unexpectedly.

But twisting an object around... that takes energy, doesn't it? Which means that the photons have to carry energy... don't they? It seems like something has to give. Is there an absolute lowest wavelength of light, or what? Anyway, that's my question. Wnt (talk) 04:20, 19 September 2010 (UTC)

Here is a paper that discusses parts of this question http://www.physics.princeton.edu/~mcdonald/examples/optics/atkinson_pr_47_623_35.pdf it shows that the frequency of the photon changes to account for the energy. I believe (but it's only a guess) that if you reduce the frequency so much that it's lower than the energy the momentum would impart, it would stop imparting momentum. Angular momentum is quantized, so it already "refuses" to impart angular momentum in some cases, and will only do it if it can transfer a whole amount. Very interesting question BTW! Ariel. (talk) 07:26, 19 September 2010 (UTC)

Also take a look at question #2 here: http://www.drtruth.org/paradoxes.html for an interesting twist on this question. (Yes, I know this site raises the "crank alert", but question #2 is still interesting.) Ariel. (talk) 07:35, 19 September 2010 (UTC)

For the photon climbing out of a gravity well to be brought to zero energy it must be below the event horizon of a black hole. We have no knowledge, and can have no knowledge, of the physics taking place below an event horizon so it really does not matter whether or not there is an anomoly with the left over spin: it is not an anomoly that can ever be witnessed, or even happen, in the observable universe. Back to the original question, is this not just another aspect of the phenomenon well known to radio engineers that long-waves ignore large objects like buildings? The longer the wave, the larger the object has to be before there is an interaction. SpinningSpark 10:38, 19 September 2010 (UTC)

I don't like this answer, conservation of momentum is VERY fundamental. If black holes violate that then there is very big hole in the theory. Ariel. (talk) 05:00, 20 September 2010 (UTC)

As you lower in frequency it gets harder and harder to transmit power, and reciprocally to receive power. The wavelength is bigger, and the near field is larger. (and photons bigger too) This means that even if you apply high power, little power is transmitted a long distance. Instead most of the power will return back to the transmitter twice each cycle, this will be photons coming back the other direction. Also your photons will have to be circularly polarized, which is extra difficult to do, you will need a phase shift between two linearly polarized antennas. Graeme Bartlett (talk) 12:11, 19 September 2010 (UTC)

I am considering an example of a spinning magnet next to a thick aluminium plate. The plate will start spinning as it is dragged by the magnet but it will not keep going faster and faster, instead it will gradually match the speed of the magnet, as no absorption will occur when the speed is matched. This is one way an electric motor can work. And my conclusion is that the angular momentum transferred can only get the absorber spinning up to the frequency of the photons. Since the energy of the spinning absorber is proportional to the square of the angular frequency it is in proportion of the power of the photons. Graeme Bartlett (talk) 12:25, 19 September 2010 (UTC)

It's true that there are "two photon polarization states", but that just means that you can write any polarization state as α |A〉 + β |B〉 where |α|² + |β|² = 1. This specifies a point on the Bloch sphere, where antipodal points represent left and right circular polarization, the equator between them represents linear polarizations in various directions, and intermediate points are elliptical polarizations. Another roughly equivalent way of saying the same thing is that you can transmit less than one unit of angular momentum per photon by emitting a mix of left- and right-polarized photons. So there's no minimum. There is a maximum, though: no angular momentum larger than nħ in a wave of energy E = nħω (where n is the photon count). In other words, no angular momentum larger than E/ω. This is a classically derivable limit, though I don't know exactly where it comes from. In short, quantum mechanics doesn't add anything to this question, except, as usual, some small-scale discreteness. It's still an interesting question, though, even classically, at least if this paper is to be believed. -- BenRG (talk) 19:01, 19 September 2010 (UTC)

C'mon Refdesk, this is a straightforward physics problem! Don't hide yourself behind the low probabilities of absorption/emission of low energy photons. Hint: consider a body floating in space that emits one photon. Consider how one accounts for angular momentum and energy conservation of this process in general. Count Iblis (talk) 14:46, 20 September 2010 (UTC)

I have to say, I'm still out at sea on this one. In the first part, I must have missed the part about the photon changing frequency to account for this momentum - I thought that the relationship between photon energy and frequency was very simple and proportional. I think that "Dr. Truth" knows even less about physics than I do - he doesn't think light has mass... Where black holes are concerned, they have spin, so they should conserve angular momentum somehow; if you shoot a photon straight up at an event horizon, then shouldn't any less-than-infinite acceleration fail to change its speed from c by the usual relativistic addition? So either it actually escapes or else it must take "infinite precision" to get it out?

Now the useful lead is that yes, low frequency radio waves affect only big things, and it makes sense that if a photon kicks a particle out of place somewhere on the rim of the Ringworld, the angular momentum that contributes to the macroscopic object will be much greater than if it kicked a particle out of place in a dye molecule. Or to take things to the opposite extreme, on the quantum scale the photon interacts with something and changes its orbital or spin quantum number and the angular momentum seems to just mean a transition in a molecule. What's oddest is that apparently angular momentum is conserved relative to any point anywhere. I wonder if you can reorient a satellite with a nuclear magnetic resonance machine... What I should know is that angular momentum isn't energy; it's not measured in J but in J s, suggesting that there should always be some way to use virtually no energy to change angular momentum, even if I'm not quite sure what it is.

The biggest mystery of all concerns the relationship of torque and energy - according to torque, apparently a "N m" isn't quite the same thing as a "N m". Since torque is the derivative of angular momentum, that is in "N m s", really, as is Planck's constant? But this last is very much a measure of energy times time. At the moment I don't understand from torque, for example, even why power (i.e. J/s) = torque x rotational speed... and so oddly enough it's the quantum physics that seems logical and the classical which seems mysterious and unintuitive. Wnt (talk) 18:12, 20 September 2010 (UTC)

Suppose for argument's sake that a molecule or atom does emit a very long wavelength photon. You can have an atom with two almost degenerate levels (the energy eigenvalues are almost identical) and a transition between the two levels is possible. The angular momentum difference between the two levels can be hbar, and an odd number of photons must then be emitted.

This example betrays what is going on: the total energy in the limit of zero energy photon doesn't change, but the state of the system does change (obviously, the angular momentum quantum number changes). Now, you can ask how it is possible for the angular momentum to change without the total energy changing. But, in general, even if the photon had energy, you cannot satisfy energy conservation with the classical equation for energy, can you? E.g. consider two photons emitted in opposite directions; due to momentum conservation the object stays in rest, so according to classical mechanics, the kinetic energy doesn't change. So where does the energy of the photons come from? Count Iblis (talk) 18:33, 20 September 2010 (UTC)

Well, mechanics per se misses a few fundamental forces, i.e. potential energy of two opposite charges, the effects of temperature etc. Actually, it's hard for me to picture gravity directly producing photons rather than gravity waves and such - you think of two objects falling together but only emitting energy when they "strike" (repulsed with some other force). But I don't know for sure there's no way at all - can someone confirm? Wnt (talk) 18:53, 20 September 2010 (UTC)

Let's take a fresh look at this problem. The issue is that you've a system that can act as a reservoir for angular momentum such that exchanging angular momentum doesn't change the energy of the reservoir (at least in some limit). The particular system you're looking at, is the long wavelength sector of the electromagnetic field. You can consider coupling some object to this using an antenna and then exchanging angular momentum with the system. The question is then how to account for the energy changes of the object, as no energy is exchanged with the system.

You can find other examples of systems that can act as an angular momentum reservoir such that the angular momentum doesn't change its energy. Consider e.g. the Earth. Suppose you want to use an engine to let a wheel spin. The engine will be connected to the Earth, so that the Earth can absorb angular mometum (otherwise, due to angular momentum conservation, the engine will start to spin in the opposite direction as the wheel).

So, clearly the angular momentum gain in the engine + wheel system is caused by an exchange of angular momentum with the Earth. However, because the Earth's moment of inertia is so huge, the Earth's energy doesn't change when angular momentum is exchanged (the change in L^2/(2I) is negligible). So, this means that the total energy in the engine + wheel system is conserved.

The case of the antenna coupling an object to the extremely long wavelength sector of the electromagnetic field, is completely analogous. As Graeme Bartlett] explained above, you need a power source and most of the power will end up back in the system. So, that's where the energy is coming from. If you look deeper and ask how it can be that the total energy is conserved despite the system starting to spin, you have to account for the internal energy of the system in rest. This changes, because the power source is being depleated. In classical mechanics, you have to treat the rest energy of an object as some ad hoc given quantity, but according to special relativity, this is m c^2, with m the rest mass (an empty batery has less mass than a fully charged battery). Count Iblis (talk) 15:37, 21 September 2010 (UTC)

I think my point about ratio of energy to angular momentum may have been missed, this can be as close to zero as you like if the rate of rotation s slow enough, and a rate of rotation below the photon frequency can accompany this. However what happens if the absorbing object is spinning faster than the frequency of the photons? I suspect they would slow down the spin instead of speeding it up. But what is happening from the photon point of view then? I was thinking that the classical treatment was bogus, but on second thoughts, if the photon hit dead centre on the absorber, it would need infinite torque to twist the object, and the further from the central axis it is the less force is needed to impart a particular amount of energy. In any case the long wavelength has a very uncertain position for the photons. The plane wave is unrealistic as it cannot fit in the universe, and a photon could be anywhere on the plane. Graeme Bartlett (talk) 21:41, 23 September 2010 (UTC)

animal maths

are humans the only living thing on earth that understands maths or do other animals do as i know allot of animals are intelligent but can they do maths. --83.71.84.246 (talk) 13:34, 19 September 2010 (UTC)

I wouldn't count on it, but there is a sub-section about animals' mathematical abilities here. Animal_cognition#Mathematics--Aspro (talk) 13:42, 19 September 2010 (UTC)

In Cormorant fishing the birds are - or so the story goes - allowed to swallow say one in ten of the fish they catch. It was said that they would refuse to dive again if they were not allowed to swallow the tenth fish due to them. Other birds I think counting, such as eggs in their nest, goes something like: one, two, lots! 92.15.17.68 (talk) 17:07, 19 September 2010 (UTC)

I am told that there was once a horse in Germany who was able to work out simple addition. Is that right ?  Jon Ascton  (talk) 18:36, 19 September 2010 (UTC)

No see Clever Hans --Digrpat (talk) 18:42, 19 September 2010 (UTC)

(ec)I googled [horse arithmetic] and a number of entries came up, including one for Clever Hans, which might be the one you're thinking about. And his Russian cousin, Sleightov Hans, was able to do card tricks. ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:46, 19 September 2010 (UTC)

That's quite an interesting story. The horse was discerning "the answer", i.e. when to stop tapping his hoof, by picking up on the body language of his trainer and others who posed questions. That horse was clever. There's more to intelligence than being able to do arithmetic. In general, animals don't have much need to do arithmetic, as very few of them are planning careers in the engineering field. But understanding body language is very important to any animal's survival. It's interesting that the scientists needed to analyze the situation to figure out that body language was tipping off the horse. For the horse, studying body language just came naturally. So who was the more "clever" species of the two? ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:03, 19 September 2010 (UTC)

In The Mathematical Brain Brian Butterworth analyses various instances of apparent "counting" in anaimals, and reaches the conclusion that many animals in the wild demonstrate "numerosity" - a sense of number that allows then to compare the relative sizes of two groups of objects. He also discusses research that shows that chimpanzees in captivity can be taught/trained to correctly match Arabic numerals with groups of objects up to about 4, and to perform simple arithmetic with small numbers at about the level of a 3-year old human child. Gandalf61 (talk) 20:09, 19 September 2010 (UTC)

What this all speaks to is the needs of various species, which go hand-in-hand with their evolutionary development. Humans are not as good with body language as other animals are, but we're "good enough to get by", because it's a less critical need for us than it is for animals. The same could be said for factors such as sense of sight, smell, etc., which are much more acutely developed in certain species, due to evolutionary pressure or whatever. Humans also use that "numerosity". If you go to an event, you don't have to know precisely how many people are there in order to get a sense of how big the crowd is. What I wonder about training chimps in arithmetic, is what would they do with that info? Is it of real use to them, as it is for humans; or might they see it as just a game they're playing with their keepers? ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:30, 19 September 2010 (UTC)

I used to hike with what I thought was a fairly smart dog. A regular route involved crossing a river that I could wade across but he had to swim. He eventually developed the habit of leaping in, finding he was being swept downstream, climbing back out and walking upstream a varying distance that always seemed to get him to the right spot directly across on the opposite bank, based on how much he got swept downstream by the the current. Did he understand vectors? HiLo48 (talk) 01:06, 20 September 2010 (UTC)

He certainly understood the concept even if he didn't know what to call it. So, was your dog named Victor? :) ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:16, 20 September 2010 (UTC)

HiLo makes a good point with his vector-dog. Math is much more than just numbers. Some see math as the (formal) study of patterns, structures, etc. In that light, many species (dolphins, chimps, New Caledonian crows) could be said to have some understanding of math, but probably not in any formal terms.SemanticMantis (talk) 02:32, 20 September 2010 (UTC)

If I am not stretching your query too far - it might be of interest to you that I recently put a question that asked ( and it turned out right to my surprise ) about baby elephants in Thailand who can paint self portraits !  Jon Ascton  (talk) 21:16, 20 September 2010 (UTC) ^{Of course, painting's no close to arithmetic, but as SemanticMantis points out it "...is much more than just numbers... (formal) study of patterns, structures, etc." that comes quite close}

The elephant in question is here. I suppose it was probably taught to paint that exact image, but my first thought was "That could be any elephant! How do we know it's a self portrait?" APL (talk) 22:12, 21 September 2010 (UTC)

You have given link to the youtube video.Here is the question I asked. The replies people gave are very creative. I strongly recommend everyone read it. Jon Ascton  (talk) 05:23, 22 September 2010 (UTC)

Here are a few more → Wow! I'm impressed. WikiDao ☯ (talk) 22:23, 21 September 2010 (UTC)

Does the bee's waggle dance count? -84user (talk) 22:03, 20 September 2010 (UTC)

An overview can also be found in "What Animals Know About Numbers", by Elizabeth M. Brannon (In Handbook of Mathematical Cognition, Psychology Press, 2005, 85-107). ---Sluzzelin talk 22:27, 20 September 2010 (UTC)

I think there are cases of animals doing advanced "math" without using numbers to do it. For example, there was an incident where a mathematician's dog "calculated" the diagonal trajectory to run in to catch a frisbee moving in a certain direction and speed when humans would need calculus to figure it out mathematically. ~AH1^(TCU) 01:55, 21 September 2010 (UTC)

science

where is the silicon controlled rectifier is demonstrated and is its first application? —Preceding unsigned comment added by Thethesarfraj (talk • contribs) 15:00, 19 September 2010 (UTC)

Does the silicon-controlled rectifier article not have the answers to your question? SpinningSpark 16:07, 19 September 2010 (UTC)

Elasticity

Who discovered the phenomenon of elasticity? —Preceding unsigned comment added by Blaze250 (talk • contribs) 15:58, 19 September 2010 (UTC)

I don't know that it was discovered as such, it has always been around, like hardness and wetness. But you are probably looking for Robert Hooke and his law of elasticity, Hooke's law. SpinningSpark 16:11, 19 September 2010 (UTC)

Ancient humans used to kill animals for food. It was probably while exploring carcasses that they discovered elasticity (intestines etc ). They used them to make bows.  Jon Ascton  (talk) 18:47, 19 September 2010 (UTC)

Your last point is incorrect, Jon: bowstrings are made of materials like Dacron which have a minuscule amount of elasticity. The power is stored in the limbs, so you want a string that will not stretch when under the 300 N or so of force at full draw. Please don't answer questions with unsupported speculation. Brammers (talk/c) 21:53, 19 September 2010 (UTC)

We are talking about how human might have discovered elasticity. Take the naturally occurring substances around him ( there was no industry there ), one thing would be flesh around him - that was my point...on second thoughts a freshly plucked stem of a herb might be a good example of elasticity as well. What I want to say is that the original technological application of the phenomenon was probably weapons he built.  Jon Ascton  (talk) 21:11, 20 September 2010 (UTC)

Well, if you want to be all 'Man, the mighty hunter!' :/ It's at least as likely that the original technological application would be slings for carrying babies and gathered foods: elastic materials will have biodegraded, so it will be hard to find evidence. Or even that people just noticed some things were stretchy, and played with them. You can see modern children do the same, pulling and tugging and squashing and ripping and poking, just to see how things behave. You can even see modern adults do it, especially when they don't think anyone's watching. There's no reason to assume the first observations or uses were associated with weapons 'he' built, anymore than there's a reason to assume that our ancestors slaughtered all the Neanderthals: strangely, those sort of interpretations seem to appeal to people and get accepted before any evidence has been found either way! 109.155.33.219 (talk) 22:51, 22 September 2010 (UTC)

Oh I am not saying that I am sure or have evidence for something - just what might be probable. And the single inverted comma you put around my (offensive) he are interesting ! Well, should I have used a she !!!!  Jon Ascton  (talk) 20:26, 23 September 2010 (UTC)

Cavemen had Dacron? :) ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:37, 19 September 2010 (UTC)

Only the ones with facilities to order it over the internet. SpinningSpark 00:38, 20 September 2010 (UTC)

To be fair, he didn't say they made bowstrings, he said they made bows - see Mongol bow. Matt Deres (talk) 14:29, 20 September 2010 (UTC)

According to Bow_string, the amount of stretch in bow strings varies. Strings with more stretch transmit less shock to the bow limbs but are less powerful. It is true that the power of the bow is in the limbs, not the string for the most part. Exxolon (talk) 01:44, 20 September 2010 (UTC)

Metal compared to ice

Ice is frozen water. Metal could be considered 'frozen' liquid-metal. Do ice and metal have a similar structure, despite being composed of different atoms? 92.15.12.54 (talk) 23:38, 19 September 2010 (UTC)

The bonding between metal atoms (see Metallic bond) and water molecules (Hydrogen bonds) are somewhat different, and the Crystal structures are also somewhat different (in general). Ice, in particular Ice Ih (which is what "normal" ice is) has a Hexagonal crystal system (and a sort of odd one at that, with a large amount of empty space). Metals have lots of different types of crystal structures, so it's hard to make generalizations. Many (most?) metals have some sort of Cubic crystal system, with generally less empty space. Some metals (Yttrium, Titanium, and Scandium, for example) do have hexagonal structures like water (though generally without the empty space like water). Buddy431 (talk) 00:15, 20 September 2010 (UTC)

I was wondering something similar: is Ice Ic equivalent in structure to lithium oxide? Wnt (talk) 00:20, 20 September 2010 (UTC)

No, not at all. In lithium oxide, the lithium ions and the oxide ions are basically charged spheres, and they "bond" by electrostatic attraction. The "bonding" is not in any given direction, but the spheres are different in size (oxide being much larger than lithium). The structure comes down to how those spheres can pack together as closely as possible – a mathematical problem – and the various solutions are taught in basic chemistry courses. In water, and ice Ic, the bonds are directional: in other words, you can represent them with lines to show where there is a bond and where there isn't one. So the hydrogen atoms are not free to go wherever they like or wherever they would fit best, they have to stay at a certain distance from the oxygens and with a certain bond angle between them. This is why ice has so much more free space than a metal or than an ionic solid like lithium oxide. Physchim62 (talk) 00:38, 20 September 2010 (UTC)

Ooops, looks like I was off base on this one - it's not even the same kind of cubic structure. Wnt (talk) 03:56, 20 September 2010 (UTC)

As an aside, the OP might find the Pykrete article interesting. Exxolon (talk) 01:41, 20 September 2010 (UTC)

Under extreme pressures water ice turns into a metal conducting electricity. Graeme Bartlett (talk) 01:55, 20 September 2010 (UTC)

Really? What exactly do you mean it turns into a metal? The atoms of hydrogen and oxygen are still hydrogen and oxygen, and they are listed in the Periodic table as being non-metals. Do you mean it behaves as a metal? — Sam 18:20, 20 September 2010 (UTC)

I imagine Graeme means that bonding between the atoms becomes metallic, that is non-directional with a sharing of some electrons. Theory has it that all materials become metallic if the pressure is high enough although, for stars composed of hydrogen and helium, there's no difference between this idea of being metallic and electron-degenerate matter. Physchim62 (talk) 19:49, 20 September 2010 (UTC)

What about the crystal structure of alkali hydroxides? ~AH1^(TCU) 01:07, 21 September 2010 (UTC)