Wikipedia:Reference desk/Archives/Science/2010 July 3

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July 3

Enthalpy-entropy chart

Hi. On this enthalpy-entropy chart, can someone please tell me what 'x' represents (ie x = 10%, x = 20%, x = 30%...).

The person who actually created the graph seems to have 'retired' from Wikipedia, and another user asked for a caption to be added to it.

Thanks in anticipation,  Chzz  ►  01:32, 3 July 2010 (UTC)

It's either Percent humidity, i.e. the ratio of actual vapor pressure to vapor pressure at the dew point, or some sort of meaure of proximity to the critical point. --Jayron32 01:38, 3 July 2010 (UTC)

I found a few descriptions that say that the x lines are "constant moisture or quality" lines. I don't know what that means, though. Looie496 (talk) 04:45, 3 July 2010 (UTC)

Constant moisture implies relative humidity, which is the link I speculated on first... --Jayron32 04:55, 3 July 2010 (UTC)

This is a Mollier Diagram for water-steam. It is named after Richard Mollier a German physicist. His water-steam diagram depicts the properties of steam and is used to estimate the performance of steam engines and steam turbines. X is called the dryness fraction of the steam. At Mollier diagram it says Lines of constant dryness fraction are drawn in the wet region ...

Dryness fraction represents the quality of steam for the purpose of doing work or supplying heat. X=90% refers to steam which consists of 90% (by mass) of dry steam and 10% colloidal liquid water. (Cloud is an example of colloidal liquid water mixed with gas. In a cloud, the gas is humid air. X is not referring to relative humidity which implies the presence of air. The diagram shows the critical point which is a property of water, not humid air.) Outside the boundary labeled as 100% the steam is superheated - it contains no colloidal liquid water, and the steam is hotter than the boiling point at the prevailing pressure.

Steam with X less than 100% is called wet steam. Steam with X=100% is called saturated steam or dry steam. When dry steam is heated it is called superheated. See Steam and Superheated steam. Dolphin (t) 06:48, 3 July 2010 (UTC)

Medroxyprogesterone versus medroxyprogesterone acetate

Hola,

I'm trying to figure out the difference between medroxyprogesterone versus medroxyprogesterone acetate. Anyone know? Anyone have any sources that can be used to distinguish the two? WLU (t) (c) Wikipedia's rules:^simple/_complex 01:57, 3 July 2010 (UTC)

Other than the obvious difference in chemical structure, what are you looking for? It appears that the 17-acetate is the only one used medically - both of the references in the MP article actually relate to MPA. The Merck Manual entry indicates that the former is "supplied as the acetate". The article in MedlinePlus uses the terms interchangeably. Many references discuss plasma levels of MPA, so my initial impression that the acetate is readily hydrolyzed is probably false. -- Scray (talk) 02:59, 3 July 2010 (UTC)

the following 4 posts were copied from WikiProject Medicine, as discussed there

(Original research alert) First, pregnanes and progestins are not mutually exclusive. Pregnane refers to the structure of the molecule, while progestin refers to its biological activity. Medroxyprogesterone is the active molecule. In order to stabilize medroxyprogesterone in a form that can be administered orally or intravenously, it's acetylated (see the final step in File:Medroxyprogesterone_acetate.png. In the body, the acetate residue is degraded and you get the active molecule back.

If you're talking about administering medroxyprogesterone as a pharmaceutical, then you're technically talking about MP acetate. If you're talking about steroid biochemistry in vivo, then it's more correct to refer to medroxyprogesterone, period. I'm not sure what implications this has for our article structure (and this is just me talking - it's been awhile since I took pharmacology, and I don't have a supporting source at my fingertips, so take it with a grain of salt because I have been known to be wrong). MastCell ^Talk 03:58, 3 July 2010 (UTC)

MastCell, thanks for correcting me (pending further corroboration/refs). That was my initial sense (see the RD/S history for my original comments if curious) but I couldn't quickly find anything reliable that supported me. I'll look again tomorrow if no one weighs in further. -- Scray (talk) 04:02, 3 July 2010 (UTC)

I wish I had some refs handy to back me up. BTW, I saw your comment at the Ref Desk about MPA levels ([1]). Are you sure those are actually looking at levels of medroxyprogesterone acetate? I'm not familiar with measuring levels of it. On the other hand, there's an extensive literature on monitoring plasma levels of mycophenolic acid (also abbreviated MPA), which is the active metabolite of mycophenolate mofetil (MMF). Like most immunosuppressants, MMF pretty finicky and has a narrow therapeutic window, so a lot of work has been done on MPA pharmacokinetics (mycophenolic acid) in the organ transplantation literature. You've probably already looked into this, but is it possible that the "MPA" levels you saw were actually mycophenolic acid, rather than medroxyprogesterone acetate? MastCell ^Talk 04:30, 3 July 2010 (UTC)

I'm very familiar with mycophenolate; the particular paper that made me doubt myself was this one, which led me to PMID 6457936, which is entitled "Medroxyprogesterone acetate in human serum". On re-reading the abstract, this may simply be sloppy terminology, since the RIA they're using might not distinguish medroxyprogesterone from the acetate. Clarity remains elusive, but I think you're probably right. -- Scray (talk) 05:01, 3 July 2010 (UTC)

end of material copied from WikiProject Medicine -- Scray (talk) 05:14, 3 July 2010 (UTC)

Can you clatify how or what property you want to distinguish, or what sort of test (chemical,biological,physical) you want to use to distinguish them.87.102.21.49 (talk) 11:27, 3 July 2010 (UTC)

I'm mostly interested in being able to find sources that distinguish between the two. Medroxyprogesterone and medroxyprogesterone acetate are both stubs and so far I haven't been able to find sources that are clear on the difference.

And my apologies for cross-posting this to the extent I did. To make things worse, I'm going to compile the posts at Talk:Medroxyprogesterone 17-acetate. WLU (t) (c) Wikipedia's rules:^simple/_complex 13:34, 3 July 2010 (UTC)

Can you expand on "distinguish between the two" - they are different compounds (the strcuture, CAS number, formula, name, molar mass etc are all different) : one is the acetate ester of the other. How do you want to distinguish between them? Chemically, physically, by effects?? 87.102.21.49 (talk) 14:03, 3 July 2010 (UTC)

Dude, the acetate probably helps with the bioavailability. When it gets to the active site, it probably gets deacetylated into your steroid. Think morphine versus heroin. John Riemann Soong (talk) 21:08, 7 July 2010 (UTC)

jail

can you have a electric space heater in jail if a relative brings it to you? —Preceding unsigned comment added by Alexsmith44 (talk • contribs) 08:15, 3 July 2010 (UTC)

It depends on the category of the jail, some low category jails may allow it, but not very likely due largely to the cost implications on the jail, I mean, just imagine all the prisoners having space heaters. In normal medium and high category prisons the possibility is zero for a range of security reasons. Richard Avery (talk) 10:11, 3 July 2010 (UTC)

what are the security reasons? —Preceding unsigned comment added by Alexsmith44 (talk • contribs) 10:17, 3 July 2010 (UTC)

The heater could have parts that could be used as a weapon.--Mr.K. (talk) 10:52, 3 July 2010 (UTC)

Or the heater could be used to start a fire. Really, I doubt any prisons would allow it. Maybe a halfway house, but that's about it. — Lomn 12:22, 3 July 2010 (UTC)

I doubt they would allow it - there are strict limits of the luxuries you are allowed in prison, and space heaters are a serious fire risk. They'll probably let your relatives bring you an extra blanket (in fact, the prison may well bring an extra blanket if you ask). A prison should be heated to a reasonable temperature anyway. If you have a medical condition that means you need a higher ambient temperature, they may well make allowances for you. (I'm assuming a prison in a developed country with a good human rights record - if either of those things don't hold, then there is no guessing what horrible conditions they might keep prisoners in.) --Tango (talk) 15:28, 3 July 2010 (UTC)

No jail in North America, at least, would permit a heater, due to fire risk, load on the electrical system, weaponization, and as a useful place to store contraband. Acroterion _(talk) 16:08, 3 July 2010 (UTC)

I imagine that's correct, but since this is the Reference Desk, perhaps someone would like to find a reference on the subject? --Anonymous, 17:15 UTC, July 3, 2010.

Do jail cells even have power sockets? This would seem unlikely to me Nil Einne (talk) 17:54, 3 July 2010 (UTC)

They might. Prisoners are sometimes allowed TV's or radios in their cells or reading lights. There might be somewhere to plug the vacuum cleaner. --Tango (talk) 18:28, 3 July 2010 (UTC)

I had a friend in a low security prison in the US. He was permitted to have some electronic devices (Specifically, I bought him a box fan). I don't know if he would have been allowed a space heater. It seems though that the heat was more of a problem than the cold, which is the case in many densely populated buildings. Buddy431 (talk) 21:08, 3 July 2010 (UTC)

It depends on the jail. In some, such as that described here, just about anythings goes. Zoonoses (talk) 12:35, 4 July 2010 (UTC)

Trapped in a fusion reactor?

Supposing a man got accidentally locked in a fusion reactor such as JET and it was switched on. Is the plasma dense enough and does it last long enough to do serious harm? We will assume he has a spacesuit so that the effects of vacuum can be ignored. —Preceding unsigned comment added by 80.1.80.3 (talk) 08:21, 3 July 2010 (UTC)

The temperatures are higher than the surface of the sun. The whole problem with fusion reactors of this kind is that the plasma has to be contained in a 'magnetic confinement' to avoid it vaporizing the walls of the reactor. So our poor test subject would likely be vaporized in a small fraction of a second. Still, that will prevent the huge amounts of radiation and energetic particles from being a problem. SteveBaker (talk) 14:39, 3 July 2010 (UTC)

We are talking about the same nuclear reaction that drives the sun, if a person could not survive on the sun, how could they survive in the nuclear reactor? The interior of the reactor has to be hot enough to keep the plasma ionized and it has to be compressed to undergo fusion, and considering people are made of matter, they would be vaporized just like the hydrogen and/or helium gases that power the reaction. In addition, some methods for heating the gases use high intensity microwaves, essentially turning the divice into a giant microwave oven. In other words, between the temperatures required for fusion, the radiation and energy produced by the reaction, and some of the ways the gases are heated, there is no way the unlucky person in the reactor would survive. —Preceding unsigned comment added by 74.67.89.61 (talk) 14:56, 3 July 2010 (UTC)

I apologise for my colleagues being so dismissive of this question. You are absolutely right that just because a plasma is very hot doesn't mean it would actually burn someone. Parts of the interstellar medium is plasma at 10,000K but obviously that wouldn't burn anyone because it is so sparse. Our article on the Joint European Torus says the plasma lasts 20-60s, so that would be long enough to burn if it is dense enough. So the question, as you say, is what is the density of the plasma? As the anonymous response says, the density has to be at least above a certain level in order for fusion to take place. Unfortunately, I can't find a description of what that density actually is. The JET website is full of interesting information, but apparently not that piece of information. The pressure inside the Sun is enormous, but the temperature there is more than 100 times the maximum temperature achieved at JET, so clearly the two situations are not comparable. --Tango (talk) 16:01, 3 July 2010 (UTC)

I believe exposure to neutrons would be a major problem, ignoring thermal energy flux for the moment, since the reaction liberates large quantities of fast neutrons (whose energy is harvested from the lining of the tokamak). Acroterion _(talk) 16:06, 3 July 2010 (UTC)

The neutron radiation could easily be a serious risk, you are right. Does anyone know the neutron flux for JET? --Tango (talk) 16:43, 3 July 2010 (UTC)

The original poster mentioned density. Is the pressure / amount of plasma sufficient? You can have very high temperature (= velocity) atomic particles, but if there are not too many of them, their combined punch will not be able to heat a cup of tea, much less vaporize anyone. I guess it's not so much about temperature than about total thermal energy. Also perhaps the type of particles matters; some kinds, while quite hot, might pass through our intrepid adventurer without sharing their kinetic energy? 88.112.56.9 (talk) 16:16, 3 July 2010 (UTC)

You can calculate the density from the pressure and amount of plasma, yes, but I can't find those numbers either. Total thermal energy would be a good way to estimate the harm, but the thermal conductivity of the plasma would be a factor too - a hot metal bar burns you more than a plastic bar with the same temperature and thermal energy, since more of the energy actually gets to you. --Tango (talk) 16:43, 3 July 2010 (UTC)

My intuition (= no sourceable fact content in this comment) suggests that gas/plasma uses a different mechanism to impart heat energy than a solid does. Gas goes: atom-skin. Solid goes: atom-atom-atom-atom-atom-atom*100000-skin. Atoms in tenuous gas/plasma do not conduct, they just hit you. 88.112.56.9 (talk) 19:54, 3 July 2010 (UTC)

Plus inside the plasma, the person would be crushed because the plasma is compressed.--dance (talk) 20:41, 3 July 2010 (UTC)

How much is it compressed by? I can't find any figures for the pressures they use. --Tango (talk) 21:15, 3 July 2010 (UTC)

The first thing they do before establishing the plasma is remove all the air with very good vacuum pumps. He will simply die of the lack of air. The person will ruin the vacuum, because the water in the body of a person takes ages to evaporate after some time hours or days they will open the chamber and find the person.--Stone (talk) 22:00, 3 July 2010 (UTC)

N.B.: per the OP: "We will assume he has a spacesuit so that the effects of vacuum can be ignored" -- Scray (talk) 22:09, 3 July 2010 (UTC)

They're going to let a multi-billion-dollar fusion reactor sit around for days with no vacuum before they bother finding out why? Boy, that fusion-workers' union — you don't mess with them, huh? --Trovatore (talk) 22:05, 3 July 2010 (UTC)

It won't be no vacuum, just a very slightly poorer vacuum. --Tango (talk) 01:07, 4 July 2010 (UTC)

For the purposes at hand, I imagine that's equivalent to no vacuum. --Trovatore (talk) 01:21, 4 July 2010 (UTC)

Thank you! I went to try and look that up (and didn't really succeed) and found just the information we need. This page says: "The densities of the hydrogen plasma that can be confined by magnetic fields are very low, about one million times lower than the density of air." I think that's low enough to be pretty harmless, even at temperatures of 100 million Kelvin. --Tango (talk) 01:33, 4 July 2010 (UTC)

No way! Just stop and think about what you're saying. The ENTIRE point of going to all that trouble with magnetic confinement is to prevent the plasma from vaporizing the walls of the reactor! If spacesuit material could comfortably withstand the heat/pressure then making a fusion reactor would be a pretty trivial problem - you'd make a simple spherical container and line it with 'spacesuit material' and you'd be done...so much for 60 years and billions of dollars of research! Our Tokamak article says "Magnetic fields are used for confinement since no solid material could withstand the extremely high temperature of the plasma."...so our intrepid experimenter is very, very dead from heat alone. You - and the others who say that the pressure is too low - need to explain why you'd need magnetic confinement if heat was not a major problem at the pressures the reactor is designed to run at. SteveBaker (talk) 03:26, 4 July 2010 (UTC)

That argument doesn't necessarily add up. I read the statement you referenced as "the magnets are needed to keep the plasma pure enough and at a high enough temperature" rather than "the magnets are needed to protect the structural integrity of the device". "Plasma vaporizing the walls of the container" isn't the same thing as "completely destroying the walls". From Magnetic confinement fusion, it appears that the magnets are to prevent "sputtering", where a portion of the wall is vaporized, thus lowering the temperature of the plasma. It's unclear how much of the container wall is actually vaporized, though. It's conceivable that a very small vaporization of the container (or man in a spacesuit) could ruin the experiment (by lowering the temperature and adding heavier element contamination), but still not compromise the structural integrity of the container (or health of the man in his spacesuit). Additionally, it appears that the plasma is only contained, heated, and compressed for a couple of seconds at a time. I don't know how much material would be vaporized in a couple seconds, and whether an ordinary spacesuit would stand up to it. Buddy431 (talk) 04:21, 4 July 2010 (UTC)

Right, that jibes with a vague memory of something I read somewhere, that the reason it is so essential to keep strict magnetic confinement is to keep the plasma hot; any failure of confinement, and consequent contact with a solid object, means your temperature instantly drops out of the working range for fusion. What happens to the solid object might also be a concern, of course, but keeping the plasma hot is a sufficient reason for confinement in itself, even if the damage to the torus were tolerable. --Trovatore (talk) 05:48, 4 July 2010 (UTC)

That's correct. If there were contact between the plasma and the walls, heat would be conducted from one to the other. A few wisps of plasma (which is all there is) will only remove a very thin layer from the walls. See the answer to the question "Q: I recently heard a sort-of scientific urbanmyth that at some point during its operation the plasma inside thetorus touched the top and the whole thing jumped up in the air. Isthere any truth to this story?" on the FAQ on the JET website (here). --Tango (talk) 06:25, 4 July 2010 (UTC)

Your person probably won't get hurt. The small amounts of plasma might heat his suit up, say 1 degree, while the plasma is no more plasma (just a gas). --Chemicalinterest (talk) 15:37, 6 July 2010 (UTC)

If the person would be fine, what's the point of making a fusion reactor? How to you use the heat if the heat is neglible?--92.251.137.196 (talk)

Metric system

What's the metric prefix for 10²⁷? --75.149.70.22 (talk) 14:28, 3 July 2010 (UTC)

There isn't one. The official list is at SI prefixes, and stops at 10²⁴. Physchim62 (talk) 14:34, 3 July 2010 (UTC)

Our article on SI prefixes mentions hella as an option (not, it seems, used outside the USA). --88.117.82.138 (talk) 14:36, 3 July 2010 (UTC)

"Hella" is an idea at the moment – it isn't used anywhere, and doesn't seem likely to be adopted anytime soon (if at all). If they chose "hella" for 10²⁷, what would be the prefix for 10³⁰???? Physchim62 (talk) 15:11, 3 July 2010 (UTC)

hellalotta- you asked for it 87.102.21.49 (talk) 15:19, 3 July 2010 (UTC)

I was thinking more along the lines of "f*ckloadsa", but we can always save that for 10^33... Physchim62 (talk) 15:38, 3 July 2010 (UTC)

I wouldn't say that it isn't used anywhere. Dismas|^(talk) 00:37, 4 July 2010 (UTC)

Isn't "Hella" the Norse name for the underworld? 67.170.215.166 (talk) 01:20, 4 July 2010 (UTC)

_{try the search box?} —Preceding unsigned comment added by 77.86.10.42 (talk) 12:19, 4 July 2010 (UTC)

That was a rhetorical question on my part -- and FYI, I knew all along that the answer is "yes it is". 67.170.215.166 (talk) 00:04, 6 July 2010 (UTC)

the sun

approximately how many fusion reactions occur within the sun every second? And how much light from it hits the moon?80.47.187.29 (talk) 17:22, 3 July 2010 (UTC)

The energy released by the creation of one Helium-4 nucleus by the Proton–proton chain reaction is 23.4MeV. The luminosity of the sun is about 4x10²⁶W. If we divide one by the other, we get about 10³⁸ Helium-4 nuclei being created every second. The amount of light hitting the moon is the solar constant multiplied by the cross-sectional area of the Moon. That's 1.4kW/m²*(pi*1700km^2)=7.5x10¹² W=7.5 TW. --Tango (talk) 17:39, 3 July 2010 (UTC)

Presuming of course that nothing is between the sun and the moon, like during an eclipse. Googlemeister (talk) 13:37, 6 July 2010 (UTC)

nucleus diameter

How scientists measure the diameter of a nucleus? —Preceding unsigned comment added by Lukyshubham (talk • contribs) 17:58, 3 July 2010 (UTC)

You can get an estimate from the Geiger–Marsden experiment. Physchim62 (talk) 18:04, 3 July 2010 (UTC)

As Rutherford himself might have illustrated, imagine you are Jesse Ventura in Diamonds Are Forever (film). Jesse "The Body" has to find a single diamond hidden in a soft-brick wall in 10 minutes, with only his minigun in hand. He fires millions of bullets spread randomly on that wall, slicing cleanly in-and-out of the clay for 8 minutes until finally he hears a loud "ping" as one of the bullet literally bounces back. "Bullseye", he says with a smirk, then spits his tobacco on the floor... before being skinned alive by The Predator.

The moral of the story is that Rutherford fired alpha particle bullets into a metal foil wall to find the size of that diamond, the nucleus, as a ratio of the clay brick wall, the electron cloud. He fires millions of alphas, and millions go clean through the foil, but occasionally one or two bounce back. The cross-sectional area of the nuclei is then the ratio of #particles bounced back divided by # particles fired, multiplied by the area of the foil wall. Jesse Ventura fired a million bullets, one bounced back, onto a 10x10-meter wall, so the length of the diamond is about sqrt(100m^2 / 1000000) meters = 1cm. SamuelRiv (talk) 03:18, 4 July 2010 (UTC)

Which Diamonds Are Forever (film) are you talking about? Jesse Ventura never was in that movie -- it was Sean Connery as James Bond, and there wasn't any scene with a Gatling gun or a diamond hidden in a brick wall (I know, I've seen it on DVD)! 67.170.215.166 (talk) 04:19, 5 July 2010 (UTC)

It's in the "deleted scenes". Actually, I thought I made it quite clear that this was a remake of Diamonds Are Forever starring Jesse The Body and replacing Kidd and Wyndt with The Predator and requiring Bond to find a diamond in a clay wall using only his minigun for some reason. AND YET IT'S STILL MORE REALISTIC THAN THE ORIGINAL! SamuelRiv (talk) 08:19, 5 July 2010 (UTC)

In case the OP wasn't talking about an atomic nucleus and instead wanted to know about a cell nucleus, you can use a microscope. Smartse (talk) 21:12, 4 July 2010 (UTC)

Decreasing metabolism?

Right now, there is great interest in increasing metabolism to help combat obesity (including interest in leptin and uncoupling proteins). I was wondering whether there is any research being done about how to decrease metabolism. Why is it that when we talk about solutions to global starvation, we never talk about metabolism – we always talk about growing more food? I should be particularly interested in any effect that would help the body burn several hundred calories less each day.174.131.45.82 (talk) 18:09, 3 July 2010 (UTC)

That's easy: do less. That's not usually an option for people facing starvation, though. --Tango (talk) 18:26, 3 July 2010 (UTC)

Doing less would burn fewer calories and prevent metabolism from increasing as a result of exercise. I was looking for a way to decrease metabolism other than inactivity or caloric restriction, however.174.131.45.82 (talk) 18:59, 3 July 2010 (UTC)

Along the lines of medication that decrease metabolism? As far as I know, that's usually considered a side effect rather than a benefit... Ks0stm ^(T•C•G) 20:08, 3 July 2010 (UTC)

I doubt that using medications would be safe, and I do not know of any medications that decrease metabolism as a side effect (many drugs cause unwanted weight gain - insulin can cause some people to gain weight, but it actually increases the metabolic rate).174.131.40.152 (talk) 20:58, 3 July 2010 (UTC)

I think the human body already has pretty well developed famine modes which it goes into when near starvation. I doubt tinkering with a few chemicals would improve what happens naturally. --BozMo talk 21:38, 3 July 2010 (UTC)

We do have drugs that can decrease metabolism, but if your metabolic range is within normal range, it is not a good idea to use these drugs. They would cause someone to develop hypothyroidism which does have have very desirable effects. —Preceding unsigned comment added by 76.91.30.156 (talk) 02:03, 4 July 2010 (UTC)

I already knew about ATD's (antithyroid drugs). What I am wondering is this: why do some people have slow metabolisms (excluding people with hypothyroidism and Cushing's syndrome)? How are they different from us? What could we learn from them? One answerer also talked about the body adapting to starvation. It is true that metabolism does slow down a little during starvation, but this occurs, of course, only when a person does not consume enough calories. Would it be possible to decrease metabolism so that a person would need fewer calories in the first place, without ever actually reaching a negative energy balance.174.131.68.138 (talk) 02:20, 4 July 2010 (UTC)

Wouldn't that just make them fat? 67.170.215.166 (talk) 07:57, 4 July 2010 (UTC)

[Geography] Locate or name the lake by picture

I've seen some different wallpapers that picture one lake.

These are links to 3 different wallpapers with this lake: (Warning: links spend traffic, these are large pictures (1920x1080 or larger))

http://i8.fastpic.ru/big/2010/0704/2a/56aa011b5871afdeece9e24644d4682a.jpg

http://www.goodfon.ru/download.html?id=39203&rash=1920x1080

http://www.goodfon.ru/download.html?id=30598&rash=2048x1536

Where's this lake, does anybody know? And does it have a name?

--Grue12 (talk) 22:13, 3 July 2010 (UTC)

Moraine Lake in Banff National Park, Canada. Deor (talk) 22:16, 3 July 2010 (UTC)

Thanks a lot! --Grue12 (talk) 22:35, 3 July 2010 (UTC)

Superball dynamics

Hi, I was reading an article on Superball dynamics, and they said that during a collision with a table or something, the normal component of the velocity will change sign without justification. Why would this be true? 74.15.137.192 (talk) 23:01, 3 July 2010 (UTC)

It's basically reflection. The kinetic energy gets converted into elastic potential energy as the ball compresses and then that elastic potential energy is converted back into kinetic energy in the other direction as the ball rebounds. --Tango (talk) 23:54, 3 July 2010 (UTC)

But why wouldn't this potential energy get converted to rotational kinetic energy or tangential kinetic energy? 74.15.137.192 (talk) 00:12, 4 July 2010 (UTC)

It just doesn't. That's the way physics works. You can't get rotation with a torque and you can't get tangential movement without a tangential force. The force between the ball and the table will always be normal to the table. --Tango (talk) 00:18, 4 July 2010 (UTC)

But if the ball is spinning and has an initial component of its velocity parallel to the table, then surely there can be some torques and frictional forces, no? 74.15.137.192 (talk) 00:31, 4 July 2010 (UTC)

Ah, if it's spinning that's a different matter entirely. A non-spinning ball with an initial parallel component to its velocity will end up with a slightly slower parallel component due to friction - that's completely independent from the normal component, though, which is why I didn't mention it. --Tango (talk) 01:05, 4 July 2010 (UTC)

Sorry for not being clear. Is your explanation still valid? 74.15.137.192 (talk) 01:36, 4 July 2010 (UTC)

Wait a minute, Tango - I think IP74 is also asking why all the force is normal, even without a rotational component but with a component parallel to the table, since that parallel part gets a bit "stuck" on the frictional table surface and thus decreases the angle of reflection. That would lower the final velocity but conserve both energy and momentum since the table is pushed in the process.

By the way, Tango, not sure if you teach undergrad or high school physics labs ever, but if you ever get to use those collision carts on the low-friction airtrack, have your students model the curve that the computer outputs at the half-second during collision - it's a nice spring-storage SHM, illustrating that springs are everywhere, even in supposedly-rigid metal carts. SamuelRiv (talk) 03:05, 4 July 2010 (UTC)

Well, the OP was asking about the normal component of velocity, which is effected by the normal component of the force between the ball and table. It's always difficult to answer "why?" questions in physics - there always comes a point where you just have to say "That's just the way it is.". --Tango (talk) 06:15, 4 July 2010 (UTC)

I was trying to answer OP's question after your first explanation, about rotation components. Regarding the great "why" of the Normal Force, I defer at that point to "this is where we introduce the Conservation of Momentum". Indeed, the Normal Force can be explained in terms of smallball-bigball for most purposes where the Normal is actually useful. When it's not useful, it's a convention issue (we can use Lagrangian mech instead - that gave one class some relief that there was some arbitrariness in this grandiose of subjects). SamuelRiv (talk) 06:31, 4 July 2010 (UTC)

What I understood from this was, the normal force doesn't depend on any tangential or spin velocity (that just adds frictional forces), and because the normal velocity reverses in a no-spin/no-tang. velocity scenario, it must always do so. Is that the right reasoning? 74.15.137.192 (talk) 07:01, 4 July 2010 (UTC)

Time Dilation

The faster something moves, the slower time passes for it, but doesn't this prove something is moving which indicates absolute space? Assume that there is a train passing a train station and bob is on the train and alice is observing the train pass on the station. If alice watches bob's actions through a window she will see what bob is doing in slow motion. Likewise if bob watches alice's actions, it will seem everything is happening super fast (assuming the train was traveling at a signifigant proportion of the speed of light). According to relativity, as long as neither person is accelerating, both alice and bob could argue that they aren't moving and the other person is, but if time changes based off speed and time was passing slower for bob, wouldn't that prove that he is the one moving because if he was trying to argue that alice was the one moving, then she would seem to be acting in slow motion. Because it is bob that time is passing slower for doesn't that prove he is the one moving which violates relativity? —Preceding unsigned comment added by 74.67.89.61 (talk) 23:53, 3 July 2010 (UTC)

Actually, both Alice and Bob see the other moving in slow motion. This seems like a contradiction, but because of the finite speed of light it does actually work. If you do the maths, it all works out. --Tango (talk) 23:56, 3 July 2010 (UTC)

It's only if they come back to compare there clocks that there's a problem. But that's hard to do. Either it takes light from one clock a finite amount of time to reach the other person (and everything works out, as 74 asserts), or one (or both) of them have to accelerate to meet the other... Which means he (or she, the one who accelerates) is at some point in a non-inertial reference frame, where special relativity is no longer sufficient, and general relativity is needed to explain the time dilation effects. Buddy431 (talk) 02:51, 4 July 2010 (UTC)

No, general relativity is not necessary to understand the correct answer to that question. There is no gravity in that question and no gravity means no need for GR. Just because you used the word acceleration does not mean that GR is required for a correct answer. What Tango said is essentially correct. The solution to the apparent paradox passes by the definition of proper time. More on that later (it's late and I'm tired). Dauto (talk) 03:49, 4 July 2010 (UTC)

One of the central principles of GR is that the effects of acceleration are indistinguishable from the effects of an equivalent gravitational field. So I don't see why you say that a problem that involves acceleration but no gravity does not need GR, since we can rephrase the problem in terms of a gravitational field with no acceleration, and the results must be the same. Surely acceleration and/or gravity => non-inertial reference frame => GR, not SR ? Gandalf61 (talk) 13:16, 4 July 2010 (UTC)

There is no acceleration involved, and no gravity. The question refers to special relativity. --Wrongfilter (talk) 15:07, 4 July 2010 (UTC)

The original question is just SR, yes. But if you read the thread you will see that Buddy431's extension, where Alice and Bob meet up again to compare clocks in the same reference frame, introduces acceleration and hence GR - it is a version of the so-called twin paradox. And this was what Dauto's reply and my own reply to Dauto were referring to. Gandalf61 (talk) 17:22, 4 July 2010 (UTC)

You don't need GR to deal with nongravitational acceleration. SR is fine. You can, if you like, use general relativity and say that in an accelerating reference frame there is a gravitational field, but that's just words. The GR predictions are the same as the SR predictions, and the mathematical framework in which they're derived is the same. There are no GR corrections. It's true that you can't accelerate an inertial reference frame, but that's an intrinsic property of inertial reference frames, not an inadequacy of the theory describing them. You can accelerate objects of finite extent, like rocket ships, and SR correctly describes the acceleration. -- BenRG (talk) 19:17, 4 July 2010 (UTC)

But if that were true, then we wouldn't need GR at all, as we could convert any problem involving gravitational fields into an equivalent problem with accelerations and no gravity, and then solve it using SR alone. Is that what you are saying ? Gandalf61 (talk) 22:51, 4 July 2010 (UTC)

No, that's not what he is saying. The only problems that can be solved that way are the ones where there is no source of gravity. GR is a theory of gravity. When there are no sources of gravity GR reduces trivially to SR. In GR the presence of Energy (or momentum) density (or flux) leads to the warping of space-time. In the absence of any (large) energy-momentum density-flux GR predicts a flat spacetime AKA Minkowski space-time. The theory that describes the motion of objects within that flat space-time is SR. The objects that are moving within that flat space-time may be accelerating but the space-time is still flat and therefore SR is enough to describe that kind of situation. Just because I used the word acceleration in that last phrase doesn't mean that GR is now required since the space-time remains flat. I hope that helps clear that point. Dauto (talk) 01:58, 5 July 2010 (UTC)

Hmmm. So an observer who notices that spacetime in the frame of reference in which they are at rest is not flat can either (a) assume they are in a gravitational field, and use GR in their own frame of reference to handle this non-flat spacetime or (b) assume they are accelerating, and use SR in an inertial frame of reference (which is accelerating relative to them) in which spacetime is flat ? And they will get the same results in either case ? Gandalf61 (talk) 09:19, 5 July 2010 (UTC)

Not quite. A curved space will be curved in any frame of reference (No matter what system of coordinates you chose to use the surface of the earth will never be a plane). But there is always a flat reference frame that is tangential to any given point of the curved spaca-time which is good enough an approximation for small enough a region around that point. The better the approximation you need the smaller the region will have to be. Within that region gravity can be replaced by an accelerated motion in that tangential flat space. That's why the equivalence principle is said to be correct only locally, but not globally. Globally the non-flatness becomes evident and shows up as tidal forces that cannot be removed by a change in reference frame. The equivalence principle is a consequence of the fact that the gravitational forces really are pseudo-forces that come about because of the choice of a non-inertial reference frame. In that sense gravity is a force similar to centrifuge force or to Coriolis force. And that's also why gravitational forces are proportional to the mass of the object in which it acts (Centrifuge and Coriolis forces also are proportional to the mass). Gravity actually belongs on the right side of the equation F=ma. The difference is that because of the curvature of space there isn't a single inertial reference frame capable to cover the whole space-time. different inertial reference frames have to be used at different points in space-time. Dauto (talk) 13:28, 5 July 2010 (UTC)

The misunderstanding is at the very beginning of the question: For the thing that is moving there is no change in "how fast" time passes. Your wrist watch almost advances at the same pace, whether you're lying in bed, riding on a train or on an interstellar spacecraft or even sliding through a wormhole (hypothetically). Your wrist watch (alternatively, all the physiological processes in your body) runs according to your personal time, so called "proper time" (as hinted at by Dauto). If Bob rides on the train, then Alice, standing on the platform, will see Bob's wrist watch run slow compared to her own watch. Conversely, Bob will see Alice's wrist watch run slow compared to his own. The situation is symmetric, which means that there is no absolute time. Sounds weird but that's the way it is and there are no logical contradictions. --Wrongfilter (talk) 15:07, 4 July 2010 (UTC)

Ah, I think this is actually where I meant to put this comment, rather than at the "neutron star" thread above.

It is important to distinguish what Alice and Bob see from what happens in Alice's and Bob's coordinate systems; these are not necessarily the same thing at all. What they see is affected by the passage of light between them, and is subject to the Doppler shift and to various distortions that are not purely relativistic (I'm sure we have an article somewhere, but the title is too long to remember; something like visual appearance of fast-moving objects?).

The safe, if slightly laborious, way to keep track of it all is to imagine that Alice and Bob have both set up a network of space probes that extend as far as necessary in all directions. The probes make sure they are at rest with respect to one another by shining laser beams at one another and making sure there's no Doppler shift, and they keep their internal clocks synched up by the laser beams as well (they transmit the reading on their clock; the neighboring probe makes sure that it's correct after subtracting the time the light takes in transit). Alice's probes are at rest with respect to Alice, and Bob's with respect to Bob. Then Alice's probes take pictures of Bob's watch as Bob passes them, and Bob's probes take pictures of Alice's watch. The "slowdown" is computed by comparing the pictures with their timestamps.

That's all in flat spacetime, pure special relativity. When you have gravitational effects it all gets more complicated, and there are more assumptions that need to be specified. Near the event horizon of a black hole these all become especially bad. --Trovatore (talk) 03:19, 5 July 2010 (UTC)