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1d
comment How does a giant walk-in fridge maintain a thin temperature gradient at the entrance?
@WetSavannaAnimalakaRodVance Thanks for that!
1d
comment How does a giant walk-in fridge maintain a thin temperature gradient at the entrance?
Is there a wind curtain? (A box/rail above the door which blows a wall of air down. They're present at many store entrances)
Nov
20
comment What is the physics of a spinning coin?
related: physics.stackexchange.com/questions/68676/… and mathandcode.com/disk I wanted to solve this problem in full generality. So my solution is undoubtedly more complicated than it needs to be. But you should be especially interested in "Partial constraint 2"
Nov
20
comment What would be the consequences of time not being “relative”
And his struggle was before he abandoned galilean relativity. You're correct that if you assume galilean relativity you can conclude that special relativity is wrong. But it seems like you're trying to show that special relativity is inconsistent in its own right?
Nov
20
comment What would be the consequences of time not being “relative”
-1 for, if no other reason, making the quote sound like it supports your post. He states in the next paragraph it was only thought by Einstein to be a contradiction when assuming Galilean relativity ("he had been assuming that the ordinary Newtonian law of addition of velocities was unproblematic")
Nov
19
comment How is strong time dilation consistent with weak tidal forces?
Mentioning spin: wired.com/2014/11/metaphysics-of-interstellar Thorne: "I went home, slept on it, did a calculation, and found that if you have a black hole that spins rapidly enough, and a planet that is very close to the last stable circular orbit, you could get the time dilation he wanted. It just amazed me." (Not that it changes your answer, just that in the movie Gargantua is in a stable circular orbit with realistic [but absurd] time dilation)
Nov
15
comment Problem in Euler-Lagrange imply Newton
@MedSaâdAlami, so certainly that term doesn't vanish. But, if it did vanish, what would that get you? (Also, $\theta=a \cos(\omega t)$ does not solve the simple pendulum problem, it solves the simple harmonic oscillator problem.) For reference I think the proof of Newton's laws from the principle of least action is given in L&L as equation 5.3. It's so simple because the first step in this would be writing $L$ in the form 5.1.
Nov
15
comment Problem in Euler-Lagrange imply Newton
@MedSaâdAlami Newton's equations are specifically written as $m\ddot{x}=-U'(x)$, where $x$ is a coordinate in your Euclidean system. If you want to prove Newton's equations, then, you'd better choose to write $L$ in terms of Euclidean coordinates first. (The pendulum adds an extra degree of complication, therefore, because there is now a force of constraint when you move back to Euclidean coordinates!)
Nov
12
comment What's the largest mushroom cloud possible from a coffee cup/grenade sized nuclear bomb?
@crclayton I'm almost certain that $1450 ft$ is just the height of the bomb when it went off!
Nov
11
comment Second derivative of dirac delta expression
@doetoe actually, scratch that, I guess it is a bit obvious they're artefacts, and only the $k=0$ behavior (which doesn't exist) is the important part.
Nov
11
comment Second derivative of dirac delta expression
@doetoe How can you be sure that they're just artefacts? (though I think you're right, since the $\sqrt{2 \pi}$ seems too specific). The process works exactly (no artefacts) for $\delta^{(n)}(x)$
Nov
11
comment Why do we have to use an integral in this scenario to figure out $v_{max}$?
In that equation $M$ is the mass enclosed in a Gaussian surface about the center of the Earth. If I remember correctly, the field turns out to go like $GmMr/R^3$ where $R$ is the radius of the Earth.
Nov
9
comment Is the explanation of special relativity in Stephen Hawking's “The Grand Design” flawed?
@bright.magus it's shorthand for $t$ arbitrarily larger than the duration of the experiment.
Nov
9
comment Is the explanation of special relativity in Stephen Hawking's “The Grand Design” flawed?
There are a lot of commonly understood tools in special relativity that have to be sorted: Filling space with clocks at $t=-\infty$ and collecting all the data at $t=+\infty$. So you can talk about "the location at time $t$ in frame $X$" and make perfect sense of it. So saying "two persons can't see the same ray of light" is a bit off.
Nov
2
comment Can action be unbounded from below?
($\int L$ being $\int_0^tLdt$ given $x=b\cos(t)$)
Nov
2
comment Can action be unbounded from below?
My thoughts: Maybe your numerical scheme doesn't implement boundary conditions properly? Ex: If $L=\dot{x}^2-x^2$, then $x(t)=b\cos(t)$ satisfies the Euler Lagrange equations, but $\int L=-b^2 \cos(t)\sin(t)$ can be made arbitrarily negative by increasing $b$.
Oct
31
comment Understanding the Jacobian Matrix
@ValterMoretti I couldn't figure out any intuition behind it! I guess you still have the eigenvalues of $A$ have to be negative (except in the worst posed problems you'll still have a characteristic equation with $n$ solutions counting multiplicity) but it looks like in the $x''=Ax$ case you can have your eigenvalue negative but your solution blowing up to infinity like $x \sin(x)$! But all of that is really far from an intuitive understanding, so I guess it doesn't belong in the answer. I'd rather just sweep it under the rug.
Oct
27
comment Is any energy required to deflect an asteroid, with force always perpendicular to its trajectory?
@stalinbeltran Hence the "if".
Oct
26
comment Is any energy required to deflect an asteroid, with force always perpendicular to its trajectory?
@stalinbeltran I was satisfied to stop my investigation here: No work is done on the asteroid. A minimum of $mv_1^2(1-\cos(\theta))$ is done on the rocket fuel. This result is due to conservation of momentum. If momentum isn't conserved (say we're modeling the sun as a fixed point w/ a $\frac{1}{r}$ potential), then this result won't hold and it might not take energy to deflect the asteroid.
Sep
28
comment Do Monte-Carlo updates have a physical significance in stat. mech?
From my very limited experience I think this is an archetypical question and the answer (in general, though specific cases can have real dynamical significance) is no. The goal of detailed balance and ergodicity is to let you get uncorrelated and unbiased samples of your configuration space. That's it.