# All Questions

1answer
41 views

### Power and frequency of photons and its relationship with biological safety

I understand that x-rays are more dangerous than radio waves because they are of higher energy, since they have higher frequency. However, it’s less dangerous to stand near a radio station with a ...
0answers
116 views

### Problem in understanding Feynman's explanation of the Dirac-Delta function [duplicate]

This is quoted from Feynman's Lectures' Normalization of the states in $x$: We return now to the discussion of the modifications of our basic equations which are required when we are dealing with ...
3answers
343 views

### Primitive unit cell of fcc

When I consider the primitive unit cell of a fcc lattice (red in the image below) the lattice points are only partially part of the primitive unit cell. All in all the primitive unit cell contains ...
1answer
36 views

### How do I calculate the speeds in x, y, and theta of a spinning rod?

Suppose the object looks like the diagram below (it is the horizontal rod). On the vertices, are two velocity vectors $v_r$ and $v_l$. The question is: how is the motion of the rod affected by these ...
0answers
137 views

### Relative displacement of a point in a deformed object

I have a 3d model (nodes and elements) and it undergoes translation and rotation at different time steps Ti. I want to know the displacement relative to some point in the model which is kept as fixed ...
1answer
74 views

0answers
41 views

### Is there any such thing as gravity? [duplicate]

So I was seeing this documentary and ended up knowing that there is no such thing as gravity. It said that we are not being pulled by Earth but being pushed by the space time curvature acc. to general ...
1answer
24 views

### Why the centered rectangualr lattice is not considered a special case of an oblique lattice

From Wikipedia, there are 5 different kinds of lattice in 2 dimension: But I am wondering how the third type (the centered rectangular) is different from the first kind (the oblique lattice). The ...
1answer
67 views

### lattice vibration

I would like to inquire about acoustic and optical branch in Phonons vibration; My question is: what is the physics description when we say what is make acoustic and optical branch appear ...
1answer
212 views

### Why is there a change in entropy in a irreversible isothermic process?

Suppose you have a irreversible isothermal process. The net change in entropy from the system to the surroundings should be 0 if the temperature does not change. So why is there an increase in entropy?...
2answers
86 views

### What are the gradient lines of the magnetic field of a cylindrical magnet?

The problem is identical to that for a finite-length solenoid. I was interested in finding the family of curves for the equipotential lines, rather than the B field itself, which is more readily ...
1answer
97 views

### How does gas spin the turbine in a jet engine?

I am confused about how the energy from the ignited gas spins a turbine in a jet engine, as pictured below                &...
1answer
56 views

1answer
83 views

### Observing light speed

Consider an observer B is moving with speed $0.8c$ relative to another observer A in standard-configuration (I think it is called that in english: that the system B is only moving along the $x$-axis ...
0answers
48 views

### What is the equation for the elementary-excitation spectrum of superfluid heluim over whole momentum space?

I have a simple question. What is the equation for the elementary-excitation spectrum of superfluid helium over whole momentum space, not just near the origin (i.e., phonon excitations $\epsilon=cp$) ...
5answers
156 views

### Pressure variation

Imagine we have a vertically orientated pressurised liquid filled closed pipe. The pipe is shaped like a spring, such that the distance between the two ends can be adjusted vertically, whilst the ...
1answer
162 views

### Completeness Relations of Polarization Vectors in QCD

What are the completeness relations of the polarization vectors of (external) particles in QCD amplitude calculation? (I assume the polarization vectors depend on the gauge and even so still have some ...
1answer
90 views

### Is entropy violated inside black holes and worm holes?

Do the laws of thermodynamics hold true everywhere in universe ? What about black holes and worm holes ?
1answer
60 views

### Does energy flowing from one place to another in an object cause displacement of that object?

Let's consider two situations: a) A solid metal box (not hollow) where some heat is put on its left side. After some time, the heat will have moved to the right and also have spread out throughout ...
2answers
74 views

### Hot bullets and shot cowboys [closed]

Please excuse the title of the question, there is physics involved here, but I can't think of a way of writing the title without it being more to the point of the question. A cowboy film is pure ...
1answer
53 views

### Wave nature of an electron

Quantum mechanics tells us that any quantum particle behaves as particle or waves in different situations. Assume that the electron is behaving like wave in some situation, how can I visualise this ...
1answer
63 views

### When can I apply Lagrangian mechanics?

I am trying to understand Lagrangian mechanics. I am having trouble capturing all of the nuances in one gulp. I can see the equations, but not necessarily the semantics behind such equations. I ...
0answers
69 views

### Source of a parabolic wavefront

The Huygens-Fresnel diffraction integral has been formulated for incident spherical waves. A source of a spherical wave is kind of intuitive, since the perpendicular rays of the sphere converge into a ...
3answers
131 views

### Path followed by centre of mass of a ball when ball breaks up in two parts due to internal stresses

A ball was moving along a parabola and it breaks up into two parts. How can I say that the centre of mass will follow the same parabola ? I know that ( for simplicity I am assuming that the ball is ...
0answers
68 views

### signal from uniformly moving charge

As a charge moves, its field changes, and this change can only be propagated outward at the speed of light. Thus the field lines will be curves that keep changing apparent source point and direction. ...

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