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  • 0 posts edited
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  • 66 votes cast
Jan
22
comment Can gravitational potential energy be released in a fire?
Keenan, your additional analysis totally neglects the effects of gravity. Your model simply doesn't include it at all. If you really want to do this rigorously, write down the partition function for the gas in a gravitational field and do the full calculation.
Jan
22
comment Why does a ballerina speed up when she pulls in her arms?
@Noldorin: You could settle it once and for all by pausing it when she has the foot on the left hand side of the image elevated and pointing directly towards you, and measuring the minimum thickness of the ankle, and then doing the same thing once she has rotated 180 degrees. That would give you the definitive answer.
Jan
22
comment Can gravitational potential energy be released in a fire?
@Keenan: The question is about whether gravitational potential energy can be released by a fire, and as Peter points out, in certain circumstances it can. Perhaps the easiest way to see this is to consider burning the fire inside a sealed vessel, and then releasing the resultant gas at different elevations.
Jan
22
comment Why does a ballerina speed up when she pulls in her arms?
As a side note regarding the new ballerina picture, it seems there is a right and a wrong answer, since the picture appears to use perspective rather than a parallel projection.
Jan
22
comment Can gravitational potential energy be released in a fire?
@Peter: I think this is a good example of exactly the problem you highlighted on meta months ago.
Jan
22
comment Can gravitational potential energy be released in a fire?
@Keenan: You need to take into effect the mass of the new gas molecules. There is an increase in the density of the gas due to the fire, which changes the potential energy, and leads to an increase in the average kinetic energy of the molecules in the atmosphere as it settles to equilibrium.
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
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
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
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.
Jan
20
comment What is the most efficient way to destroy the universe?
Or simply refuse to accept the axioms of logic. Then you can insist whatever you want is true.