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 Jul 27 asked Explanation for $E~$ not falling off at $1/r^2$ for infinite line and sheet charges? Jul 26 answered Why are bricks typically constructed to have six faces at, or near right-angles to each the other? Jul 26 comment Rotation, cats landing on their feet, and conservation of angular momentum that only works because of friction which is an external torque and so the momentum of me and the bar stool isn't conserved. Your thought experiment wouldn't work on a frictionless bar stool Jul 26 comment Rotation, cats landing on their feet, and conservation of angular momentum sure, likewise for the cat's body you can have one half rotating in one direction, the other in the opposite. But they realign so there is no angular displacement between them, and that's my point. I can sit on a bar stool and twist my body in one direction, the bar stool going in the other. But when I realign my body with the bar stool, I end up pointing in the same direction in the room as I started out with. Jul 26 comment Rotation, cats landing on their feet, and conservation of angular momentum do you honestly think cats have a spine where each half of the body can rotate n*360 degrees independently of the other? Jul 26 comment Rotation, cats landing on their feet, and conservation of angular momentum but every part in the body of a cat has rotated by the same angle. Jul 26 comment Is the stress finite at the centre of a spherical continuous charge distribution? @RonMaimon from the electric field of the charges acting on the centre. I thought someone might come up with a nice simple argument on the limiting process of $dr^3$ and how it affects the answer. Jul 25 asked Is the stress finite at the centre of a spherical continuous charge distribution? Jul 15 comment Significant Figures @JohnRennie this question is a homework problem, rather than a general question about why physicists work to so many significant figures or decimal points which would be a good question to ask. Jul 13 awarded Yearling Jul 13 comment Is this a valid understanding of Newtonian mechanics? You can define force as the cause behind the compression of a spring. You can quantify it by the length of the compression. Jul 13 comment Why did we need relativity to derive $E=mc^2$? @JerrySchirmer think of two masses on the ends of the compressed spring and the change in kinetic energy of the system when the spring is released - is this change in kinetic energy, and therefore change in potential energy in the the compressed spring, invariant? I would say yes, but if you say no then maybe I need to have a closer look :) Jul 12 comment Why did we need relativity to derive $E=mc^2$? @jerry potential energy, such as that stored in a compressed spring, is invariant though, right? Jul 12 comment Why did we need relativity to derive $E=mc^2$? Since his first derivation is what's causing the confusion then really you should be commenting on on that, don't you think? Jul 8 comment Why did we need relativity to derive $E=mc^2$? Do you think you could stick with the original derivation of Einstein since that's what the OP is referring to? Jul 8 comment Why did we need relativity to derive $E=mc^2$? @ron kinetic energy is zero when v is zero. The constant c is to do with the arbitrariness of the measurement of potential energy to within a constant and this is the internal energy of the mass. Jul 7 comment Why did we need relativity to derive $E=mc^2$? Stachel is just saying that the internal state is like the mass, in being defined in the rest frame and all other observers agreeing upon this value. On page 217 they also remark that Einstein was not correct in stating that the change in kinetic energy is independent of the qualities of the body - its internal state $S_0$ before, and $S_1$ after. Jul 6 comment Why did we need relativity to derive $E=mc^2$? @RonMaimon have a look at the paper and please tell me if you think it's reasonable to keep C the same in both equations, because to me it looks as if he's assuming the internal energy remains constant: fourmilab.ch/etexts/einstein/E_mc2/www Jul 6 comment Why did we need relativity to derive $E=mc^2$? My point was why shouldn't the internal energy contribute to the radiation, rather than the mass? Isn't this a physical assumption? And since energy is frame dependent, I'd expect potential energy to be also. Jul 5 comment Why did we need relativity to derive $E=mc^2$? @RonMaimon if you look at the derivation, the constant C doesn't change during emission of radiation and so the internal energy is constant - why should this be so?