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Physics : Recent Questions and Answers (Page 4)

Find answers to your questions about Physics or help others by answering their Physics questions.

Can we solve the general eigenfunction of $L_x$?

Consider the orbital angular momentum in QM, labeled by $L$ ($mathbf{L}=mathbf{r}timesmathbf{p}$). In spherical coordinate, the operator can be expressed as:begin{equation*}left{begin{aligned} L_x&=frac{hbar}{mathrm{i}}left(-sinphifrac{partial}{partial theta}-cosphicotthetafrac{partial}{partial phi}right)\...

Asked on 12/31/2021

2 answer

Is the Birkhoff Theorem valid for the FLRW metric?

The FLRW metric, in the case of positive scalar curvature, is: $ds^2 =- c^2 dt^2+a(t)^2left(dw^2+sin^{2}w(dtheta^2+sin^2theta dphi^2)right)$. The Birkhoff’s theorem states that "any spherically symmetric solution of the vacuum field...

Asked on 12/31/2021

1 answer

Can you 'derive' mathematical approximations made from Taylor approximations from limiting cases in real life?

Here, the natural length of the string is $l_o$, and pulling the string up by $x$ increases its...

Asked on 12/31/2021

1 answer

Newton's third law (action and reaction) and work done with a wheel

I'm aware of the common misunderstanding about action and reaction canceling each other; they don't because they don't act on the same object. This is about a more subtle...

Asked on 12/31/2021

2 answer

How does Fermat's principle make light choose a straight path over a short path?

This is a thought experiment where I have made a "C" shaped hole inside diamond. The refractive index $(mu)$ of diamond is 2.45....

Asked on 12/31/2021

4 answer

Why are the principal planes where principal stresses occur perpendicular to each other?

Equation of principal angles: $$tan 2theta_p=frac{2tau_{xy}}{sigma_x-sigma_y}$$ Equation of principal stresses:$$sigma_{max}, sigma_{min} = {sigma_{xx} + sigma_{yy} over 2} pmsqrt{ left( {sigma_{xx} - sigma_{yy} over 2} right)^2 +...

Asked on 12/31/2021

1 answer

Climate Change and Degrees of Freedom of Gases

I studied a sciences of climate change module with the OU a while ago, I'm half remembering something that's bugging me and wonder whether any could help provide some clarification?...

Asked on 12/29/2021

1 answer

What exactly is a basis in the context of crystalline solids?

For the purpose of this question, I'm making the assumption that atoms are fixed in space, and as such possess no vibrational degrees of freedom. A crystalline solid is a...

Asked on 12/29/2021

1 answer

Capillary Perpetual Motion

Can anyone figure out what is wrong with this perpetual motion machine? What part of it violates physics? I found it on a website a while ago, and I couldn't...

Asked on 12/29/2021

4 answer

Mathematical coincidence of the Schwarzschild radius of the Universe?

I read here that a black hole with a mass of the observable universe, $M=8.8times10^{52}kg$, would have a Schwarzschild radius of $r_s=13.7$ billion lightyears. I immediately...

Asked on 12/29/2021

1 answer

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