Talk:Physics Exercises/Physics problems

'Removed the question about anti-matter rocket. After thinking about it for a while, I realized that energy conservation is of NO CONSEQUENCE in rocket propulsion--the type of "collision" occuring in rocket propulsion is exactly the kind of collision (i.e. completely inelastic collision, in reverse, of course) that does not conserve energy (unless we include thermal energy, that is). Besides, having antimatter as "energy source" will give no propulsion as annihilation reaction will give off two photons, going in opposite direction (so that momentum is conserved), and no propulsion for the rocket unless there's a way to redirect those photons (and since annihilation reaction gives gamma rays, I'm not sure how those photons will be redirected--it goes right through most materials).

I know it was only a hypothetical question, but since it is unsolvable without more assumptions, please make sure that it can be solved (and is consistent with conservation of momentum) before reposting it. novakyu 13:23, 7 Dec 2004 (UTC)
 * I don't know what I was smoking there, but energy is always conserved, even in inelastic collisions. It's "inelastic" because it fails to conserve kinetic energy, but if we include energy in all its forms (including as rest masses), energy is always conserved. Looking at the date, I probably said that loooong before I understood relativity better. novakyu (talk) 07:57, 17 December 2007 (UTC)

For the question What is the average density of a planet if an orbital period of a spacecraft on a low orbit is N minutes ? I tried doing it, but I don't think it can be solved unless we are told what "low orbit" is. Is there a definition of "low orbit" or should we add that information as given (at a distance R or something like that)? novakyu 10:49, 14 Dec 2004 (UTC)

The term low orbit generally refers to an orbit whose radius is approximately R (from the centre of the earth). This is the standard assumption that many courses make in finding orbital velocity, $$ \sqrt(gR) $$. I think it can be given though. studiesrule 10:49, 14 Dec 2004 (UTC)

all questions merged in
I merged in all "relevant" questions to appropriate sections in Physics Exercises and therefore am turning the module into a redirect. For future reference, these are the questions I did not merge in and why (feel free to add them in, if you can resolve issues I brought up here):


 * 1) What devices are affected by 0 gravity: common electric bulb, candle, mercury thermometer, spirit thermometer, minimum thermometer, hair hygrometer, psychrometer, mercury barometer, aneroid barometer, hypsometer, absorption refrigerator ?
 * 2) * Because this question didn't seem to be ... very quantitative, and it is very vague. To some degree, all these things are affected: electric bulb, when it burns out, it'll probably have blackened marking that's different from if it burned out under gravity; mercury barometer is obviously affected... on the whole, this question does not require much physical insight---more of a bunch of memorization of trivial objects that does not help the student understand the important matter. On the whole, this looks like one of those questions that is easier to come up with than it is to answer (I consider those to be a bad question, because it also means that the question-maker did not really work it through herself).
 * 3) What is the minimum height of a shot tower for a given shot size ?
 * 4) What is the maximum size of a shot that can be produced in a shot tower ?
 * 5) * Above two problems are excluded for the same reason, as follows: not enough information. The student has to look up a lot of information (like ... heat conductivity and emittivity of lead, assuming lead is used), and the essential physics is really "how long does it take for a shot to fall" (the other part, of how fast a spherical object would cool down under a "wind" is too complicated a problem for the student to calculate and should be stated far more clearly, rather than hidden in the term "shot tower").
 * 6) A simple lead-acid battery charger consists of a variable transformer, a rectifier, an ammeter, and a fuse. Why the fuse blows well before the current according to the ammeter reaches its rated value?
 * 7) * Again, not enough information. Questions such as "How are these elements connected? What does ammeter read?" should have been answered on the question. Also, the question seems to essentially shooting for the fact that most ammeters would read the average value, and assuming the rectifier hasn't been connected with a capacitor to smooth out the ripple, the maximum current would be higher than what ammeter reads. But if that's the case, the question-maker missed a far more practical fact of ... that there are such things as "slow-blow" fuses. And even assuming slow-blow fuses are not used, fuses essentially work by thermal heating, so if it's anything like 60 Hz input voltage, essential current that fuse "feels" would be the averaged current that is felt by the ammeter. On the whole, it's a sloppy question that requires more work by student than the questioner. In the case that this question was being asked by an actual mechanic who is wondering why this happens ... a simpler answer might be that the averaging time of the ammeter is far too long. Suppose the averaging time for the ammeter is 30 seconds and the current rises faster than that time scale---in that case, when th ammeter "reads" 10 A, actual current (say, averaged over 1 s) may be 15 A. In any case, this is a bad question; offers no physical insight at all.