# All Questions

85 views

### Entangled event horizons

Assuming it is possible in principle to entangle the degrees of freedom of the event horizons of two black holes, and that this is something that can be done, either after the black hole is formed, or ...
697 views

### Electric Field due to a disk of charge. (Problem in derivation)

This might be a really silly question, but I don't understand it. In finding the electric field due to a thin disk of charge, we use the known result of the field due to a ring of charge and then ...
298 views

### Self-dual Maxwell equations, the second homology group, and topological invariants of a four manifold

In Witten's paper Quantum Field Theory and the Jones Polynomial, he mentioned that: Geometers have long known that (via de Rham theory) the self-dual and anti-self-dual Maxwell equations are ...
530 views

62 views

### Finding dimensional formula

$$y(x,t)=2A\sin(Kx)\cos(\omega t)$$ $A$ and $x$ are in metre, $\omega$ is angular frequency. Then find dimensions of $A$ and $K$. In this equation how can I find the dimension of $K$?
143 views

### Description of the heat equation with an additional term

I have the following equation: $$\frac{\partial U}{\partial t}=k\frac{\partial^2 U}{\partial x^2}-v_{0}\frac{\partial U}{\partial x}, x>0$$ with initial conditions: $$U(0,t)=0$$ $$U(x,0)=f(x)$$ ...
64 views

### Bending of a spline

The introduction section on this page states: "the spline will take a shape that minimizes the bending", while showing the formula for curvature of a real curve. Question is, what is the formula for ...
18 views

### Bass and acute tones. Perceiving their direction

In a modern sound system, you put the sub-woofer somewhere on the floor, not worrying much about its position, but putting the loud speaker around my ears, to generate a sound-surround effect. I ...
56 views

### How to calculate frequency in the following question [closed]

A police car with a siren of frequency $8$ kHz is moving with uniform velocity $36 \mathrm{km}\, \mathrm{hr}^{-1}$ towards a tall building which reflects the sound waves. The speed of sound in air is ...
76 views

Say, in the example below, the weight $mg$ of the object is $800N$. To find $R$, the conventional method is to use $R\sin(28^\circ) = 800$. But why isn't it possible to use instead the component of ...
161 views

### A question about causality and Quantum Field Theory from improper Lorentz transformation

Related post Causality and Quantum Field Theory In Peskin and Schroeder's QFT p28, the authors tried to show causality is preserved in scalar field theory. Consider commutator  [ \phi(x), \phi(y) ...
252 views

### Effect of Earth's magnetic field on aeroplanes?

As an aeroplane accelerates through the Earth's B-field, it experiences a changing flux $d\phi \over dt$ and a potential difference is induced along its wings. Given the wings are made of metal and ...
59 views

### Is there quantitative theory of cutting with edge or blade

I wonder if there is some simple theory of what determine efficiency ( speed, energy end force required ) of cutting by edge ( blade , knife, sword ) At least something phenomenological like in ...
155 views

### Tension in string and gravitational force on bob relative strength

When calculating time period of simple pendulum (an approximation of SHM at small amplitudes) we take gravitational force greater than tension in string and resolve gravitational force in two ...
101 views

### Could the dark matter sector be boring?

People always talk about finding good dark matter (DM) candidates of beyond Standard Model (SM) theories. To be a proper candidate the particles should not decay into the SM particles and could ...
37 views

### Evaluating angle of refraction [closed]

Can someone show me how to evaluate the angle of refraction for white light where the angle of incidence is 30 degrees if medium A is air and medium B is water. The answer is 22 degrees. I don't know ...
61 views

### Optimal Airplane trajectory

The last time I took a plane the following problem crossed my mind. Setting: take the Earth and neglect its rotation around the Sun. It then only rotates on itself with angular velocity $\Omega$. ...
97 views

### Why is a material such as plastic air-tight?

I'm not too familiar with fluid mechanics, but i always wondered how let's say plastic containers would be able to contain a fluid, such as gas or water. How does plastic prevent the molecules of the ...
172 views

### Geostationary orbit and rotation speed of a planet

Assume for planets similar to Earth but with different rotation speeds (including zero), and we put a satellites at altitude of 35 786 km/hr (similar to the Geostationary orbit or earth), with speed ...
41 views

### Could one estimate the size of air-molecules based on analysis of the sound?

Say one knows air is composed of molecules (atoms). Could one estimate the size of molecules (atoms) by analysing the sound properties as one perceives them (clearness, speed, etc.)?
73 views

### Intuitively, why does removing solutes cost $k_B T$ of free energy per molecule?

I can calculate that if you want to, for example, desalinate water, you will have to pay a free energy cost of $k_B T$ for each ion you remove. In other words, removing an ion from a volume of water ...