# All Questions

2k views

### Why are tensions in the pulley different when the pulley has a mass or moment of inertia?

When two blocks are connected by a string passing over a pulley whose moment of inertia is given (means pulley is not massless) then why does the string not have same tensions?
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### Why do $H$ and $L^2$ commute in spherically symmetric potential?

In this PDF document (a lecture by Shivaly Reddy, page 13), he says that $L^2$ is independent of $r$; therefore it commutes with any function of $r$. This seems related to a problem in ...
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### What do people mean with a vev=diag( -2,-2,-2,-2,-2,-2,3,3,3,3)?

For example, in this paper on page 21 the authors write the vev that breaks $SO(10)$ to $SU(4)\times SU(2) \times SU(2)$ $$<54>= 1/5 \cdot diag( -2,-2,-2,-2,-2,-2,3,3,3,3) \omega_s$$ where ...
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### What is meant by invariant under change of coordinates **to first order**?

I am studying elementary Lagrangian mechanics, and I'm a bit confused about the what's meant by invariance of the Lagrangian under change of coordinates to first order. More specifically, Noether's ...
3k views

### How to explain independence of momentum and energy conservation in elementary terms?

I'm trying to explain to someone learning elementary physics (16 year old) that linear momentum and energy are conserved independently. I'm not a professional physicist and haven't tried to explain ...
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### Fixing time in Feynman phase space path integral

The phase space version of Feynman's path integral expression for the free particle propagator involves a (formal) sum over paths in phase space with fixed $q$ endpoints and (as far as I'm aware) ...
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### How are aberration levels measured?

I was reading reading the paper: Predicting subjective judgment of best focus with objective image quality metrics when I come across this statement: Through-focus visual acuity (VA) was ...
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### Parity of $n$-photon system

The $C$-parity (charge conjugation) of an $n$-photon system is given by $(-1)^n$. If I'm not totally wrong, the intrinsic parity of a photon is $(-1)$. What is the parity $P$ of a system of $n$ ...
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### How is hydrogen stored in hydrogen bombs?

Fusion bombs obviously fuse hydrogen atoms, using the pressure from a fission bomb. But how much hydrogen does it contain, and how is it stored? AFAIK hydrogen easily drops its electron and escapes as ...
229 views

### Is it possible to calculate the work function of a metal with basic lab equipment?

ive been given a problem in physics, its to prove if a lump of metal we have been given is real gold or not. one way to to do it would be to work out its density, which is fairly easy, mass/volume. ...
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### Question about intensity of EM waves

For electromagnetic wave if it's reflected from a perfect conductor standing wave can be form. I wonder why Poynting vector can be used to describe the intensity of standing EM wave. (see p.19 of ...
343 views

### Is it true that spring has more force acting on it at its positive maximum amplitude than than at the negative one?

Am I missing something? It seems obvious to me that at $+A$ and $-A$, the spring has restorative forces equal in magnitude but opposite in direction. But since gravity is always pulling it down, ...
185 views

### How are standing waves a result of constructive and destructive interferences?

For constructive I can understand. But destructive I can't. I can not picture the shape of two pulses or waves maybe that form the resulting standing wave. The places where waves are canceled just ...
### Square root of a matrix appears in massive gravity. How to solve $\sqrt{A+B}$ perturbatively
$A=\text{diag}\{\lambda_1,...,\lambda_n\}$, where $\lambda_i$ can be any number and not necessarily a small number, $\lambda_i>0$, $B$ is a positive definite symmetric matrix, and ...