Wikipedia:Reference desk/Archives/Science/2009 October 21

= October 21 =

How does the speed of light speed limit jive with physics shortly after the big bang?
I heard on a TV show that a microsecond after the big bang, the universe was about the size of our galaxy. That's a lot more than 3*10^8 m/s. 71.161.59.133 (talk) 01:43, 21 October 2009 (UTC)


 * Two answers. One is that the fabric of the universe itself is not limited to the speed of light. That is, special relativity does not prohibit the universe expanding faster than the speed of light, even if objects within the universe are prohibited from doing such. (Relevant article: metric expansion of space.)
 * But wait! If that's the case, why is the universe basically homogenous? That is, if it is expanding faster than the speed of light, then how did the initial light get spread throughout the existing universe pretty evenly? (Which it is.) For the currently accepted as the tentative answer, see the article on inflation (cosmology). Basically the most popular theory is that for a brief period, the universe expanded extra fast, and then slowed down again. This seems to jive with experimental evidence, though why it inflated, and why it stopped inflating, are not really understood (and there is a certain ad hoc aspect to the theory in general—it looks a lot like a "patch", even though it is pretty well accepted these days). This allowed the universe to stay rather small for awhile longer, becoming homogenous, and then to expand real quick-like. --Mr.98 (talk) 02:01, 21 October 2009 (UTC)
 * Just by the bye, I think the word you want is jibe. Jive is something else. --Trovatore (talk) 02:05, 21 October 2009 (UTC)
 * Hmm, you're right. Though it's funnier if they are really jiving. --Mr.98 (talk) 02:10, 21 October 2009 (UTC)
 * The Wiktionary article usage notes state that jive and jibe are interchanged in the U.S. - I have never heard the term "jibe" - so regardless of the authoritative "correct" usage, it seems that "jibe" doesn't jive around here... Nimur (talk) 13:35, 21 October 2009 (UTC)
 * It says "most sources consider [jive] an error". I am American, and also consider it an error.  It's a mis-hearing, like "boldface lie" and "you've got another thing coming". --Trovatore (talk) 19:38, 21 October 2009 (UTC)
 * I see, the balloon analogy at metric expansion of space helped. So two objects on the surface of the "balloon" of space can move apart from each other at a rate faster than the speed of light if the "balloon"'s expansion is what's causing (or contributing to) their displacement. Too bad we don't know of a way to contract the fabric of space between two destinations so we can travel a long distance while still obeying the cosmic speed limit of c. 71.161.59.133 (talk) 22:03, 21 October 2009 (UTC)
 * Indeed. If we had some matter with negative mass (which we don't) we could make something like an Alcubierre drive, which is basically what you've described. --Tango (talk) 22:07, 21 October 2009 (UTC)
 * The Alcubierre drive is pure pseudoscience, though. All that Alcubierre did was write down a warp drive metric, plug it into the field equations of general relativity, get out a stress-energy tensor that, not surprisingly, makes no sense (the negative energy is the least of its problems), and then say "if we could create this stress-energy tensor that makes no sense, then we'd have a warp drive". That's the same as coloring the pixels on a computer screen so that they make a picture of the starship Enterprise and then saying "if we could make that in real life, we'd have a warp drive". You can write down any metric just like you can make any picture, but relatively few of them describe things that can happen in the real world. The Alcubierre drive definitely doesn't; the evidence for that is roughly as strong as the evidence against fairies in Dawkins' garden. -- BenRG (talk) 00:57, 22 October 2009 (UTC)


 * So what you're saying is, if you want to get to, say, Alpha Centauri in one week, your plan is to shrink the whole universe so that Alpha Centauri is right on our doorstep, pop on over, and then re-inflate the universe? At least no one can accuse you of thinking small.... --Trovatore (talk) 22:08, 21 October 2009 (UTC)


 * The way I was imagining it, you wouldn't have to deflate the whole balloon if you could just locally pull the rubber together in an area. But you'd probably want to contract an area of space without much mass in it, because that mass, if contracted a whole lot into a very small volume, might collapse into a black hole. 71.161.59.133 (talk) 22:18, 21 October 2009 (UTC)


 * The balloon analogy is perilously close to being outright wrong. When cosmologists say that the universe is expanding they just mean that the stuff in the universe is moving apart. Space, as such, doesn't expand. There is not and never has been (even during inflation) any "faster than light" expansion in any sense that matters. Faster-than-light speeds show up simply because the speeds used in cosmology are defined differently. Special relativity has "coordinate velocity" which is limited to c, but it also has "proper velocity" which has no upper limit. It's not a different kind of motion, it's just a different definition of the word "velocity". (The difference is that coordinate velocity is measured by synchronized clocks at the starting and ending points, while proper velocity is measured by a clock you take with you. Because of time dilation the clock you take with you measures a shorter time, so the velocity you get that way is higher.) The velocities in cosmology are analogous to proper velocities. Hubble's law says that galaxies at the edge of the visible universe are moving away at about 3 times the speed of light in every direction, and there are probably galaxies much farther away (that we can't see) moving at a billion times the speed of light in every direction—but you can nevertheless fit all of that inside a light cone, with the outermost galaxies moving slower than the boundary of the cone. It's just a different definition of speed. There's no violation of the light speed limit when the universe expands. There's nothing special going on at all. It's just stuff moving away from other stuff. -- BenRG (talk) 00:57, 22 October 2009 (UTC)
 * I'm not completely sure what you mean but as far as I can tell SR doesn't allow for anything like this. If you're moving at speed v relative to another object, you will perceive the object to be moving at speed v relative to you, and if that's how you measure your "proper velocity" then it still can never be greater than c regardless of time dilation.  Cosmology requires some much bigger guns than just SR. Rckrone (talk) 16:19, 22 October 2009 (UTC)
 * Sorry, I didn't realize that proper velocity was a thing. Still, I'm pretty sure that when it's said that the edge of the universe is moving away from us at some speed greater than c, they aren't talking about proper velocity.  The universe is expanding faster than SR on flat space-time would allow. Rckrone (talk) 18:42, 22 October 2009 (UTC)
 * It's not exactly the same as coordinate and proper velocity, but the difference between superluminal and subluminal motion in cosmology really is just a matter of definition. The Milne universe is the zero-density limit of big bang cosmology. In Minkowski coordinates the Milne universe is finite in size, spatially flat, and the edges are expanding at the speed of light, but in cosmological coordinates it's infinite in size, spatially hyperbolic, and recessional speeds are given by Hubble's law. (Also, in Minkowski coordinates the redshift is given by the SR redshift formula, while in cosmological coordinates the same redshift is given by the cosmological redshift formula.) When the matter density isn't zero you can't embed the universe in Minkowski space any more because spacetime isn't flat any more, but you can still take any finite spherical portion of the universe and put a Schwarzschild vacuum around it with the Schwarzschild m equal to the total enclosed mass. The edges stay inside the Schwarzschild light cones even if they have speeds higher than c in cosmological terms. -- BenRG (talk) 20:44, 24 October 2009 (UTC)


 * Then, of course, there's the possibility that c is actually slower than it was immediately after the Big Bang. Given 15 billion years, and the further possiblity that the rate of deceleration is itself also declining, the speed limit may have been considerably higher way back when. Perhaps a century from now, a light-year may be a proton's width shorter. B00P (talk) 07:00, 22 October 2009 (UTC)

Blood pressures & winter jackets

 * We probably should not answer that here. If we told you one thing and it was wrong, that could affect the effectiveness of your work.  Ask the people you work with or a doctor.  Falconus p  t   c 11:29, 21 October 2009 (UTC)
 * Hmm, I don't see how it's medical advice, but I managed to find two studies  that examine the difference between bare arm and clothed arm blood pressures. My question was more about how exactly the sphygmometer measures the pressure its exerting and whether or not having clothing in the way would mean some of the pressure is being dispersed. -- MacAddct1984 (talk &#149; contribs) 16:18, 21 October 2009 (UTC)
 * It is clearly medical advice because it is seeking advice about carrying out a medical procedure, the failure or success of which could affect a patient's condition or treatment. Further, what the heck an Emergency Medical Technician is doing asking for technical advice on the RD is beyond me. OK, I do understand s/he may not be what they appear to be. But any intelligent responder should realise that medical technicians have recourse to professional advice within their work ambit. Richard Avery (talk) 17:34, 21 October 2009 (UTC)
 * It's not a medical advice question in the usual sense, which is where someone asks about what they should do for their own health (for example it basically satisfies kainaw's criterion). There is still a liability issue here and it's the same sort of concern that motivates the rule about medical advice questions.  I don't think this is the right place for the question to be answered.  Rckrone (talk) 18:20, 21 October 2009 (UTC)

WWII era German HEAVY tank
Hello. One of my friends mentioned a heavy (truly heavy) tank designed by some German scientist in the late WWII. The tank was designed to be inpenetrable, and could withstand huge amounts of enemy fire because of it's thick armor. My friend also said that the tank was so heavy, it would've had to use submarine engines as it's propulsion device, and it could've withstood even artillery shells or bomb hits. The production never begun because of the tank's heavy weight - it would sink slowly in the ground (or quickly if it was muddy) and become immobile very soon. The entire story sound a lot like an urban legend or a alternative history description to me, but I just want to be sure - was there such a tank or plans to build one? 88.112.62.154 (talk) 07:11, 21 October 2009 (UTC)


 * What you describe is pretty much true. See Landkreuzer P. 1500 Monster and the smaller Landkreuzer P. 1000 Ratte. For more crazy tank ideas, some of which were actually built, see Super-heavy tank. Someguy1221 (talk) 07:15, 21 October 2009 (UTC)


 * Thanks a lot! It was that Ratte tank. He did mention the name too, I just forgot it. Thank you so much! 88.112.62.154 (talk) 07:20, 21 October 2009 (UTC)
 * While not WWII, there's always Killdozer -- MacAddct1984 (talk &#149; contribs) 17:15, 21 October 2009 (UTC)
 * As tanks go, that's not very impressive. A foot of concrete gives about the same protection as an inch of armor steel, so at a guess, the Killdozer has about as much armor as a late-model Panzer IV. --Carnildo (talk) 23:53, 21 October 2009 (UTC)

Coiled rope
I had a homework question that went as follows: a uniform rope of linear mass density λ is coiled on a smooth horizontal table. One end is pulled up with constant speed v. Find the force exerted on the end of the rope as a function of the height y.

At first I decided to look at the total energy of the system, which I found to be (λyv^2)/2 + (λgy^2)/2. Then I said that F=dE/dt, getting (λv^2)/2 + λgy as my answer.

But then I took a different approach, instead looking at the net force acting on the rope: ΣF = dp/dt, F - Mg = (dm/dt)*v, F - λyg = λv*v, F = λv^2 + λgy.

Each answer seems plausible, but which is right?


 * Neither is right. The speed is constant, so what it actually is is irrelevant. You just need the weight of the rope that isn't on the table to equal the upward force in order to have zero resultant force and thus maintain constant velocity. --Tango (talk) 10:52, 21 October 2009 (UTC)


 * I disagree with Tango, a segment of the rope is accelerating all the time, from being static on the table to being in upward motion so v is needed and the second answer is correct. In the first I assume you mean Fv=dE/dt in the above. Anyway E=(λyv^2) + (λgy^2)/2 not (λyv^2)/2 + (λgy^2)/2.  You have a mass of λy moving at v m/s so the second /2 is in error. --BozMo talk 10:56, 21 October 2009 (UTC)


 * I agree with your disagreement, and actually I had meant F=dE/dy...but for the total energy, we both agree on the potential energy, so that's not an issue; but I don't see why there wouldn't be a /2 for the kinetic component. Isn't K=(1/2)mv^2=(1/2)(λyv^2)?  —Preceding unsigned comment added by 24.202.172.103 (talk) 11:03, 21 October 2009 (UTC)


 * Ok I was reading the ^2 as a half in the formating I am not used to keyboard notations. But where does F=dE/dy come from? That's not physics. Force times velocity is energy expended rate of change of force with height is garbage. Which I think answers your question, the second is right. --BozMo talk 11:06, 21 October 2009 (UTC)


 * W = ΔE, for small displacements dW = dE, dW = Fdy, leading to F=dE/dy, right? —Preceding unsigned comment added by 24.202.172.103 (talk) 11:16, 21 October 2009 (UTC)


 * Ok. In fact Fv=dE/dt gives the same answer so I will have to look at the second version more closely. Trying to do too much at the same time--BozMo talk 11:20, 21 October 2009 (UTC)


 * SO to be clear on notation in (2) you write net force is rate of change of momentum with time, which is λv of rope accelerated to v. ?--BozMo talk 11:27, 21 October 2009 (UTC)


 * Had to use a bit of paper for this one. I think the energy version works and the net force one does not. The reason is that there has to be some tension in the rope at the point of contact. With gravity you do not notice, it just pushes slightly less hard down on the table. In zero gravity free space I think the whole coil would start to move towards you.--BozMo talk 11:45, 21 October 2009 (UTC)
 * I don't think so. I solved the problem a totally different way, and I got the second answer, F=λv2 + λyg.  The way I did it was to use the equation Fnet=macm, where Fnet is the net force on the rope, Fnet=F-λyg, m is the total mass of the rope, including the part of the rope on the table, and acm is the acceleration of the center of mass of the entire rope.  The center of mass of the entire rope is at (y/2)(λy/m) = λy2/(2m), where again m is the total mass of the rope, so acm=λv2/m.  Red Act (talk) 12:36, 21 October 2009 (UTC)
 * Hmm. But that uses the net force again, in effect? In your terms the force upwards on the rope by the table is actually unknown. You do not know how much rope is off the table but not travelling upwards at V and it may matter. I think this curved segment of rope which is being accelerated is significant (as in non zero) even for a ultra thin rope. --BozMo talk 12:48, 21 October 2009 (UTC)


 * Agree with Red Act. Second answer, F = λv^2 + λgy, is correct. You can't use conservation of energy because energy is "lost" to thermal energy when each part of rope is "instantaneously" accelerated from rest to speed v - this is like an inelastic collision in reverse. The total work done in lifting the rope can be found by integrating F dy from y=0 to y=L, the length of the rope - you get λLv^2 + λgL^2/2. Of this, λgL^2/2 is potential energy of the rope, λLv^2/2 is kinetic energy of the rope, and the remaining λLv^2/2 is lost to thermal energy. Gandalf61 (talk) 12:53, 21 October 2009 (UTC)


 * Surely not? You can get ropes which are pretty elastic and in a real physical system there is no need for the acceleration to be instantaneous, the rope would curve upward at the elbow. The upward curve would reduce the amount of rope on the table hence the missing force. --BozMo talk 13:02, 21 October 2009 (UTC)


 * But this is an ideal problem with an ideal inextensible rope. In real life, yes, the rope would stretch and the acceleration would not be instantaneous - but then you would have to account for the energy stored as elastic energy in the rope as it extends under its own weight. Gandalf61 (talk) 13:16, 21 October 2009 (UTC)


 * Forgive me but I still think this is wrong. Even for an inextensible rope the proper net force solution requires introducing tension as a function of height. As long as the rope has a non-zero mass per unit length this is needed. You could solve the equation for the curve shape of the rope assuming it was completely flexible and its instantaneous acceleration around the curve was given by the tension and curvature. You would find that a significant part of the rope was not in contact with the table which would be the missing force (do you remember deriving Catenary curves that way at school)?. You could force the elbow to be on the table e.g. by fitting a pulley but then that would exert net force up on the table. I don't think anything here introduces an energy loss to the first order in flexibility, etc. The curvature of the rope at contact with the table scales like v^2/g which is a length. --BozMo talk 13:26, 21 October 2009 (UTC)


 * And I still think that energy is lost and you are wrong - but rather than go back and forth on this, let's wait to see what other folks think. Gandalf61 (talk) 13:44, 21 October 2009 (UTC)


 * I agree with RedAct. Regardless of tension or elasticity or internal energy or anything else, the net force on the rope has to equal the change in momentum per time.  The force applied to the end of the rope has to counter gravity on the dangling end of the rope, which is λgy (technically the upward force is transmitted by the normal force at the corner of the table).  Then as the rope rounds the bend, the vertical change in momentum is caused by gravity (the force on the end of the rope is in the wrong direction to have an effect) and the horizontal change in momentum which is λv2 must be supplied by the force on the end of the rope. Rckrone (talk) 16:54, 21 October 2009 (UTC)
 * So how did you calculate y for the length of a non-straight bit of rope with a v^2/g curve at one end? If the rope is thin and inelastic then it will not descend to a right angle bend. Nor would a small link chain for example. If it has some marvellous non Newtonian properties (e.g. a very rusty chain) it might descend to a right angle bend and then basically Gandalf61 would be correct, it could lose energy in a corner. However in general the acceleration upward will be along a curve of radius order of v^2/g giving rise to an additional weight of rope off the floor which has to be supported by the force. This means if you do the net force calculation properly you have a smaller acceleration than expected and you end up with the energy version of the equation. --BozMo talk 17:21, 21 October 2009 (UTC)
 * The spirit of the problem is that the rope is coiled very tightly (think of a very dense rope), such that any curvature at the end can be neglected. Or even with a less dense rope, you can always specify a large enough value of v that the rope at the end can't possibly have as large of a radius of curvature as you're requiring of it.  If the diameter of the rope coil isn't considered negligible, then there's energy going into making the rope oscillate, which ultimately turns into heat as the oscillation gets damped.  Red Act (talk) 20:27, 21 October 2009 (UTC)
 * The curvature is not to do with the rope thickness but to do with the challenge of accelerating a finite mass in zero distance. Anyway, the OP gave too different calculations which were apparently inconsistent. A system which conserves energy resolves the inconsistency by a pick up on the rope, the rope joins the ground at a curve of about v^2/g. A system which maintains no pick up has to introduce an energy loss associated with flexing the rope or an extra downwards force to stop a pick up. Either way it breaks energy conservation or adds more force. It seems to me that we have no inconsistency (but actually I do not believe there is a force free way of suppressing the pick up. --BozMo talk 20:51, 21 October 2009 (UTC)

I find it dubious that energy loss is the solution. For simplicity, let's say we aren't in a gravitational field, and that instead of pulling on a macroscopic rope, we are pulling on an infitessimal rope (a point particle in effect). What is the force necessary to accelerate this point particle to a speed v? Again, using either forces or work yields each yields different but plausible solutions, but with a point particle, how is it that energy can be lost? —Preceding unsigned comment added by 24.202.172.103 (talk) 21:00, 21 October 2009 (UTC)


 * Does this not resolve in exactly the same way when you introduce the fact that the force has to be applied for a time and a distance. It is only if you think you can apply the force without the particle moving at all that you get a different answer. Just as for zero curvature above--BozMo talk 21:13, 21 October 2009 (UTC)


 * The key difference between a rope and a point particle is that a point particle is rigid. Treating the rope as a point particle would be as if the rope was superglued into a rigid shape, so that the mass being accelerated is always the entire mass of the rope.  Eliminating the variable-mass aspect of this problem would turn this into a completely different problem.  Red Act (talk) 22:32, 21 October 2009 (UTC)

The fundamental flaw with the first "solution" is that it relies on the work-energy theorem. But the work-energy theorem only applies to a rigid object. "If the object is not rigid and any of the forces acting on it deforms the object, then the Work-Energy Theorem will no longer be valid. Some of the energy transferred to the object has gone into deforming the object and is no longer available to increase or decrease the object's Kinetic Energy." Turning a coiled rope into a straight rope is deforming the rope, and some energy is generally required to do that. The only way to straighten out the rope without expending any energy in the process would be to move the rope at an infinitesimal speed. Straightening out the rope in a finite amount of time requires a finite amount of energy. So the work applied to the rope in this problem must be greater than the kinetic energy plus potential energy that the rope gains. Additional work must be applied in order to supply the energy required to straighten out the rope in a finite amount of time. Red Act (talk) 23:14, 21 October 2009 (UTC)

To better see where the problem lies, change the experiment to eliminate gravity, and eliminate kinetic energy at the end. Supposed you're floating in space, with a coil of rope at rest relative to your spacecraft. Your goal is to straighten out the coil of rope in a finite amount of time, such that at the end, the rope is still at rest relative to your spacecraft. No matter how carefully you plan how you nudge parts of the rope, you're going to need to apply some energy to straighten out the rope. The minimum amount of energy required to straighten out the rope is dictated by how little time you have to complete the task, and the initial shape of the rope. But it's a nonzero amount of energy, and it's energy that the first "solution" above neglects to consider. Red Act (talk) 23:34, 21 October 2009 (UTC)

Cool, thanks.


 * Just a little caution. There are a number of ways of idealising physical situations in order to model them. Using deformation energy to consider a "chain" (a flexible rope with linear density) is not a standard one and not one a maths examiner would typically recognise. Generally the work energy theorem is considered to apply to flexible chains, which are not considered to require energy loss in deformation. --BozMo talk 07:14, 22 October 2009 (UTC)
 * To sum up: in mathematical modelling we can only take something as effectively zero if it can be made arbitrarily small as the control parameters (rope thickness, deformation viscosity etc) become arbitrarily small. In this problem there is no choice of control parameters which make the curvature as the rope reaches the ground arbitrarily small so you have to consider the curved part of the rope. For a given geometry the energy loss flexing a real rope is linearly proportional to several physical constants in the rope and so this does tend to zero as the control parameters tend to zero (provided we are confident that the limit is well behaved and the geometry doesn't change, unlike for turbulent flow as viscosity tends to zero say). Therefore, in the standard treatment we would take a completely flexible rope with no energy loss but consider the fact the the rope accelerated over a finite corner. The finite corner gives more hanging rope for the force to support, so the net force argument does not work. There is no energy loss so the energy argument does. --BozMo talk 08:35, 22 October 2009 (UTC)
 * I found a problem very similar to this one in this textbook. The only difference between that problem and this one is that in that problem, the top of the rope is being pulled upward by being attached to a mass that's been given some initial velocity, instead of the top of the rope being pulled up at a constant velocity as in this problem.  But both problems involved a coil of rope, whose mass is not taken to be negligible, being pulled upward.  Notice several things about the problem in the textbook:


 * 1) The problem has the student start off with F=dp/dt. That is, the textbook points the student in the direction of the second solution above, the solution that I believe is the correct one.


 * 2) The problem states that "This is an example of a variable mass system for which the principle of conservation of energy does not hold...", and also points out that the work-energy principle does not hold here because this is a variable mass problem. That is, the textbook leads the student away from the first solution above, the one that you believe is the correct one.


 * 3) The problem assumes that you can solve the problem with the information given, without mentioning anything about having to mess around with the curvature of the rope. Indeed, insufficient information is given in the problem to figure out what the curvature of the rope would be, information like what the diameter of the coil of rope is.  The tacit assumption is that the portion of the rope that's off the table is simply straight.  Red Act (talk) 11:13, 22 October 2009 (UTC)


 * It is clear in the "flexible chain" formulation that the shape of the rope becomes independent of the thickness of the rope as the thickness of the rope tends to zero, so that is not a problem. You also do not need to know the exact shape, its existence just explains why the net force approach gives a different answer, and why there is a well posed problem which conserves energy. I agree with you that if the portion of the rope off the table is simply straight your answer is the correct one. However it is rather unclear to me that any physical circumstances would approximate to that (they might, I just cannot think of any), whereas treating the problem as a flexible chain does correspond to a real physical limit. If there is any real physical limit of parameters capable of yielding a right angle in the rope (as some internal property tends to zero) I would be interested. --BozMo talk 13:02, 22 October 2009 (UTC)


 * BozMo - I don't understand why you continue to insist that work done on rope = kinetic energy of rope + potential energy of rope when it clearly isn't. Applying Newton's second law (and ignoring gravity and p.e., since we all agree on the λgy term) then F dt = dp = λv dy, so F dy = Fv dt = λv2 dy, and so work done on rope = λv2y, whereas k.e. of rope is only λv2y/2. If you insist that there is no "lost" energy then you have to explain why Newton's second law does not apply to the rope. Gandalf61 (talk) 13:51, 22 October 2009 (UTC)


 * Hmm. I have done too many problems treating chains as point weights and rigid connectors I guess. There just isn't a friction to absorb any heat (of course is it idealised but the limit is a good one in mathematical terms, for a given set up you can halve the friction by changing materials and that cannot change the geometry). If you take the obviously geometry with the chain ascending upwards above the coil centre and the coil feeding round in a circle the rising chain gets horizontal rotational kinetic energy and angular momentum. That might disipate into your heat in time but there is absolutely no way it is plastic deformation at a corner. --BozMo talk 14:25, 22 October 2009 (UTC)
 * The alternative way to see this geometrically is to lay the end of the rope flat along the ground and pick it up at v m/s but this time you have to run sideways at v m/s. Your upwards force is still F dt = dp = λv dy, so F dy = Fv dt = λv2 dy, and so work done on rope = λv2y, but now the k.e. of rope is λv2y. Are you really claiming that I am missing an energy loss in the corner?
 * One more note. If your rope stays exactly above the point in the coil where it instantaneously is unwinding then the whole rope is travelling around in a circle of radius r (which cancels) and at a velocity v. Therefore there is λv2y/2 in vertical kinetic energy and λv2y/2 in horizontal kinetic energy. --BozMo talk 14:42, 22 October 2009 (UTC)


 * Hmmm ? Energy is energy - it is not vertical or horizontal. But if you impart a horizontal component of velocity to the rope then you have to change both force/momentum and energy equations, not just one of them, so there is still lost energy. I think you are tying yourself in even more knots than the rope - I am done with this nonsense. Gandalf61 (talk) 14:57, 22 October 2009 (UTC)

Anyway to recap when this has got to. Flexible chains are energy conserving so there is no internal loss to plastic deformation and the like. However in this case some half of the energy has to turn into horizontal kinetic energy as the initial movement of the rope has to be not only v upwards but also v along the coil so the rope can continue unwinding without snapping. The upward force is F dt = dp = λv dy, so F dy = Fv dt = λv2 dy, and so work done on rope = λv2y, and the k.e. of rope is λv2y. Further up in the air this additional horizontal energy (and angular momentum) presumably disipates. I happily concede I was wrong about which equation gave the right answer. However it is quite clear that everyone else was wrong about what happened to the energy. It cannot be dissipated in the corner as it still has λv2y energy just above the corner. --BozMo talk 15:02, 22 October 2009 (UTC)
 * Who said that all of the energy dissipation was necessarily right at the corner? What I said yesterday was "If the diameter of the rope coil isn't considered negligible, then there's energy going into making the rope oscillate, which ultimately turns into heat as the oscillation gets damped."  In any realistic rope, that circular oscillation with the extra λyv2/2 of kinetic energy will get damped, and that energy will be converted into heat.  In the limit as r->0, the initial oscillations will have a smaller amplitude but a higher frequency, which I think would result in the oscillations getting damped into heat faster.  In fact, if you get down to a small enough value of r, the oscillations would even have a high enough frequency that they would already count as heat initially. Of course, it's impossible for a real rope coil to have a radius anywhere near that small, so it does take some time for the oscillations to get damped into oscillations that have a frequency high enough to count as heat.  At any rate, that extra energy does eventually wind up as heat, so an equivalent way of saying why solution #1 is incorrect is to say that it fails to take into consideration the heat that's produced as a result of deforming the rope.  Saying otherwise would require a rope that can sustain energy in the form of a macroscopic oscillation forever, without it getting turned into heat.  Such a rope does not exist.  Red Act (talk) 15:43, 22 October 2009 (UTC)
 * "Who said that all of the energy dissipation was necessarily right at the corner?" Who? see above "energy is "lost" to thermal energy when each part of rope is "instantaneously" accelerated from rest to speed v - this is like an inelastic collision in reverse" was not your comment I agree. Then you were right all along I just misunderstood you, also when you said in your illustration "No matter how carefully you plan how you nudge parts of the rope, you're going to need to apply some energy to straighten out the rope." I thought you were implying the energy loss was somehow in straightening the rope. All clear now, you did not mean that. --BozMo talk 18:33, 22 October 2009 (UTC)
 * Yes, I said that. I assumed an ideal inextensible rope, and with that assumption my statement is correct - each part of the rope is accelerated instantaneously and energy is lost to thermal energy instantaneously. You objected that the inextensible rope is not a reasonable model of the real world. Red Act points out that even with a deformable rope, where energy goes initially into transverse or longitudinal oscillations of the rope, this energy is eventually lost to thermal energy anyway. In either case the assumption that work done on rope = kinetic energy of rope + potential energy of rope is incorrect. Gandalf61 (talk) 09:53, 23 October 2009 (UTC)
 * No, you are still in error about this. With an ideal inextensible rope as each element unfurls it has a vertical velocity of v and a horizontal velocity of v, to match the horizontal speed at which the pick up point has to travel sideways. An ideal inextensible rope is incapable of spontaneously losing energy. --BozMo talk 10:47, 23 October 2009 (UTC)
 * "An ideal inextensible rope is incapable of spontaneously losing energy" - oh really ? Attach an ideal rigid mass to one end of an ideal inextensible rope; attach the other end of the rope to an ideal perefctly rigid support; release the mass from a point vertically below the other attached end of the rope. When the mass comes to the end of the rope it stops dead - instantaneously - because the rope cannot extend. Where has the kinetic energy of the mass gone ? Gandalf61 (talk) 11:47, 23 October 2009 (UTC)
 * Sorry am I missing something? With an ideal inextensible rope the mass would bounce upwards, same as with any rigid elastic rod. --BozMo talk 12:12, 23 October 2009 (UTC)

I agree with Red Act where he has written that $$F=\lambda v^2 + \lambda y g $$

I arrived at this conclusion by considering the force in two components. Firstly there is the component of force necessary to support the weight of the rope. That is:

$$F_w= \lambda y g\,$$

Secondly there is the component of force necessary to supply the steady increase in momentum:

$$F_m=v \frac{dm}{dt}$$

$$\frac{dm}{dt}=\frac{dm}{dy}*\frac{dy}{dt}=\lambda*v$$

$$F_m=v*\lambda*v\,$$

$$F=F_w + F_m = \lambda y g + \lambda v^2\,$$

In this question, mechanical energy is not conserved. Half the power provided by the rising force goes into kinetic energy of the rope, and half is lost to irreversibilities and ends up as heat. Dolphin51 (talk) 10:50, 23 October 2009 (UTC)

Yes, as above Red Act (and you) are correct about this. The issue is how the half is lost to irreversibles and the claim that this could be limited to a tiny area around the kink for a (near) perfect chain. It could not. Initially the rope picks up at the point with v up and v sideways and the v sideways gets lost in galloping some time later. --BozMo talk 10:57, 23 October 2009 (UTC)


 * I am now having second thoughts about this. Perhaps half the power goes into kinetic energy, and half goes into gravitational potential energy.  I need to think about it.  (I agree that the idea that half goes to irreversibilities is not intuitively convincing.) Dolphin51 (talk) 11:02, 23 October 2009 (UTC)


 * Half instantly into irreversibles is obvious nonsense as a perfect chain can be modelled by stiff light rods and elastic weights. That was my starting point but to avoid you making the same mistake which I made above, draw a little picture of the rope at lift off. You can see immediately as the lift off point has to move sideways at v so does the rope, otherwise it would snap. I have seen a video of a helicopter picking up steel cabling (to lift a tree trunk at the far end of the cabling) and the galloping is massive. As you I thought this must to take the extra energy into PE but actually it is to absorb the sideways kinetic energy. --BozMo talk 11:10, 23 October 2009 (UTC)


 * Y'all are forgetting that the lost energy doesn't need to be instantly transformed in heat. For an ideal rone no heat is produced but the extra energy can be radiated away as waves on the rope that (for an inextensible rope) will propagate at infinte speed and (apparently) disapear.


 * PS: Please BozMo, stop talking about sideways energy. It's getting on my nerves. Dauto (talk) 14:52, 23 October 2009 (UTC)


 * Agree completely that it doesn't need to be, and certainly isn't instantly transformed in heat... that is the point. Sorry you do not like the phrase "sideways energy" but as kE is quadratic, the two directions are orthogonal and Pythagorus was right it does seem to me to be a shorthand with a unique and obvious meaning... FWIW the whole problem for a flexible elastic chain is far from solved here (and does not need to be). There is still the question of whether the rope induces tension in the coil or not and what the pick up curve actually is. I would need a long car journey to do those. --BozMo talk 14:59, 23 October 2009 (UTC)

Acid attack neutralization spray?
In certain backward cultures, acid attacks are a problem for women. Could a spray can or face cream be devised to neutralize the acid immediately after contact and limit tissue damage without being corrosive or toxic itself?80.1.88.12 (talk) 10:50, 21 October 2009 (UTC)


 * Certainly something which was a form of Buffer solution would help.--BozMo talk 11:04, 21 October 2009 (UTC)


 * Bicarbonate, since it can either gain or loose a proton at physiological pH (bicarbonate is actually the buffering agent in blood), is frequently used for neutralizing both acids and bases. Sodium Bicarbonate is readily available to most people as baking soda, and can be used either as a solution in water or directly as the powdered solid. -- 128.104.112.179 (talk) 14:41, 21 October 2009 (UTC)


 * I think the real difficulty here is in being able to apply the neutralizer within a useful amount of time. I suspect a technical solution is not the answer to this particular problem. --Mr.98 (talk) 15:54, 21 October 2009 (UTC)


 * Note also that neutralization of an acid typically involves an (often violent) exothermic reaction - i.e. heat. Simply washing the acid off with large amounts of water is almost certainly preferable. Using a thick, inert cream layer before a suspected attack may protect somewhat, I'd think. Does e.g. Vaseline react with commonly used acids? --Stephan Schulz (talk) 16:12, 21 October 2009 (UTC)


 * It's a tricky problem because even washing it off with water will initially spread the acid over a larger area before it becomes dilute enough not to be harmful. As Stephan says - you need large amounts of water - like a bucketful. SteveBaker (talk) 17:56, 21 October 2009 (UTC)


 * And I imagine it also requires some knowledge of what the scarring substance in question is. If it is lye, you don't want to apply water to it. --Mr.98 (talk) 17:30, 22 October 2009 (UTC)


 * Finding something to neutralize the acid isn't hard -- as noted, baking soda will do quite nicely -- the problem is that it needs to be applied immediately, within a few seconds of the attack. That's why anyone working with acids will have an emergency shower in the same room. --Carnildo (talk) 23:57, 21 October 2009 (UTC)

Lotions and aluminum starch
I've noticed many hand and body lotions contain "aluminum starch" (usually within the top 5-10 ingredients). What is it and why is it added to lotions? I see the word aluminum and I'm a little worried about rubbing into my skin every day. --68.103.141.28 (talk) 13:44, 21 October 2009 (UTC)


 * It's a binding and anti-caking agent (full name Aluminum Starch Octenylsuccinate and is a modified starch). Here is a safet report from PubMed.  It is also approved for use in food.  Sp in ni  ng  Spark  16:02, 21 October 2009 (UTC)

Experiments
What are the five steps for doing an experiment?--Mikespedia (talk) 15:56, 21 October 2009 (UTC)


 * I suspect whatever it is you are looking for (homework, whatever) is covered in scientific method, even though the idea of there being a defined five steps that are actually used by practicing scientists is kind of absurd. Actually, the definition at Simple Wikipedia is probably closer to what you want than the long philosophical/historical exegesis at the regular English article. --Mr.98 (talk) 15:58, 21 October 2009 (UTC)


 * 1) Do your own homework
 * 2) Read the textbook provided by your science teacher
 * 3) Pay attention during class when your science teacher lists the 5 steps
 * 4) Write what your teacher says down in a notebook so you can refer to it later
 * 5) Read our article on scientific method, which contains several different ways of conducting experiments. There is no one single "5 step" method, but rather many different ways of thinking about the overall set up of an experiment.  That's why # 1-4 are important; it is likely that your teacher has a specific 5 steps in mind, and there is absolutely no way that people who did not sit in your class will know which 5 steps your teacher taught you.  -- Jayron  32  16:03, 21 October 2009 (UTC)


 * Quite right assuming you mean some variant on Aim/Apparatus/Method/Results/Conclusion --BozMo talk 16:05, 21 October 2009 (UTC)

Particles in Brand New Water Bottle
This MON I bought me a 32 oz. bottle of EVIAN water, left it in the freezer to chill while at work. Left work early and forgot about the water. TUES I take solid ice bottle out of freezer, and place at my work station to thaw. I come in this WED morning to see a sealed bottle filled with white flakey floaties. Looks like dandruff. After the initial gross out period, I concluded that the freezing of the water expanded the bottle a bit, and this expansion stretched the thin plastic, and after the thaw, a think layer of weak plastic (skin like) broke apart inside to make flakes--A sort of Evian Snow Globe. Do you concur? If not, explain. Thanks. --i am the kwisatz haderach (talk) 16:02, 21 October 2009 (UTC)
 * Was the bottle still factory-sealed on WED ? Most water bottles have a break-to-open ring under the screw top. Cuddlyable3 (talk) 22:08, 21 October 2009 (UTC)
 * Yes, Factory Seal still unbroken. I'm putting back in the freezer, see if I can incubate more in a Freeze/Thaw/Repeat sequence. --i am the kwisatz haderach (talk) 22:38, 21 October 2009 (UTC)
 * Most plastic bottles I'm familiar with use wax or a wax-like substance to improve the seal between the cap and the bottle. It could be that part of this flaked off when you froze the bottle. --Carnildo (talk) 00:00, 22 October 2009 (UTC)
 * From the ice cube article: "Melting ice cubes sometimes precipitate white flakes, commonly known as 'floaties'. This is calcium carbonate which is present in many water supplies and is completely harmless." Red Act (talk) 00:38, 22 October 2009 (UTC)
 * Red Act got it right. Same thing happened to me.  I called the number on the bottle to ask about the white flakes, and got basically the same answer.  Other than the "yuk factor", you have nothing to worry about (add usual disclaimers here). Bunthorne (talk) 19:55, 26 October 2009 (UTC)

Oil-based scum in me coffee
My last question made me think of STARBUCKS or COFFEE BEAN, just regular BLACK COFFEE from any/random cafes. I don't use milk or sugar, and like all Americans, I get my coffee in their togo cups w/plastic lids. Sometimes if I sit down and relax in the cafe, I'll take the lid of, and blow on the hot coffee, take sips, the usual. I've noticed that there's this thin layer of oily shine on the surface water of the coffee. What could this be? I'm thinking some sort of chemical cleanser used by the cup/lid manufacturing company. That's just speculation. What do we think? Cheers, --i am the kwisatz haderach (talk) 16:08, 21 October 2009 (UTC)
 * It's most likely the oil from the coffee itself. That oil is a large part of the flavor/aroma of the beverage.  --Sean 16:13, 21 October 2009 (UTC)
 * Or something from your saliva perhaps? Googlemeister (talk) 17:44, 21 October 2009 (UTC)
 * If the water is hard (contains a lot of calcium ions), both coffee and tea will form thin layers of something that looks like oil, but is some scum formed from calcium, coffein, and some other substances. It's harmless, but ugly, and the reaction spoils the taste of tea and, to a lesser degree, coffee. --Stephan Schulz (talk) 19:20, 21 October 2009 (UTC)
 * o/~I had a dream there were clouds in my coffee...o/~ Tevildo (talk) 00:11, 22 October 2009 (UTC)

How does resistance change on increasing the tempreture of the conductor?
From the ohmslaw i hit a doubt. Law states: the electric current flowing through a mettalic wire is directly proporstional to the potential difference 'v' across its ends provided its tempreture remains the same. Sescond part of the the law states that temp must be same, but if tempreture is changed conductor's resitance will increase or decrease. frfom the formulae R is directly proportional to the Heat so Heat is directly proportional to R this gives that resistance should increase on heating but after heating there will be more free ions to conduct electricity, so resistance should decrease. —Preceding unsigned comment added by Myownid420 (talk • contribs)


 * Read Electrical_resistance. Toodles.  -- Jayron  32  16:16, 21 October 2009 (UTC)
 * Current in a metal is not carried by ions, but by electrons. What makes a metal a metal is the fact that the crystal has an energy band that is partially empty and partially full. That means that the electrons in that band can essentially move freely through the metal. Thus metals are good conductors. As the temperature increases, atoms in the lattice increase their vibration and hence interact more often with electrons, slowing them down. This increases resistance. Semi-conductors, on the other hand, have completely filled or empty energy bands, but have only a small band gap between the highest full and lowest empty band. Increased temperatures allow electrons to jump that gap and move into the conducting band. Thus, for semi-conductors resistance drops with temperature. --Stephan Schulz (talk) 16:27, 21 October 2009 (UTC)


 * The OP first stated Ohm's law correctly. However there is no "Sescond[sic] part of" Ohm's law concerning temperature. Resistance, the 'R' in Ohm's law, being proportional to temperature is a property of the conductor material. The OP should not confuse Heat with Temperature. Heat is the energy that must be moved in order to raise or lower the temperature of any thing. Cuddlyable3 (talk) 21:57, 21 October 2009 (UTC)

why isn't sodium chloride that hygroscopic
What makes magnesium sulfate, calcium chloride and lithium chloride more hygroscopic than sodium chloride? I'm confused, sodium chloride appears to be in the middle of them (w/regard to periodic trend)... John Riemann Soong (talk) 17:23, 21 October 2009 (UTC)


 * It's a complicated property that has little to with the sort of thing that can be explained easily via periodic trends. Hygroscopicity (is that a word?  it is now...) is likely related to the binding energy between the ions in question and water molecules.  My guess is that the +2 ions will create stronger bonds with water than will the +1 sodium; while the smaller Lithium +1 ion will likewise bind tighter than the larger Sodium +1 ion; but that is just an educated guess.  There are a LOT of factors to consider, solvation is an exceedingly complex process, and so while that is one factor to consider, it likely is not the only one.  The solvation article contains a rough overview, and shows exactly how many things need to be considered when looking at a problem like this. -- Jayron  32  02:55, 22 October 2009 (UTC)

CNS & PNS cell types.
I was wondering if someone would be kind enough to assemble a list of the main cell types found in the Central and Peripheral Nervous Systems, and what it is that each type does? Many thanks, Colds7ream (talk) 17:59, 21 October 2009 (UTC)
 * Sensory afferent --> pseudounipolar, motor efferent --> multipolar. ANS --> both sympathetic + parasympathetic would be multipolar.  Bipolar exists only in the retina eyes and a few other places.  DRosenbach  ( Talk 18:15, 21 October 2009 (UTC)
 * Ahh...forgot about ancillary cells. Glial in the CNS, consisting of various types of astrocytes.  Then there would be Schwann cells in the PNS and oligodendrocytes in the CNS.  DRosenbach  ( Talk 18:16, 21 October 2009 (UTC)
 * Thanks very much! :-) Colds7ream (talk) 21:48, 21 October 2009 (UTC)
 * I was sort of in a hurry when responding earlier today -- so I just threw together a bunch of info without properly organizing it. But if you check the links I included above, as well as some of the other, unlinked-to "buzzwords" of neurophysiology, you should be able to get all that you need from Wikipedia.  If you have any other specific points, though, you can restate them.  DRosenbach  ( Talk 02:47, 22 October 2009 (UTC)

Shoulder problems

 * It is easily subject to injury because the ball of the upper arm is larger than the shoulder socket that holds it.

What does this mean? How could the ball be larger than the socket and still be contained inside?  DRosenbach  ( Talk 19:04, 21 October 2009 (UTC)
 * The implication is that the socket embraces less than half of the ball, in the same way (though not to the same degree) that a golf ball is larger than the cup of the tee supporting it. 87.81.230.195 (talk) 19:47, 21 October 2009 (UTC)




 * The pictures (right) explain this better than words can. The ball is spherical - the inside of the socket has the same curvature as the ball - they both have about the same radius - so the ball fits perfectly into the socket - but unlike normal industrial/mechanical ball-and-socket joints - and unlike the hip joint, the shoulder socket is less than a hemisphere.  If there were no muscles, tendons and whatnot holding it in place, it would just fall off.  SteveBaker (talk) 00:24, 22 October 2009 (UTC)
 * Wonderful! Thanks Steve.  DRosenbach  ( Talk 02:44, 22 October 2009 (UTC)
 * The analogy used in the anatomy classes I took is that the shoulder is like a teacup sitting in a saucer. Yes, it's a ball and socket joint, but like SteveBaker wrote, the socket is not a hemisphere. In fact the glenoid fossa of the scapula (where the ball of the humerus articulates) is surrounded by a lip of collagenous material called the glenoid labrum which increases the concavity of the socket. The muscles and tendons of the rotator cuff are critical in maintaining shoulder integrity, along with the tendons of the surrounding larger muscles. -- Flyguy649 talk 02:45, 22 October 2009 (UTC)




 * To maintain joint integrity and stability,the center of rotation of the ball in any ball & socket/cup joint must remain relatively static in translation(horizontal movements) with respect to the socket/cup. The large socket with ligaments at the hip maintains this better than the less stable shoulder joint. It is a bit like one of those compression and tension mechanical equilibrium setups.Sjschen (talk) 03:57, 22 October 2009 (UTC)

Sunrise and sunset on a perfect sphere
I was reading an article about the International Space Station, and it got me thinking about how astronauts perceive sunrise and sunset from various orbits, and how you might figure this out for a general case of an observer on a sphere. After musing over it for a while, it seems like there are quite a few things you'd have to know.

In the simplest case, we might have the following:


 * There is an observer $$O$$.
 * $$O$$ is standing on the equator of a sphere $$E$$ with radius $$r$$.
 * $$S$$ is a point source a distance $$r_{orbit} >> r$$ away from $$E$$.
 * $$E$$ is rotating in space about its axis and completes a full revolution every $$t_d$$ time units.
 * $$E$$ exhibits no precession in its rotation.
 * The axis of rotation of $$E$$ is perpendicular to the plane containing the center of $$E$$, the center of $$S$$, and $$O$$.
 * $$E$$ does not rotate around $$S$$.

Here the only variables seem to be $$t_d$$, $$r_{orbit}$$, and $$r$$. If $$O$$ starts out on the point on $$E$$ closest to $$S$$, is there an equation that describes when $$O$$ will experience sunrise and sunset?

Moving to a more realistic real-world case, you can see how there might be a number of additional different variables:


 * $$O$$ is some height $$h_E$$ above $$E$$ in geosynchronous orbit (as if O were standing on a mountain with height $$h_E$$).
 * $$O$$ is not on the equator of $$E$$, but rather at an angle $$\theta$$ above the equator. (Is this equivalent to $$O$$ simply being on the equator of a sphere of size $$r \cdot cos(\theta)$$?)
 * $$E$$ rotates around $$S$$ with a period of $$t_y > 0$$ time units.
 * $$E$$ has an axis of rotation which is tilted at an angle $$\phi$$ from the vertical.

That's four more additional variables now: $$h_E$$, $$t_y$$, $$\theta$$, and $$\phi$$. You could get even more complex (for example, $$O$$ might not be in geosynchronous orbit and instead completes a rotation around $$E$$ every $$t_O$$ time units), but I think this covers a lot of useful cases.

Is there an equation which completely describes when O experiences sunrise and sunset in this case? —Preceding unsigned comment added by 216.30.180.104 (talk) 19:48, 21 October 2009 (UTC)


 * You'll likely find answers to your question by digging in two directions (though I doubt a single equation will answer each and every one of the issues you raise):


 * Astronomy: Astronomers have been preoccupied with this problem for centuries, though accurate solutions have been found only after the laws of motion and gravitation have been mastered. Check, for instance, the [|JPL Horizons] on-line system.


 * Remote sensing: Some Earth orbiting space platforms embark limb scanning instruments that aim to characterize the vertical distribution of chemical compounds in the atmosphere by measuring absorption processes during occultation. See, for instance, the article on Upper Atmosphere Research Satellite.

Michel M Verstraete (talk) 20:39, 21 October 2009 (UTC).


 * The generic term here is occultation, and it occurs whenever the nearer two bodies' positions are closer (by angular distance) than their angular diameter. In general, you can set up an equation to define the angular position of both bodies (as viewed from whatever stationary or moving point of reference you like), and then solve for all times when the angular distance is less than the angular diameter (for a partial occlusion).  In your case, you seek the angular diameter of earth and sun, as well as their relative angular positions, while viewed from the moving frame of the ISS (or some other orbit).  In the most general case, even the angular diameters of both objects are time-varying (as their relative distances might change).  Nimur (talk) 20:53, 21 October 2009 (UTC)
 * (For a grazing occultation, angular distance is less than the sum of the two angular diameters). A total occultation is when the angular separation is less than the nearer body's angular diameter.  Nimur (talk) 20:58, 21 October 2009 (UTC)


 * The geometry here isn't too bad. For someone on the Earth, the sine of the apparent angle of the sun above the horizon plane is
 * $$s(\psi) = \cos\psi\cos\phi\cos\theta - \sin\phi\sin\theta$$
 * where ψ is the angle corresponding to the time of day, with ψ = 0 at noon. Note also that φ here is not strictly the tilt of the Earth's axis, but the tilt of the axis toward or away from the sun.  For Earth this value varies over the course of the year between 23.5° in December and -23.5° in June (I arbitrarily chose φ to be positive in the direction of the north pole pointing away from the sun).   To find the time of sunrise and sunset on Earth for a given θ and φ, you would find the values of ψ that satisfy s(ψ) = 0.
 * In orbit we can use the same formula, except now ψ is the angle through the satellite's orbit, with ψ=0 at the closest point to the sun, θ = 0 since the plane of the orbit has to pass through the center of the Earth, and φ is the tilt of the satellite's axis of revolution toward or away from the sun. So now $$s(\psi) = \cos\psi\cos\phi$$.
 * Above the Earth, the sun is still visible when it passes below the plane of the horizon. The angle at which it passes out of view is at -cos-1(r/rorbit) so to find the angle ψ for sunrise and sunset solve for s(ψ) = -(1 - (r/rorbit)2)1/2.  In either case, to get from an angle ψ to a time t, t = ψ*T/2π where T is the period.  Rckrone (talk) 21:17, 21 October 2009 (UTC)


 * In the relatively low orbits of the ISS and Shuttle, the atmosphere plays a big part in what they see. Rayleigh scattering and Mie scattering dominate what they see - just as they do for us down here on earth.

Stop bleeding
Does anyone know the how small a blood vessel has to be before it can stop bleeding by itself if cut? For example, if I slice and tiny capillary or arteriole it will stop bleeding relatively quickly, however, the same cannot be said of something like, say the aorta. I can't seem to find any information on this... Sjschen (talk) 21:57, 21 October 2009 (UTC)
 * The aorta will eventually stop bleeding when pressure drops due to hypovolemia and overload of compensatory mechanisms.  DRosenbach  ( Talk 02:33, 22 October 2009 (UTC)
 * And, depending on where the hemorrhaging occurs, formation of a hematoma may have a tamponading effect. In general, though, I don't think any lacerated or incised arterial vessel except a very small one would stop bleeding on it's own.  A dentist who cuts into the greater palatine artery needs to apply pressure and may need to suture to the palatal bone to ligate in order to stop bleeding.  Similar measures are neccesary if one incises the sublingual artery.  If I would cut the artery coming out of the mental foramen and just leave it, I think it could very well lead to life-threatening blood loss.  DRosenbach  ( Talk 02:42, 22 October 2009 (UTC)
 * The reason I ask is when a surgeon resects some sort of pathology from say, the brain, there are some vessels that must be cauterized, while others can be left to bleed while the surgeon continues to use the aspirator to suck out more lumps of tissue. Is there a guideline somewhere to say when bleeding should/must be "manually" halted and when it can just be left as is (and it is safe to close up the craniotomy/incision)? Sjschen (talk) 03:19, 22 October 2009 (UTC)
 * I'd say that any arterial vessel proximal enough to spurt blood in concert with the pulse should be dealt with, rather than leaving it to clot on its own.  DRosenbach  ( Talk 03:32, 22 October 2009 (UTC)
 * No guideline that I know. For any given operation, ligating specific named vessels may be part of the procedure (like ligating the uterine artery during a hysterectomy). Beyond that, if a vessel is actively bleeding in the OR, it simply must be stopped. Hemostasis is constantly on a surgeon's mind; neglecting it can easily be deadly. It's guided more by the surgeon's feel and knowledge of anatomy, not breaking out a little ruler and looking up a guideline. - Draeco (talk) 04:03, 22 October 2009 (UTC)

Rabbit insertion through human cervix: Reality check
Today's featured article, Mary Toft claims that in 1726 the woman miscarried and for some reason, inserted a cat or rabbit through the cervix into the womb, then delivered it in the presence of a man-midwife. Is this remotely possible? What could a poor woman have used as a speculum or dilator in 1726 to place such an object in the womb, and lived to tell of it? Edison (talk) 23:29, 21 October 2009 (UTC)
 * I don't know the answer, but I'd be very suspicious of this account, which (as is stated in the article) is purely Toft's own story. The later "births" observed by people with any sort of medical knowledge were of relatively small pieces of animal which had been placed in her vagina. Warofdreams talk 01:39, 22 October 2009 (UTC)


 * You either didn't read the article, or didn't read it closely. An accomplice inserted a few animal parts, not entire corpses, immediately after her miscarriage while the cervix was still dialated. Further "deliveries" were just her (or someone else) cramming odds and ends into her vagina. It's all in the article. 218.25.32.210 (talk) 01:35, 22 October 2009 (UTC)


 * I'm kinda dumbstruck by this article.. I'm sorry I know the reference desk isn't for discussion or opinion, but seriously. I thought I'd pretty much heard about the depth of human depravity and then an article comes up like this and proves me wrong yet again.. I've heard of people doing things which are more "sick" for some sort of perverted gratification, but this just seems so perverse yet so remarkably pointless and stupid, it's just stunning! I almost sympathise with the doctors who fell for it because they would have undoubtedly asked them selves: why on earth would someone stick rabbit parts up them self?!? The alternative at the time, that she was giving birth to the animal parts, so completely ridiculous actually made more sense to them then the truth!Vespine (talk) 03:56, 22 October 2009 (UTC)


 * I don't know, it's not too different than balloon boy. People do goofy stuff. --Mr.98 (talk) 12:33, 22 October 2009 (UTC)


 * antiwikipedia Rant (sorry): I was baffled too! I know getting a science featured article or DYK is nearly impossible as there is too much technical jargon, yet there seems to be an endless list of useless featured articles and DYK such as "did you known that someone with a stupid porn name is actually called something else?" or facts like this, how is this remotely noteworthy? it is not cultural (yet, geek culture gets destroyed dy deletionists) nor entertaining nor scientific! Uncyclopedia describes |wikipedia as "has over 3 million articles, mostly about bands no one has ever heard of, one-time Naruto characters, and rare diseases mentioned in passing on House", which is painfully true... --Squidonius (talk) 13:14, 22 October 2009 (UTC)


 * Do you have evidence getting a science related DYK is actually 'nearly impossible'? The requirements for DYK aren't particularly high. Also DYK isn't hooks aren't really meant to be noteworthy. They're more meant to me interesting if possible or at least hopefully catch someone's attention. Ultimately of course DYK hooks can only be whatever is in the article and supported by references so even if it isn't that interesting, there's little choice. Ultimately of course many science DYK hooks are likely to have a similar problem since people have already written an article for the core stuff therefore only the more obscure is likely to be lacking an article. For example I see "that Aliquandostipitaceae members have the widest hyphae in the Ascomycetes" which while I understand it (unlike I expect 99% of visitors to the main page) is not exactly that interesting. In terms of this particular article, the fact that people are still mentioning in in the 1990s and 2000s (as shown by the sources) means it has had more staying power then one time Naruto characters, bands no one has ever heard of, rare diseases mentioned in pass on House, most of 'geek culture' and balloon boy Nil Einne (talk) 17:05, 22 October 2009 (UTC)
 * DYK is particularly annoying in it's requirements. The article has to be reasonably long - and well referenced - yet it has to have been created fairly recently.  This is a difficult set of rules to match.  Hitting any two out of the three is easy.  The goal of the DYK project clearly isn't to produce a set of interesting (and verified) oddities for the front page - it's an effort to get new articles pushed rapidly to a particular standard.  That goal is a worthy one - but it doesn't help readers of the front page very much! SteveBaker (talk) 20:20, 22 October 2009 (UTC)
 * I'm well aware of the requirements. But that's precisely my point. DYK is set up to show off new and recently updated articles. (Whether you feel that's a worthwhile goal or likely to be of interest to readers is somewhat irrelevant to my point.) It's not particularly biased against science related articles and I'm not aware of any great difficulty getting science related articles on it. However it is somewhat biased in favour of the more obscure articles (nowadays anyway) since they are ones most likely to not exist or to be stubs. Having said that, there is still a lot of areas in the developing world particularly the non-anglophile and African world that we are sorely lacking in articles that could fill DYK with less obscure stuff if there were the editors for them Nil Einne (talk) 20:44, 23 October 2009 (UTC)


 * The particular hoaxer in question is very well documented and very notable. Any book on 18th century hoaxes would be badly incomplete without a chapter on Mary Toft. When it was happening it was very famous.
 * You have picked a bad example for your rant. Mary Toft is undeniably notable and worthy of inclusion in the encyclopedia. Entire books have been written about her. APL (talk) 19:37, 22 October 2009 (UTC)


 * To APL - for what it's worth ... I don't think that anyone in the above discussion is saying that Mary Toft is not notable or that she is not worthy of inclusion in this encyclopedia. Thanks.   (64.252.124.238 (talk) 18:02, 25 October 2009 (UTC))