# All Questions

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### Equation of motion of a simple pendulum

Consider a simple pendulum of length $l$ moving in $(x,y)$ plane. Assume its point of attachment is being jiggled by external forces with prescribed acceleration $a(t)=(a_x(t),a_y(t))$. Let $\theta(t)$...
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### Charge on capacitors in series

'When there are 3 capacitors connected in series, charge across each capacitor is the same.' Okay so say we consider three caps made of plates of the same conducting materials and same dielectric ...
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### The Feynman propagator and the $i\epsilon$ prescription

The Feynman propagator is usually represented in the i-epsilon form and texts solve the integral in this form (as opposed to doing the Feynman (time-ordered) contour on the real axis). Restricting ...
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### Susskind's Theoretical Minimum, Error in General Symmetries Section?

On page 136 of Leonard Susskind's The Theoretical Minimum, Suppose the coordinates of an abstract dynamical system are $q_i$. The general idea of an infinitesimal transformation is that it is a ...
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### Diagonal lorentz invariant matrix

Is there any general way to show that diagonal Lorentz invariant matrix must be proportional to $g^{\mu \nu}$? I can show it is true, but my proof is not clean enough for such a simple problem.
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### Would it be possible to pull Mars and Earth closer each other?

The burden of travelling between Mars and Earth comes from the distance. If the planets were closer, life could spread to Mars more easily, ensuring the survival of the intelligent lifeforms. What ...
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### Is a light wave's amplitude stretched, along with the “red shift” stretch - making it brighter?

When light waves are stretched and "red-shifted", is the amplitude of the light wave stretched as well, affecting the intensity/brightness of the light wave?
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### Coleman-Mandula theorem and mass gap

I had a couple of naive questions about Coleman-Mandula theorem. One of the assumptions of the theorem is the non-existence of massless particles in the spectrum. Since we do have massless photons ...
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### Bose-Einstein condensate effect

Does every boson gas go through the condensate effect? Could there be a boson gas with high energy (photons for example) that would have enough energy to be at different quantum states even at 0 K?
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### Is there a delay in the effect of the gravitational or electromagnetic force if a secondary body suddenly appears? [on hold]

If the sun were to suddenly vanish in our solar system, it would take about 8 minutes for the Earth to exit its orbit. This is because the gravitational field of the sun would vanish at a rate of $c$. ...
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### Why would speed of light be directional if spacetime is discrete?

In Feynman's Simulating Physics with Computers, Feynman states that "we might change the idea that space is continuous to the idea that space perhaps is a simple lattice and everything is ...
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### What is the time for which this plane is in danger of being hit by this cannon shell? [on hold]

A fighter plane is flying horizontally at a height of $250~\mathrm{m}$ from ground with a constant velocity of $500~\mathrm{m/s}$. It passes over a cannon which can fire a shell at any time in any ...
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### gauss law and electric flux

Can you please explain me what is electric flux. I know it is $\oint\mathbf{E}\cdot\mathrm{d}\mathbf{S}$, but in many place its given flux means to flow. But how does electric field "flow"? Also if we ...
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### Parallel combination of cells [on hold]

In this circuit suppose that $E_1 >E_2$. This means that the positive terminal of $E_1$ will have a greater potential than the positive terminal of $E_2$. Then current should flow from the ...
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### Use my example to explain why loop diagram will not occur in classical equation of motion?

We always say that tree levels are classical but loop diagrams are quantum. Let's talk about a concrete example： $$\mathcal{L}=\partial_a \phi\partial^a \phi-\frac{g}{4}\phi^4+\phi J$$ where $J$ is ...
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### Is the deformation of spacetime, elastic deformation or plastic deformation?

Is the deformation of spacetime, elastic deformation or plastic deformation?
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### Swirl velocity from circulation - Vortex

I have a vector field in cartesian coordinate $(x,y)$ with coordinate origin at vortex core. For calculating the swirl velocity $v_\theta$ I followed below steps, Step 1: Find the circle coordinates ...
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### Calculating effective dielectic constant

I have a system of metallic nano-particles supported on glass where I have observed a difference in the in-plane dielectric constants of the metal ($\varepsilon_x$ is different from $\varepsilon_y$ ...
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### Why do we get a diffraction pattern with a slit whose width is not comparable to the wavelength of the incident light?

In single slit experiments with sodium light whose wavelength is of order micrometer and slit width of order of hundreds of micron, we still observe the diffraction pattern. How do we explain this?
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### Acceleration of a pulley system

The figure above shows a pulley system consisting of 3 masses ($m_1$, $m_2$ and $m_3$), a homogeneous wheel (radius R, mass M) and 2 massless pulleyes which are connected by a massless rope. Mass $m_1$...
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### Infer the shape of a solid given the moments of inertia

Let's say we have a solid with uniform mass distribution, but we don't know its shape. However, we know the moment of inertia of the solid with respect to as many axes of your choosing as you want (e....
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### Fin area and heat transfer direction

So, I was studying some fin design in a heat transfer course , and then came the part where the efficiency is to be calculated, then I noticed that when he calculated the surface area , the sides of a ...
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### Electrostatics of hollow conductors

My textbook says: Inside a conductor, electric field is zero. The interior of a conductor can have no excess charge in static situation. Electric field just outside a charged conductor is ...
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### How does the LHC explore extra dimensions?

The Large Hadron Collider (LHC) has been smashing particles for a long time and sometimes people say that it has found new dimensions. How is it even possible for a particle accelerator to find new ...
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### Bidirectional jerk motion on a stopping vehicle

A stopping vehicle (say a car) has an apparent retardation (which may/may not be constant in magnitude) when force via brakes is applied. I travel by subway trains, and I noticed an odd phenomenon. ...
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### Does a “capacitor” for light exist? [duplicate]

If I have a light that is flickering at a frequency low enough to be perceived by the human eye, is there any type of material that exists that will smooth out the appearance of flickering? Similar to ...
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### Motion in one dimension and relative velocity [on hold]

Two bodies $A$ and $B$ are separated by $4~\mathrm{km}$ and $A$ is due west of $B$. If $A$ moves at a speed of $6~\mathrm{km/hr}$ towards the south, what is the relative velocity of $A$ w.r.t $B$?
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### Find the energy of an x-ray photon which can impact a maximum energy of $50~\mathrm{keV}$ to an electron [on hold]

This is an exercise question from the text book Concepts of Modern Physics by Arthur Beiser. Find the energy of an x-ray photon which can impact a maximum energy of $50~\mathrm{keV}$ to an ...
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### Why 3D nonlinear waves (e.g Faraday waves) create vortices on the fluid surface that interact and lead to the development of 2D turbulence

When wave became three-dimensional as a result of modulational instability, a floater can be forced to move towards the wave source.
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### Thermodynamic system, phonon and electron temperature and dissipation

Consider the following system (rudimental bolometer): A material (imagine a metal) A is initially at $25°\mathrm{C}$ and connected through Van der Waals forces to a material B (metal as well), which ...