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I'm a physics graduate student.


Jan
22
comment Placing an object in a tub changes the weight of the tub?
If the buoyant force is $W$, then the object pushes down on the water with a force $W$ by Newton's third law. The water is not accelerating, so the net force on it is zero, so the force upward on the water from the tub must increase by $W$. So the force from pressure on the tub increases by $W$ by Newton's third law.
Jan
22
comment Placing an object in a tub changes the weight of the tub?
Because the water level rises, increasing the pressure.
Jan
22
answered Placing an object in a tub changes the weight of the tub?
Jan
22
awarded  Notable Question
Jan
22
comment Is there a thermodynamic limit on how efficiently you can solve a Rubik's cube?
This is an obvious point that wasn't the intent of the question. What if the computer needs to solve a very large number of cubes and doesn't have enough memory to store them all?
Jan
21
comment Is there a thermodynamic limit on how efficiently you can solve a Rubik's cube?
Yeah, I think that's right. We can look at the set of all microstates of the entire system and partition it based on the state of the cube in each microstate. Then by the cube's approximate symmetry, each state of the cube should have about the same number microstates of the entire system associated with it. Thus the entropy reduction based on counting microstates is roughly the same as that based on counting the information in the cube.
Jan
21
awarded  Good Question
Jan
21
comment Is there a thermodynamic limit on how efficiently you can solve a Rubik's cube?
@IlmariKaronen It seems this argument is predicated on being able to say that the state of the cube is identifiable with microstates somehow. E.g. if I could write the state of the system as the tensor product $\mathrm{system} = \mathrm{cube state} \otimes \mathrm{everything else}$, then the argument would work b/c reducing entropy in $\mathrm{cube state}$ forces me to increase it in $\mathrm{everything else}$, but how do I know that I can speak about the state of the cube in such a way?
Jan
21
awarded  Popular Question
Jan
21
accepted Is there a thermodynamic limit on how efficiently you can solve a Rubik's cube?
Jan
21
comment Is there a thermodynamic limit on how efficiently you can solve a Rubik's cube?
@IlmariKaronen Okay, yeah, I think that sounds convincing. Thank you.
Jan
21
comment Is there a thermodynamic limit on how efficiently you can solve a Rubik's cube?
Sure, but the question is, supposing we are not going to record the information on a second cube (or anywhere else), can we then justify the conclusion that the machine must give off heat?
Jan
21
comment Is there a thermodynamic limit on how efficiently you can solve a Rubik's cube?
How do we know that it is impossible to solve the cube without recording the position in the machine's state?
Jan
21
awarded  Nice Question
Jan
21
asked Is there a thermodynamic limit on how efficiently you can solve a Rubik's cube?
Jan
15
comment Alternate young double slit experiment
The question isn't clear to me. Light comes from a laser, hits a screen with a small circular hole, goes through the hole, then hits a second screen. The second screen is solid, and we observe the interference pattern on the second screen? Is that the entire setup? If so, you will see an Airy pattern.
Jan
15
answered Is diffraction affected by interaction between photons and electrons?
Jan
15
revised Relative velocity between a ball hitting a rod
edited tags
Jan
15
comment Conservation of angular momentum in a free rod
@BillN My first response had a typo in the figures which led to some back-and-forth which I deleted as unproductive.
Jan
15
answered Measuring mass zero G