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I'm a physics graduate student.


2d
comment How does a hovercraft hover, if it has low pressure underneath it?
I don't know, sorry. Wikipedia is usually a good first place to look.
Apr
28
comment Is there any disadvantage to sending rockets straight up?
Yakk, yes, good point.
Apr
3
comment Relativity of Work
If the duck is already in motion, from one perspective you do positive work and the ground does equal negative work and the duck's energy doesn't change. From another perspective you do zero work and the ground does zero work and the energy doesn't change.
Mar
31
comment Why can we ignore self energy?
Right; as long as it's continuous that integral works.
Mar
31
comment Why can we ignore self energy?
It breaks my heart that someone is reading Purcell and using a bunch of $\epsilon_0$ everywhere.
Mar
25
comment How does anything move?
Good point. I decided to change it to "can become positive" because I think the mere possibility is what was motivating the question.
Mar
25
comment How does anything move?
...except those higher than six, of course, so the statement in my answer assumes the function is smooth. But worry about details like that is missing the point.
Mar
25
comment How does anything move?
The phrase "when the object starts moving" is a colloquial one, it doesn't have to mean the same thing as t=0. In your example, all the derivatives are positive at some time arbitrarily close to t=0.
Mar
19
comment Noether Theorem and Energy conservation in classical mechanics
I find it hard to understand your question. The numerical value of the Lagrangian is not constant. It is not constant for infinitesimal periods. It is not constant for finite periods. The function form of the Lagrangian is usually constant. So if the Lagrangian is $\frac{1}{2}mv^2 - mgh$, it stays that, and doesn't become $\frac{1}{3}mv^2 - mgh$ at some later time or anything like that. The actual value of the Lagrangian of course changes because $v$ and $h$ change.
Feb
22
comment Why do flat sheets of paper twist more than paper cones as they fall?
What is the name of the theorem you mentioned? A cursory search hasn't turned it up.
Feb
22
comment Why do flat sheets of paper twist more than paper cones as they fall?
Evidently "flutter" was a distracting word. I'm referencing only the "twisting" of the paper, I guess. The thing that a flat playing card does.
Feb
17
comment Height dependancy when adding volume from below to a fluid column
If you make the screw thinner $F_a$ and $F_b$ are reduced by the same factor, so the relation between them holds.
Feb
17
comment Height dependancy when adding volume from below to a fluid column
I'm not sure what "other take" you want. It's a simple problem and this is the answer.
Feb
2
comment Kinetic energy of an expanding sphere
It says that if, for example, the sphere is expanding at 2cm/s on the outside and you look at a point half way from the center to the outside, that point is expanding at 1 cm/s.
Jan
27
comment How thick is the “skin” formed from surface tension?
There's no point in being super-accurate for this question. We just want to know whether surface tension is an effect of mainly the top layer of atoms or not. If a very simple argument works, then why bother trying to be sophisticated about it?
Jan
23
comment Is there a thermodynamic limit on how efficiently you can solve a Rubik's cube?
That was the intent of physics.stackexchange.com/questions/160585/…
Jan
23
comment Is there a thermodynamic limit on how efficiently you can solve a Rubik's cube?
Intuitively, I think this is right. I think it is also not perfectly rigorous because the information stored in the cube is not obviously the same sort of information stored in microscopic degrees of freedom. Conservation of information is a theorem we can derive from Hamiltonian dynamics. If we want to apply it to the Rubik's cube, we need some justification that our ideas about Hamiltonian systems can be applied. There needs to be some definite link made between states of a Rubik's cube and the microstates for which we have relevant theorems about information.
Jan
22
comment If a place a spring in a box and drop the box, what happens?
No. When you change initial conditions, the amplitude also changes.
Jan
22
comment If a place a spring in a box and drop the box, what happens?
Your question says the system starts at equilibrium. Are you now asking a different one?
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.