Joe Fitzsimons
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 Jan 22 comment Can gravitational potential energy be released in a fire? @Georg: Your objection is incorrect. As long as the same amount of fuel is burned, you are essentially dumping the same mass of gas into the atmosphere, whether at ground level, or at elevation. The effect of these new molecules falling on average, as they come into equilibrium with the atmosphere heats it. Jan 22 comment Don't heavier objects actually fall faster because they exert their own gravity? @Jerry: This isn't true in the earth's atmosphere, which is why people find it so counter intuitive in the case of a vacuum (a caveat which you should probably add). Jan 22 comment Transfer function of an RLC circuit This looks very like a homework problem, so I doubt you will find anyone to give you the answer here. For this reason I have also voted to close the question. However, I will point you in the right direction: What you are looking for is Kirchoff's laws (en.wikipedia.org/wiki/Kirchoff%27s_circuit_laws). Jan 22 comment Why does a ballerina speed up when she pulls in her arms? You're definitely better at expository answers +1. Jan 22 comment Double slit experiment near event horizon @kakemonsteret: Yes, I know it's only a practical consideration. Jan 22 answered Why does a ballerina speed up when she pulls in her arms? Jan 22 comment Double slit experiment near event horizon @Columbia: That is nonsense. The evaporation of the hole will effect the state inside the state sent in, so you are not violating the no-cloning theorem. Jan 22 comment Temperature of a System of molecules Actually temperature is usually defined as $T = (\frac{\partial S}{\partial E})^{-1}$, where $S$ is the entropy of the system. Jan 22 answered To calculate the correlation functions of an XX spin chain, Wick's theorem is used. But is it valid for a chain of any size? Jan 22 answered Is it possible to separate the poles of a magnet? Jan 22 comment Is it possible to separate the poles of a magnet? Monopoles are prohibited by Maxwell's equations, specifically $\nabla \cdot B = 0$. If you want monopoles you need to modify this particular equation. Jan 22 comment Double slit experiment near event horizon @kakemosteret: You won't make it across the event horizon anyway. The gradient of the gravitational field will rip you apart first. Jan 22 comment Does decoherence need non-determinism? Good answer! Jan 22 comment Does decoherence need non-determinism? This isn't technically correct. Decoherence can be the result of an entirely deterministic (and unitary process), since you only care about the reduced density matrix for the system in question. Open quantum systems -do- decohere, even though the entire process is unitary. The larger wavefunction of the entire system is still pure, but the state of the local system becomes mixed. Jan 21 revised Does decoherence need non-determinism? added 807 characters in body Jan 21 answered Does decoherence need non-determinism? Jan 21 answered If a 1kg mass was accelerated close to the speed of light would it turn into a black hole? Jan 20 revised Using delayed choice interference experiments as a computing device deleted 2 characters in body Jan 20 comment Where's the best place to add weight to a Pinewood Derby car? @Mark: Sure, but since the question presumably relates to a real car, it it would be more reliable to simply measure them for the car in question. Jan 20 comment Where's the best place to add weight to a Pinewood Derby car? @Mark This isn't possible without knowing more about the car. The change in potential energy is $m \Delta H$. To calculate the value of $\Delta H$ we need to know the position of the front axle as well as how low down the weight can practically be placed (the lower the better), we also need $m$ the mass added. Even then, although we can calculate the potential energy, this doesn't translate into a specific speed increase (even if it is all converted to kinetic energy). This is because the kinetic energy scales quadratically in velocity, so we need to know the velocity without the extra weight.