# All Questions

0answers
93 views

### Time between two collisions in a 2D gas

I'm trying to code a simulation of a 2D gas in a box. The molecules are represented by circles of radius $R$ having elastic collisions with the walls only (for now). I read that the average time step ...
0answers
33 views

### Does chemical formula of matter influence on how would it be charged? [closed]

Suppose we rub glass ($\mathrm{SiO_{2}}$) against paper(which is made from cellulose ($\mathrm{C_6H_{10}O{5})n}$). We receive positively charged glass and negatively charged paper. Is that because of ...
1answer
43 views

### Clarification about first spherical harmonic

If the quantum numer $l$ is equal to zero, each components of L has eigenvalues $m=0$. So the states that have $l=0$ are simulaneous eigenvectors of each components of L. But, my textbook says, it is ...
0answers
39 views

### What is ferromagnetism?

Simple question. As far as I've understood it; a ferromagnetic material is one where all the electron spins are alligned parallely - that is, the wavefunction has an antisymmetric spacial part and a ...
0answers
24 views

### Properties of materials _not_ dependent on fermi surface?

So I'm studying a second solid state physics course where we've covered calculating things like magnetic susceptibility, specific heat and resistivity by considering excitations of electrons around ...
0answers
65 views

### Quantum harmonic oscillator, clarification about the period

We can write the general state |A> at time $t=0$ as $|A, 0>=\sum a_n |n>$ where |n> are the eigenvectors of the oscillator. In my textbook there is written that if the $a_n=0$ for every n=even ...
0answers
16 views

### Distance needed to double the moment of inertia of a cylinder

The moment of inertia formula for a solid cylinder is $0.5MR^2$. How would I need to change the radius in order to double the moment of inertia of the solid cylinder?
1answer
73 views

### Modeling a pool ball rolling and slipping given initial velocity

I am trying to model a pool game. Currently my physics is faked by giving it an initial velocity and just subtracting from it every second. To make it more realistic I want to use physics to determine ...
1answer
142 views

### How can we calculate the maximum height of a fountain? [closed]

There are two limiters in this question. First is drag of air. Second is the viscous forces of pipe.(belong to the tube where water hitts up) The question is how can we calculate the maximum height ...
1answer
71 views

### Why do we need theorems like **Parallel Axis Theorem**

In rigid body pure rotation, quantities like $\omega, \tau, L, I, r_i$ (symbol with usual meaning) are axis dependent. Assume rigid body to be sphere rotating about the axis passing trough the center. ...
1answer
27 views

### How drag force relates to velocity in the air?

As you know in the formula of drag $$F=0.5 *d*A*C*v^2$$ in different velocities we have different Reynolds number. and the Reynolds number relates to coefficient of drag. my question is how can we ...
1answer
82 views

### When are energy, mechanical energy, momentum, and angular momentum conserved?

I am in AP Physics and my only real hangup is knowing when the said quantities are conserved. Please define what is the SYSTEM in your answer. I kind of have the basic idea. For example, if there ...
0answers
72 views

### Two dimensional spacetime and the Gauss Bonnet theorem

Generally two dimensional spacetimes are deemed to be static, as the Gauss Bonnet theorem implies that the Einstein Hilbert action would be a constant independent of $g$. But as far as I can tell, ...
0answers
18 views

### Interferometer based on atoms and gravitational field detection

I know that ultra cold atoms can be used to measure a gravitational field, but how does this work exactly. More specifically, I know that an interferometer based on atoms can be used to make very ...
3answers
124 views

### Why does Torricelli's law seems to fail when water speeds up after I put my finger in a hose?

We are taught that the speed of the fluid through a hole in a water filled tank obeys Torricelli law: $v=\sqrt{2gh}$, thus the speed is independent of the size of the hole. Why is it then that making ...
1answer
126 views

### Can entropy be regarded as energy dispersal?

In several answers here the claim has been made that thermodynamic entropy can be regarded as energy dispersion. See, in particular here, and here and here. This is apparently the pet theory of a ...
0answers
48 views

### Calculus/Vector Calculus and so on in special relativity book recommandation

I want to learn calculus 1,2,3 vector calculus, analysis and so on in special relativity. But I never found a good and clear book/source on it. Can someone recommend an easy book or internet source on ...
1answer
89 views

### Wave reflection, boundary conditions?

When a wave-pulse is reflected such that initially it was moving in the denser medium, and towards the other side of the interface lay the rarer medium, will there have been a phase change post ...
0answers
48 views

### Are these two equations correct? [closed]

I know this is a pretty simple question, but like there should be room for those as well. I have the following two formulas describing how long it'll take for an object to decelerate. For example if ...
0answers
51 views

### Is there a theoretical maximum speed for wind?

What we call wind is actually molecules moving into an area of lower particle density due to their thermal motion. A lower particle density offers less collisions (resistance), so more molecules move ...
1answer
37 views

1answer
41 views

### Calculate the relativity of time of an event occurred in different frames

Imagine we have a train which is moving with a velocity and there's an observer inside of that train and there is an observer that is outside of the train observing the train moving. There's a ball ...
3answers
82 views

### Violation of newton's third law in case of fluids

If a steel ball hits an iron surface it bounces back due to the Newton's third law of motion. Then why does'nt it bounce back when it hits a water surface ? or for that matter even air ? The ...
1answer
116 views

### Confused regarding sign convention of silvered plano convex lens

I'm totally confused while finding the sign convention of a silvered plano convex lens. I know that equivalent power of such a system can be found by adding the power of the mirror and twice the power ...
1answer
56 views

### Do physicists subscribe to Occam's razor? If so, how do you define 'all things being equal'? [closed]

The question here is the title: Do physicists subscribe to Occam's razor? If so, how do you define 'all things being equal'?
1answer
58 views

### Back Emf in Motor

Why does back emf tend to decrease as the rate of doing work increases. When load increases it reduces the angular speed of motor as a result induce current due to back emf also decreases because the ...
1answer
64 views

### Calculating velocity of a spacecraft between moon and earth [closed]

I'm in high school, I'm not very good at physics, and I have this project (year long). I'm trying to determine the accuracy of a simulator (Universe Sandbox 2) with theoretical velocities vs. the data ...

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