Martin Büttner
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 Mar 9 comment Why does matter exist in 3 states (liquids, solid, gas)? One very interesting point (in the opposite direction of "there's more than three states of matter") is that at sufficiently high temperatures/pressures (i.e. beyond the critical point) liquids and gases become indistinguishable, so there isn't even necessarily a clear distinction between the two. Might be worth mentioning that in one of the answers, and yours seems nice and comprehensive, yet accessible. Mar 14 awarded Citizen Patrol Sep 24 awarded Autobiographer May 28 awarded Notable Question Dec 5 awarded Popular Question Sep 15 awarded Yearling May 10 comment Simple conservation of momentum and frame of reference problem If you are going to solve it in one of the objects' rest frames you will need to stick to it, because by switching from one to the other, you are obviously changing things. Usually it's easiest to work in the centre of mass frame. Also, you will need to consider conservation of energy in addition to conservation of momentum, and specify whether your collision is supposed to be elastic or inelastic (or to what degree), to figure out the exact distribution of speeds after the collision. May 6 comment Why can't we accelerate objects past the speed of light? If you are not questioning the mathematical reasoning, but just the overall universal fact that there is a speed limit... I think the question might be pointless. Because you could think of it as not there being a speed limit, but of us having a wrong perception of velocity addition. After all, you can accelerate something further and further. And for someone travelling at the $0.99c$ there can still be something $0.99c$ faster. It is just that in our non-relativistic environment we are used to velocities adding up normally - but that's just not how the universe works with velocities. May 6 comment Why can't we accelerate objects past the speed of light? @Anthony initially it's just based on experimental evidence. In particular, we cannot accelerate the speed of light. Mounting a laser on a cart that moves with 1 m/s does not make the laser light move with c + 1m/s. So, you start from the assumption that the speed of light is the same in all inertial frames of reference. This is really the only starting point you need to derive special relativity. And it tells you that adding velocities $v_1$ and $v_2$ does not give you $v_1 + v_2$. May 6 awarded Critic May 6 comment Angular momentum after elastic collision @DavidZaslavsky there you go May 6 answered Angular momentum after elastic collision May 5 comment How are qubits better than classical bit? Just a little nitpicking: NP-complete is a specific class, where checking a solution has polynomial time-complexity, while finding one has exponential time-complexity (for all we know!) - and there are often approaches that are better than trying every single candidate solution, but they are still exponential. However, there are other complexity classes, where checking a solution is significantly faster than finding one - so this applies to all of these. May 5 comment Angular momentum after elastic collision @joshphysics but in the presence of friction angular momentum might be transferred between the two May 5 awarded Commentator May 5 comment Angular momentum after elastic collision No. Without friction, the forces during the collision are applied exclusively through their centres of mass - hence there is no torque and hence the angular momentum cannot change. Wikipedia has a force diagram that illustrates what I mean: upload.wikimedia.org/wikipedia/commons/2/2c/… Apr 30 awarded Scholar Apr 30 accepted Faraday's law - does the induced current's magnetic field affect the change in flux? Apr 30 comment Faraday's law - does the induced current's magnetic field affect the change in flux? Thanks a lot guys, that's really helpful! I'll play around with Mathematica a bit; I guess some plots will help my understanding as well. (Also, wow, did you actually make an account just answer that? Welcome to physics.stackexchange.com!) Apr 30 awarded Student