Wikipedia:Reference desk/Archives/Science/2008 July 2

= July 2 =

Infantry vs. tank
[This is really eight related questions, some of which may share answers.] Consider either (a) one or (b) eight soldiers armed with normal-caliber machine guns and an arbitrary amount of any kind(s) (armor piercing rounds?) of ammunition for them. They are free to walk the top of a tall concrete wall that is, say, 75m on a side. Trapped inside the wall on open ground is either (a) one unarmed enemy tank or (b) one enemy and one friendly tank, armed only with their main cannons. All tanks are modern main battle tanks; the unarmed tank may be considered to be repairing supposedly-minor damage to its anti-personnel weapons in (vain) hopes that it may return fire, and the armed tanks fire only at each other. How may the infantry in each of the four situations disable or destroy the enemy tank (a) in the least time or (b) using the least ammunition?

The purpose of the first bifurcation is to increase the infantry's effectiveness in the situation where there is a friendly tank, as well as to allow them to surround the enemy. The purpose of the second is to distinguish between an enemy that is fighting (albeit not against the infantry) and one that is doing nothing but trying to evade/weather fire. The purpose of the third is obvious, but in the case where there is a friendly tank present the "least ammunition" option should be taken to require some material contribution to the battle. I would also be interested in hearing about the mechanism by which (a large amount of) machine gun fire damages the high-grade armor against which it is not frequently used. Thanks for the myriad answers. --Tardis (talk) 01:11, 2 July 2008 (UTC)


 * Good luck. Modern tank armor (e.g. Chobham armor) is really, really tough. I'm no expert, but I doubt that anything fired from a machine gun would have a chance of doing any real damage. You're probably better off having these guys break out their shovels and cover the trapped tank with dirt. Clarityfiend (talk) 06:49, 2 July 2008 (UTC)
 * They could try and open the hatch and shoot the people inside. Trying to damage the armour would be in vain, I imagine. --Tango (talk) 14:04, 2 July 2008 (UTC)
 * Yeah- small arms fire against modern armor would be useless, no matter what ammo you're using. You'd need to get the hatch open somehow.  Trap the tanks somehow, and wait for the people inside to get hungry?  I don't know. Friday (talk) 14:39, 2 July 2008 (UTC)


 * 75m square? What sort of ground? Discharge five 40 tonne road tankers of diesel into the area (so 3cm deep without bothering to do the maths) and ignite it? The thermal conduction of armour is pretty high and baking the people in it cannot be too hard. --BozMo talk 14:55, 2 July 2008 (UTC)


 * Best you could with small arms would be to try to blind and deafen them by destroying the optics and antennas. --—— Gadget850 (Ed)  talk  -  14:58, 2 July 2008 (UTC)


 * Even with an arbitrary amount (and duration; give the tanks infinite fuel if needed) of sustained fire? Surely something would happen to the armor, even if it shattered or reflected the bullets, and it wouldn't get stronger&hellip;  --Tardis (talk) 15:02, 2 July 2008 (UTC)


 * I suppose if you had a billion rounds of machine-gun bullets, you could eventually wear a hole in the armor, but that's just getting a bit silly, the occupants of the tank would starve to death first. There may be a few weak spots, though, like the exhaust port, which might be more vulnerable to small arms fire. StuRat (talk) 15:38, 2 July 2008 (UTC)


 * I suppose water dripping on the tank would eventually wear through the armor, too. Friday (talk) 17:28, 2 July 2008 (UTC)


 * Removable armor plates are common. If the tank is disabled and just sitting there, you could rush up and start removing armor.  Eventually, the guys inside will try to come out to stop you and it will be man vs. man instead of man vs. tank. --  k a i n a w &trade; 17:33, 2 July 2008 (UTC)

(EC)
 * What about MacGyver solutions? Take the gunpowder out, make a kind of bomb, could at least blow the tracks off.

Could destroy the main gun in a similar way. With no movement or firepower you could burn the people out...--Shniken1 (talk) 17:37, 2 July 2008 (UTC)

I don't know if the Barrett M82 would be useless against the tracks of a tank. It have caliber .50 and the latter are the Achilles' heel of the tank. Just give it a try on a tank that you don't need anymore. Mr.K. (talk) 09:54, 3 July 2008 (UTC)


 * The infantry could lay siege to the tank until the crew starve or surrender. D  aniel  (‽) 19:39, 3 July 2008 (UTC)

Evolution - Woman as Well?
Today I was speaking to a Jehovah's Witness that came by my door. We got into talking about everyone's favorite topic, evolution. He brought up an objection to the theory that I had not heard before. I don't remember exactly what it was, but it was along the lines of "even if a man evolved into a human, what are the chances that a woman evolved too, at the same place, and the same time?". I'm sure there is an easy rebuttal to this, but I was wondering if this objection has a name and how I can rebut it in layman's terms. (In a somewhat related query, does anyone know where one can find very old (circa 1894) copies of the Watchtower magazine?) Thanks! Ab e g92 contribs 03:56, 2 July 2008 (UTC)


 * Let me see if I understand the argument first. The idea is that if a male evolves from Species A to Species B, then there has to be a female that goes from Species A to Species B as well, otherwise there would be no-one to breed with? I can see what's wrong with the argument, but I'm finding it hard to put into words, except to point out that (a) evolution isn't sudden jumps, it's a gradual process (at least gradual in the sense that new species don't suddenly crop up in a single generation), (b) speciation occurs when an entire hereditary line becomes so genetically different to another that they cannot reliably interbreed, so it's actually the combination of a man and a woman who are, by necessity, genetically different, passing a particular combination of those different genes onto their offspring that leads to the creation of a new species. I'd try to explain it in terms of the historical development of language, but presumably the Witnesses believe that all happened when God struck down the Tower of Babel. I will leave the understandable explanation to someone with more experience in the subject matter. Confusing Manifestation (Say hi!) 04:56, 2 July 2008 (UTC)


 * Put another way, your Jehovah's Witness friend is mistaken in thinking that the process of speciation is something that occurs in an individual. Populations become species, individuals do not. - Nunh-huh 05:06, 2 July 2008 (UTC)


 * In addition to speciation, the population genetics article may also be helpful. --arkuat (talk) 05:35, 2 July 2008 (UTC)


 * The entire mechanism of reproduction involves males and females sharing their genomes again and again. The amount of genome that differentiates males and females is tiny. Your friend seems to think that males and females are terribly genetically different. They aren't. And again, all members of the species are products of their mating. So the genome isn't going to get out of sync between the two of them. --98.217.8.46 (talk) 13:53, 2 July 2008 (UTC)


 * If you want a more technical answer, consider how traits are distributed upon reproduction. Say I have a hypothetical gene for red hair. (Hair is a bit more complicated than one Mendelian trait but for our example it is concrete.) It sits somewhere on one of my chromosomes. I mate with a woman who had a gene for brown hair. It sits on one of her chromosomes. Inside the sperm, egg, etc., half of my genes are randomly thrown together with half of her genes. It's entirely possible that all of our male or female children could have my gene expressed, or half with her gene expressed. The resulting children are a mix-and-match of our two gene pools. It isn't that the "female gene pool" is separate from the "male gene pool" and has to "evolve" at the same time. They'll evolve together, as long as none of the evolutionary changes are great enough to make evolving with another human impossible. The gene pool of course allows for a certain amount of flexibility (if it didn't, even simple reproduction with genetically different members of the same species would be impossible). Beneficial mutations have a certain chance of being passed down to children of either sex. Males and females are, again, genetically (and developmentally, up to a certain point) almost identical—they are both humans, and it is the firing off of different hormones at different times that differentiate the two physically, mentally, etc. --98.217.8.46 (talk) 14:09, 2 July 2008 (UTC)


 * The number of chromosome is different in man and chimp, lets asume that the last comon ancestor had the same number than one of the two species. So sometime ago a chromosome split or paired with another to get to the different number. This happened in one single individuum not to the whole group.What I know from genetics this makes interbreeding very complicated what it is seen in donkey and horse. Has this problem of genetics been solved yet?---Stone (talk) 14:25, 2 July 2008 (UTC)


 * There are lots of theories as to how this can work. Needless to say, yes, it seems pretty clear that it can work. Even in individual humans you can do weird things like have too many chromosomes (e.g. XYY syndrome, trisomy 21). It is not usually positive in such cases though, but the point is, it can happen—it doesn't mean automatic death of the organism or inability to reproduce. --98.217.8.46 (talk) 15:24, 2 July 2008 (UTC)


 * I haven't seen a definite resolution of this "problem", but from genetic sequencing of the human and chimp genomes, it's pretty certain that the Human chromosome 2 is equivalent to a fusion of two chimp chromosomes, including residual portions of the centromere and telomeres that are no longer functional. My best guess is that at some point in the past, a fusion event happened in a human ancestor, creating chromosome 2. At this point, all breeding partners to this individual still had the two separate chromosomes, but as the fused chromosome still had all the regulatory apparatus of both chromosomes, the cells were still able to line up and divide normally (although perhaps at reduced efficiency). Eventually for some reason (potentially unrelated to the chromosomal fusion) the fused chromosome came to be dominant (This can happen very rapidly - see founder effect). At that point the secondary centromere/etc. was superfluous, and could be lost through mutations, leaving us with a single chromosome. (See for a related discussion - no endorcement implied, just the first decent result I got in searching) -- 128.104.112.147 (talk) 19:54, 2 July 2008 (UTC)


 * There seems to be a false assumption that males and females evolve completely independently of one another. This is untrue.  For the most part, only the X and Y chromosomes evolve independently, in humans.  I can try to make this into a reasonable objection to evolution:

"Since, in humans, the X chromosome determines (or at least triggers) female characteristics and the Y chromosome male characteristics, the two must evolve in parallel so that any change in females matches a corresponding change in males. For example, the tendency in females towards nurturing, homemaking, and gathering required a corresponding change in males toward hunting, to ensure that all the child-rearing and food provision needs of the group were met, utilizing a division of labor."


 * I suppose this is true, but, in cases where both sexes developed noncomplimentary traits, like everyone going hunting and leaving the children unattended, those traits would not be likely to be passed down. StuRat (talk) 15:30, 2 July 2008 (UTC)


 * Basically, he doesn't understand how evolution works, because he is making assumptions that aren't true. ScienceApe (talk) 04:55, 6 July 2008 (UTC)

He thinks there is a point where something is human, and the previous generation is not, but an individual does not change, it's an entire population. Eriorguez (talk) 22:30, 8 July 2008 (UTC)

What is the process of decompossition of the human body?
Do you need further information for that question? —Preceding unsigned comment added by Ros1701 (talk • contribs) 04:43, 2 July 2008 (UTC)
 * Have you read Decomposition? Gwinva (talk) 05:04, 2 July 2008 (UTC)

Cavity in a Unifomly charged solid sphere..
consider a uniformly charged sphere with a cavity inside. the electric field inside the cavity is non zero and uniform... but if we draw a gaussian surface inside the cavity, the charge enclosed will be zero. so field zero on the surface... why the contradiction???? —Preceding unsigned comment added by 117.201.49.226 (talk) 07:46, 2 July 2008 (UTC)


 * The electric field is zero inside. I think you are confusing electric field and electrostatic potential. The latter is uniform inside and may or may not be zero, as you're free to define a point where it's zero. Icek (talk) 08:50, 2 July 2008 (UTC)
 * Maybe they were thinking of Shell_theory - inside the 'solid' of this shell..
 * Shell_theory esp. Shell_theory may help here with the maths..
 * Shell theory explains the field (works for any inverse square relation ship) both inside and outside hollow spheres..


 * Note at the centre of the cavity the field is zero. I've no idea what you mean by "but if we draw a gaussian surface inside the cavity..." if you wan't the field then shell theory has a mathematical method of how to get it.

CLARIFY:Did you actually mean a solid sphere, or thick walled sphere with a hollow cavity inside or something else?87.102.86.73 (talk) 12:03, 2 July 2008 (UTC)
 * One answer "... inside the cavity, the charge enclosed will be zero. so field zero on the surface.." - the surface is not inside the cavity - it's on the boundary - which is different..87.102.86.73 (talk) 15:10, 2 July 2008 (UTC)

I hate to quibble, but nothing in the initial question stated that either: A) the cavity is spherical or B) the cavity is centered. Hence it could be inappropriate to jump straight to shell theory. Dragons flight (talk) 23:44, 2 July 2008 (UTC)
 * quote "consider a uniformly charged sphere ..." - it's easily to miss..87.102.86.73 (talk) 12:05, 3 July 2008 (UTC)
 * That says nothing about the shape of the cavity within the sphere. Dragons flight (talk) 16:04, 3 July 2008 (UTC)
 * Actually, even if the field is neither centered nor spherical, it will still have zero electric field within, although yes, shell theory wouldn't apply. Someguy1221 (talk) 23:57, 2 July 2008 (UTC)


 * That's not true of a uniformly charged (in a volume sense) insolator. You are correct if you assume the sphere is a conductor, which also is not stated. Dragons flight (talk) 00:04, 3 July 2008 (UTC)

Can I get fully solved IIT-JEE question papers in Physics (mechanics only) of past 10-20 years?
I want to know whether I can get fully solved Physics question papers of past 10-20 years of the IIT-JEE examination from you. If you do this small favor for me,I'll be very very thankful to you.Please send them (if possible) as an attachment to your response mai

--Prateekgreat (talk) 10:22, 2 July 2008 (UTC)prateekgreat


 * I guess "Do your own homework" needs to be expanded to "Study for tests on your own." (I presume this person wants this as part of a review for his or her exam - not sure what that ITT-JEE exam is they're asking for.)
 * And...an attachment? I *really* hope the person means a link to a site with such, because I don't know how a person could even send it to the questioner personally.209.244.30.221 (talk) 21:54, 5 July 2008 (UTC)

What defines a yeast?
I posted this at Talk:Yeast but thought I should place it here too.

What defines a yeast? Do yeasts have some intrinsic trait that qualifies a particular species as being a "yeast" rather than just generically being a "fungus"? What is special about all (or most) of the species of yeast that qualifies them as yeasts? --Alecmconroy (talk) 15:07, 2 July 2008 (UTC)
 * According to yeast, yeast is unicellular. Otherwise, fungi are usually multicellular. I think that's probably the main distinction. — CycloneNimrod talk? contribs? 16:08, 2 July 2008 (UTC)

How fast does the "wind" go when we blow on something?
Just blew on a crwling bug (likely an ant) to get it off my book outside, and it made me wonder - how fast does that "gust" blow that we creat when we blow?

I'm thikning, just from experimenting, maybe 50 MPH or so at first, but then of course air resistance will slow it down a lot. Plus, the amoung of saliva probably affects things. Holding my hand about a foot away from me (like when blowing out candles on a birthday cake) it still feels gusty, but not too much; maybe 20 MPH or so?209.244.30.221 (talk) 17:35, 2 July 2008 (UTC)


 * Sneeze gives some speed estimates. I would expect a sneeze is about the upper limit for how fast we can blow.  So, somewhere less than the speed of a sneeze, which is itself the subject of widely varying estimates.  Friday (talk) 17:43, 2 July 2008 (UTC)

Tree identification
My parents had a tree in their back yard, which was about, if I recall, about 3-5 metres tall, and had purple waxy leaves. It shed them in the winter, and was able to survive winter temperatures of -50 degrees celsius. I think it may have lived for about 15 years, and produced wrinkled, cherry like fruit. —Preceding unsigned comment added by Sliver Slave (talk • contribs) 20:25, 2 July 2008 (UTC)


 * First guess, purple leaf plum? Seems to survive in the cold, not sure about -50C though. --Wirbelwind ヴィルヴェルヴィント (talk) 21:45, 2 July 2008 (UTC)

Crude oil estimates
Where/how do they come up with estimates for how long our fuel is going to last us? I mean how can they know how much there is when its all underground? --212.120.246.239 (talk) 22:06, 2 July 2008 (UTC)


 * Directly, geologists can use techniques like seismic mapping to figure out how large an oil pocket is. Indirectly, they can measure how fast oil is being pumped from the ground -- once the rate slows down, about half the available oil has been pumped out.  Interesting articles to read are Hydrocarbon exploration, Exploration geophysics, and Hubbert curve --Carnildo (talk) 22:40, 2 July 2008 (UTC)


 * And, of course, how long oil lasts also depends on our rate of consumption. If we conserve energy and switch to alternative energy sources, oil should last longer. StuRat (talk) 03:54, 3 July 2008 (UTC)


 * They also have to account for the oil we haven't discovered yet, which can be estimated based on how commonly we find it, and they have to pick an arbitrary point where oil becomes too expensive to extract, and thus not counting things like tar sands. They also have to pick a point for not having enough fuel, as their will always be some left, but the speed at which we extract it will approach zero. — DanielLC 15:51, 3 July 2008 (UTC)


 * The traditional point of "too expensive to extract" is when the energy return on energy investment drops below 1: it takes more energy to extract the oil than can be gained by burning the oil. --Carnildo (talk) 21:46, 3 July 2008 (UTC)
 * Tar sands are counted as crude oil reserves, that's how Canada leapt at a stroke to become the nation with second-highest crude reserves. And in the case of the tar sands, the resource is at the surface, so it's relatively easy to estimate just how much is there. Franamax (talk) 01:04, 4 July 2008 (UTC)

Drafting
When driving on the interstate highways, if one wants to "draft" behind a fast moving eighteen wheeler, how close does he have to follow? WSC —Preceding unsigned comment added by 75.85.203.191 (talk) 22:29, 2 July 2008 (UTC)


 * Mythbusters did an episode on this. The benefit depends on how close you are: at a safe following distance, you get about a 5%-10% increase in fuel economy; if you're following at ten feet, your fuel economy doubles. --Carnildo (talk) 22:43, 2 July 2008 (UTC)


 * A word of caution: A truck driver once told me he really worries when someone drafts behind him. For one thing, you're driving in his blind spot when you're close enough to draft, and furthermore, truck tires sometimes throw their treads (you can see truck tire treads occasionally lying on the shoulders of freeways). These tire treads are heavy steel-belted rubber things that can fly right through your windshield. =Axlq 23:07, 2 July 2008 (UTC)


 * I keep hearing "If you can't see my mirrors, I can't see you", warnings about following too closely. I would expect that the truck driver would be more concerned about what's in front of him and to either side, since that is what controls his ability to maneuver.  The obvious danger of drafting or tailgating is that the trucker slams his breaks on (presumably for good reason) and you don't, but that danger exists whether the trucker can see you or not.


 * A related question I've always wondered about: Has anyone ever studied the effect of drafting of the truck?  I would think that the car would either (1) cause an increased low pressure behind the truck, thus causing additional drag and effectively stealing the trucks energy, or (2) the car would act as a tapered tail to the truck, thus decreasing the drag in the truck.  -- Tcncv (talk) 23:33, 2 July 2008 (UTC)


 * A word of caution: with the Mythbusters episode in question, even the farthest distance tested was too close to constitute a safe highway-speed following distance.
 * As for the question of what drafting does to the truck's fuel economy -- interesting. You can't get something for nothing, but I don't know if car+truck would closely approximate a closed system for those purposes or not. &mdash; Lomn 00:11, 3 July 2008 (UTC)
 * The truck is already losing energy through air resistance - the car may well just be using that energy, so the truck doesn't lose any more. --Tango (talk) 00:15, 3 July 2008 (UTC)


 * At 55 mph, they got an 11% reduction in fuel usage at 100 ft and 39% reduction at 10 ft (Mythbusters' wiki). Whether or not you personally consider 100 ft to be safe may be something of a matter of opinion.  It's not that uncommon on highways where I live to see cars following big rigs within that distance (which is about 6 car lengths).  A big rig isn't capable of stopping on a dime, so even if he did slam on the brakes (which let's be honest isn't that common on freeways), you'd have the benefit of his slower stopping time to react and stop or dodge.  Back when I was growing up 100 ft was actually the recommended distance at 60 mph, but I understand that people now recommend 150 ft.  So you be the judge of whether the small reduction at 100 ft is worth the (in my opinion) small risk.  Dragons flight (talk) 00:26, 3 July 2008 (UTC)


 * Given the huge mass the 18-wheeler carries, is it possible that even if you include reaction time that you would be able to brake safely in a normal vehicle?--droptone (talk) 12:14, 3 July 2008 (UTC)


 * By this reasoning a bicycle would out-brake a car, which is clearly not the case. Heavier vehicles have more powerful brakes, plus more wheels = more brakes to dissipate energy. Gandalf61 (talk) 16:17, 3 July 2008 (UTC)


 * Empirical evidence says otherwise. I haven't compared my 15-0 stopping distance to that of a bicycle, but I have compared 60-0 stopping distances with a semi (I was passing it when a car ahead of both of us lost control), and my car stopped well before the semi did. --Carnildo (talk) 21:54, 3 July 2008 (UTC)
 * The limiting factor in braking a bicycle is not the brakes. It's the tires, geometry, and weight distribution. The question is not whether you can slow down the wheels, but whether you can keep control of the bike when you do. Optimal braking is supposed to be around 70-30 or 75-25, weighting the front brake more, because the front tire presses into the pavement during braking and is therefore less prone to skidding. But hit the front brake too hard and you go over the handlebars. --Trovatore (talk) 22:15, 3 July 2008 (UTC)
 * Oh, that said--I think the drafting thing is seriously a bad idea. At highway speeds you should not cruise closer than 3 seconds behind the vehicle ahead of you. At 60 mph that's 264 feet. Of course you will have to get closer than that at times; you just shouldn't stay there. And if you don't stay there, you're not going to get much benefit in fuel economy. --Trovatore (talk) 22:47, 3 July 2008 (UTC)

Biting fly
Hi. What species of biting fly lives in southern Ontario, has a mildly painful bite, thrives near midday in June-July, and leaves a barely noticeable reddish bump about 1mm wide? I'm not asking for a diagnosis, just a vague species identification. The article doesn't seem to help, as it wasn't a mosquito, Tetse flies live in Africa, horse and deer flies are yellowish in colour, and black flies should be far more painful? I think it was grey-black in colour, had rufous-brown eyes, and was less than 1cm long. Its bite was that of a mild sting. It might sound like a blackfly, but aren't their bites more severe and larger? Thanks. ~ A H  1 (TCU) 22:35, 2 July 2008 (UTC)


 * Around the Great Lakes, I've been bitten by many basic black biting flies. It is a mild sting - no worse than a mosquito bite.  I've been bitten by deer flies also - it is severe.  One caused my whole ear to swell up.  So, I see no reason for it to be anything more than a basic biting fly. --  k a i n a w &trade; 01:03, 3 July 2008 (UTC)

Why are Galaxies disc shaped?
Just curious why some galaxies are disc shaped and not 3D collections of stars (like how I imagine Nebulae are). --70.167.58.6 (talk) 23:30, 2 July 2008 (UTC)
 * Rotation - and nebulae are gas, not stars. Rmhermen (talk) 00:03, 3 July 2008 (UTC)
 * He wasn't saying nebulae were composed of stars actually. He was just referring to the shape. ScienceApe (talk) 18:50, 3 July 2008 (UTC)


 * Nebulae often contain stars as the condensing of the gas is what causes the formation of stars. Its why we can usually see them, the gas is illuminated by the stars in there. EagleFalconn (talk) 00:14, 3 July 2008 (UTC)
 * Galaxy formation and evolution and spiral galaxy are probably good places to start. As Rmhermen says, it's all to do with the rotation - when you rotate something, it generally flattens out. Just watch someone showing off while making pizza - rather than roll out the dough, they spin it round and it flattens out nicely. The reason for the difference in shape between spiral galaxies and nebulae, I would guess, is the speed of rotation (possibly, relative to their overall mass - it might be angular momentum that's actually important) and the length of time they've existed. It's going to vary from galaxy to galaxy and from nebula to nebula, though. Plenty of galaxies aren't disc shaped (see elliptical galaxy and irregular galaxy), and there may well be some disc shaped nebulae (I honestly don't know!). I hope that helps a little. --Tango (talk) 00:13, 3 July 2008 (UTC)