# All Questions

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### How is the EM force exchanged over long distances?

The Situation Imagine we place two charged objects a very far distance apart, essentially making them point charges. How does the EM force interact between the two point charges if virtual photons ...
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### Can I use high frequency sounds to trigger an infrared motion sensors

Currently, I am doing a research whereby I trying to trigger the infrared motion sensors using sounds. Well first of all, I will blocked the whole motion detector using a black box and then use a high ...
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### Rotation matrix of Euler's equations of rotation relative to inertial reference frame

I was playing with simulation of Euler's equations of rotation in MATLAB, $$I_1\dot{\omega}_1 + (I_3 - I_2)\omega_2\omega_3 = M_1,$$ $$I_2\dot{\omega}_2 + (I_1 - I_3)\omega_3\omega_1 = M_2,$$ ...
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### Recreating an image from a photometer or similar light-detecting device?

I'm thinking if it is possible to recreate an image from data from this kind of device. It is known analog signals theoretically have infinite resolution, but since we use them in discrete systems ...
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### definition of Hamiltonian in statistics

Is there any advantage to use $\hat H-\mu \hat N$ instead of $\hat H$ as the hamiltonian in statistical mechanics?
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### In 1d criticality, what is the relation between the universality class and central charge?

I want to know how to obtain the universality class of the phase transition from the central charge "c" in one dimensional model. If c is less than 1, there is a one-to-one correspondence. But if c is ...
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### Will two opposing magnetic fields cancel out?

When a conductor induces eddy currents that creates a magnetic field opposing the change that created it, would the two fields at some point cancel out? Imagine the change to be so great, it ...
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### Angle sum of triangle in Schwarzchild solution

Curvature of space is often intuitively explained as angles of a triangle not adding up to 180 degrees. My questions concerns that. Suppose you have a perfectly spherical star of uniform density - so ...
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### Zero current induced in a loop

Consider the following two cases, Case 1 – A coil moving through a constant magnetic field Case 2 – A conductor moving through a constant magnetic field Now, in Case 1 (a coil moving through a ...
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### Converting spectra to rgb colour

I am having trouble obtaining RGB colour values from the spectra of an object. I have the specific flux $F_\nu$ as a function of $\nu$ for each pixel of the image I'd like to make, but am unsure of ...
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### Magnet rotor diameter effect on power output in an axial flux generator

I'm trying to determine which parameters of the magnet rotor have the greatest effect on power output of an axial flux generator. Obviously a very large diameter will have a greater tangential ...
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### Conservation of momentum in Heisenberg's microscope

In working through Heisenberg's microscope, conservation of momentum for the photon and electron tells us that \begin{align} \frac{h}{\lambda}=\frac{h}{\lambda'}\sin\theta+p_x\,, \end{align} where ...
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### Stretch as a function of time

A cylinder of neglectable mass, initial radius $R_{0}$ and initial length $L_{0}$ is attached to a ceiling by one end. A mass $M$ is attached to the other end of the cylinder. Following is also ...
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### Black holes – Irrotational vortex or Rigid-body-like vortex

As light enters a Black hole – does the light enter as “Irrotational vortex” or as “Rigid-body-like vortex”? https://en.wikipedia.org/wiki/Vorticity Thank You
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### Is a Plutonium gun-type atomic bomb really “impossible”?

I caught a pretty well done 2 hour documentary on atomic bomb history yesterday on the local PBS station. In it, they go over the paths taken for design of the first bombs, including the Thin Man ...
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### Is dark energy homogeneous and/or isotropic?

Shouldn't there be at least some fluctuations of dark energy in various regions, analogous to the fluctuations we observe in other forms of energy like matter?
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### Conical train wheels

I've been reading about how the conical shape of train wheels helps trains round turns without a differential. For those who are unfamiliar with the idea, the conical shape allows the wheels to shift ...
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### 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, ...
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A rocket passes the earth and synchronises its clock with the earth. Years later, a rocket passes it going to the earth and synchronises its clock with the first rocket. When it reached the earth, ...
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### seperate engines working vs single engine

What is more fuel efficient: Two identical cars running at 50mph or one being tied to the other and only the first dragging both at 50mph? Or what requires less fuel: Two cars to start from stop and ...
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### Flow in diverging channel is unstable according to Bernoulli's equation

So let's say we have a circular pipe with a variable cross section with an inviscid incompressible fluid flowing through it. The areas of the two ends are $A_1$ and and $A_2$. Let's say we also ...
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### Can something actually be air tight? Sub unit of energy?

I thought about space travel and wondered if space crafts are 100% air tight or not. How would we even know if something is air tight or not. There could be a leak that would allow small amounts of ...
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### General proof of formulas of geometric optics?

In most lf textbooks formulas of geometric optics like lens maker formula and base formula for that are proven (or rather verified from my point of view) by taking specific case (ray diagram) and ...
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### Does spacetime curvature increase when an attractor's potential energy is converted to kinetic energy?

Imagine an asymptotically flat spacetime with nothing but two stars at a certain distance. They fall into each other and form one big star, so their potential energy is converted to kinetic energy. ...
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### Quantum Entanglement - How To Interpret

I have thought about quantum entanglement for some time, and I still don't quite understand the reasoning behind the conclusion that entangled particles somehow can communicate their state to each ...
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### Why do motorcycles have bigger wheels than mopped? [on hold]

I am from India. Here bikes and mopeds are used almost everywhere for the same purpose and terrain.
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### is there a situation where $|\frac{d\vec{v}}{dt}|$ is non-zero while $\frac{d|\vec{v}|}{dt}$ is zero?

Here $\vec{v}$ represents velocity and $\frac{dv}{dt}$ gives the instantaneous acceleration. $|\vec v|$ gives the magnitude of velocity. When acceleration is zero, then change in magnitude of velocity ...
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### Determination of the ground state of a field theory

Consider the Spontaneous symmetry breaking in the theory $$\mathcal{L}=\frac{1}{2}\partial_\mu\phi\partial^\mu\phi-\frac{\mu^2}{2}\phi^2+\frac{\lambda}{4!}\phi^4.$$ By the ground state of a ...
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### Counting modes Rayleigh-Jeans

In the derivation of the Rayleigh-Jeans Law, we count the number of EM modes in a square cavity. After calculating the number of allowed modes due to boundary conditions, we multiply it by a factor of ...
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### Can the sign of metric change physics?

Consider the Lagrangian of a massless real scalar (classical field) in $\phi(\textbf{x},t)$: $$\mathcal{L}=\frac{1}{2}\partial^\mu\phi\partial_\mu\phi$$ The Hamiltonian density in two different ...
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### What causes a eutectic point?

What causes the melting point depression known as a eutectic point? The temperature depression of a eutectic point can be calculated from the enthaplies and entropies of fusion of the two ...
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### Why does the vortex form in the von Karman Vortex Street?

Why does the vortex form in the von Karman Vortex Street? So far I have this for the reasoning: 'It occurs when a fluid flows around a cylindrical object. The pressure on a fluid particle rises to ...
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### Virtual work (generalized forces) for rotation with Euler angles

Here is what we know from virtual work: $$\delta W=\sum_{i=1}^N{\vec F_i\cdot\delta\vec r_{i}}$$ Where $N$ is the number of bodies in the system. I am considering a ...
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### Crying in microgravity

So I looked around the net and found that you really cant "cry" as we do on earth in space floating around, because the tears would just stay near your eyes, because the pull of gravity is not strong ...
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### Does buoyancy change on smaller scales?

For baseball size objects on the order of centimeters across we have experience with what floats and what doesn't. A stick will float and a rock will not. But what if the rock were a grain of sand? ...
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### What maximum theoretical speed can reach spaceship with EM Drive in space? [on hold]

There are several studies (by Institute of Aerospace Engineering, Technische Universität Dresden and by NASA) that are concluding that the EM Drive produces thrust. So I am wondering: if the EM Drive ...
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### Area under v-t and a-t graphs

I know that the magnitude ( i.e. considering all area as positive) of area under v-t graph gives distance travelled by the particle in the given time. But I've been told that magnitude of area under ...
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### One Pion Exchange Potential properties for a two-nucleon system

I'm going through my Nuclear Physics book, and has come across a section called "Properties of OPEP for the two-nucleon system". It start out by considering the n-p system in a singlet spin state ...
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### Using Lagrangian to get the equations of motion [on hold]

The thin hollow cylinder rolls on the plane without slipping. I want to write the Lagrangian, but am confused about the degrees of freedom of the motion and the coordinate axes to consider for ...
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### Pressure change with a pipe diameter change

I have a question regarding a water line from the street to a house. What happens to the pressure in a water line if it goes from a 1" diameter pipe (at street) and increases to a 1.5" diameter pipe ...
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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 ...
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### Which centripetal force equation should I use for centripetal motion in uniform magnetic field, and why?

So far I learn two equations, doing the same thing: $$F_c = m\frac{v^2}{r} = qvB$$ It leaves me undecided when a question asks "by what factor is the force increased when velocity is double". The ...
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### Visible wavelengths of air fluorescence info - needed for school demo laser telephone design

I'm seeking information about fluorescence in visible wavelengths (390-700nm) of any of the main constituents of air ( $N_2, O_2, CO_2, H_2O, CO, etc.$) that can be excited with a low-cost? COTS ...
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### Calculating the drag force provided by a laminar flow [duplicate]

If I drop a ball into viscous fluid (e.g. oil), by calculation I know that the fluid flow is laminar. So which equation do I need to calculate the drag force provided by fluid? The drag equation, ...
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### Does entanglement have to be verified synchronously

I am not a physicist so, first of all I like to apologise if I my question is stupid. I just want to know where I am going wrong here. But if I have understood anything about the entanglement ...
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### Calculate constant acceleration for a length of track with start at rest and final velocity known [on hold]

My maths and physics are rusty... I'm a bit ashamed, but I have this problem: Let's have a 1000m linear accelerator (mass driver) propel a payload at constant acceleration, with a final velocity of ...
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### Setting up superlattices in VASP based on two POSCARS

Does anyone know how to setup a superlattice in VASP/QE using the atomic simulation environment based on two materials/POSCARS?
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### When we ride a bicycle, why do we tend to move left when we turn it right?

When we ride a bicycle, we tend to move our body left if we are taking a right turn and vice versa. Why is it like that?