# All Questions

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### Net force equal to mass multiplied by acceleration of centre of mass

f=ma if a=0 then definately net force applied will be also zero(0). then, in this case m=f/a. if both net force and acceleration(of centre of mass) is equal to zero(0). then, isn't mass must be equal ...
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### EM: Maxwell relations, waves

I have found the following formula in my lecture notes but I don't know from where it is derived: $i\vec{k}$ x $\vec{E} = iw\vec{B}$
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### Does a normal torch emit entangled photons?

I was reading a sciencenews.org post about three photons being entangled. My question here is, why is the chance of producing an entangled pair once in a billion times? Isn't every particle produced ...
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### Superficial degree of divergence for scalar theories

I have a few questions regarding the derivation of the degree of divergence for feynman diagrams. The result is $$D = [g_E] - \sum_{n=3}^{\infty} V_n [g_n]$$ (following notation in Srednicki, $P118$) ...
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### How can I prove that the euler-bernoulli beam pde is hamiltonian?

I'm having trouble with the above. I have the hamiltonian: how can I prove this is hamiltonian in structure?
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### Rotation of a vector

Is a vector necessarily changed when it is rotated through an angle? I think a vector always get changed because it's projection will change an also it's inclination with axes will always change. ...
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### EM: Conducting media

I have a question concerning the following exercise: The physical electric field associated with a plane wave in a material has the form: $\vec{E_{phys}}= E_0e^{-a_1z}\cos(a_2z-a_3t)\vec{y}$ ...
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### How does change in medium affect object distance/image distance?

Say,we have a container filled with a liquid of refractive index $7/5$ upto a height up $H$. There exists a plane glass mirror at the bottom of the container. Now if a fish were placed at a height of ...
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### Bell's theorem and fluid-mechanical experiments using droplets: are local hidden-variable theories possible after all?

Recent fluid-mechanical experiments by the groups of Couder in Paris and Bush at MIT, mimic a surprisingly wide range of quantum effects. The essential ingredient of these fluid-mechanical systems is ...
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### Does a photon follow a straight trajectory in a thin transparent film?

Does a photon follow a straight trajectory in a thin transparent film if the source has emitted a photon inside the film with impulse parallel to the film surface. (lambda greater than thickness)? ...
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### How do I model the motion of a particle changing acceleration vector (2D)?

I want to model a particle with an arbitrary initial velocity, and estimate the time it takes to reach a final point given a constant magnitude of acceleration. It should take the quickest path to the ...
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### Formation of nucleus

Is it even remotely possible for a nucleus to be formed in the following way? Consider two nucleons approaching each other at extreme speeds. Could the relativistic increase in mass be so high to ...
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### Is gravity the current limitation to laser cooling?

This is something I've been wondering for a while. I've read about the experiment with NASA to reach unprecedented cold temperatures in space because of the microgravity environment not being such a ...
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### How fast should 6 liters flow out of the sink through a 6cm pipe?

I had 6 liters in the sink when I pulled the plug. It took the water about 30 seconds to get through a pipe which has about 5cm in diameter. I think the pipe might be blocked a bit, causing it to have ...
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### What is quantum foam?

Can someone please explain me what quantum foam is? Is it the space-time fabric or just any other field? Also please explain this image
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### Speed of light obtained by equating electric field and magnetic field

$$\vec{B}=\mu_0\epsilon_0v\vec{E}$$ If electric field and magnetic field are equal then $$v=\frac{1}{\mu_0\epsilon_0}$$ Here as I remember $$v=1/\sqrt{\mu_0\epsilon_0}$$ Which is speed of light ...
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### Exact definition of momentarily comoving reference frame (MCRF)

Consider a particle $P$ in the framework of special relativity with position $r(t)=(ct,x(t),y(t),z(t))$ respect to an inertial reference frame $\Sigma=(ct,x,y,z;O)$. I need to know if the following ...
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### Decompression sickness

In decompression sickness why does nitrogen gas come out as bubbles? Is it because under high pressure the nitrogen gas was converted into liquid nitrogen and during the process of coming to the ...
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### Can anyone explain that “the human body is made of energy (photons)”? and how many photons it contains?

The modern research has shown that our body is basically made of energy not matter in a sense that atom itself is basically consists of energy waves, so it is nothing but energy. How we can explain ...
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### Deep Inelastic Scattering - electromagnetic current

When one tries to compute the deep inelastic scattering for the process: where $l$ is a lepton with incoming momentum $k$ and outgoing $k'$, $h$ is an hadron with momentum $P$, $q$ denotes some ...
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### Entanglement in Quantum field theory [duplicate]

How is entanglement represented in a field theory? For instance how can I represent a maximally entangled state such as a Bell state? Would such an approach also apply in a Conformal field theory ...
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### Have I found all of the zero order laue zone points for an FCC crystal being imaged along the [111] zone axis?

Since the equation for laue zones is hu+kv+lw=0 and the electron beam is incident in [uvw]=[111] can I say that the first order laue zone points are [hkl]=[-110],[1-10],[-101],[10-1],[0-11], and ...
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### Where can I learn about band theory?

I'm studying quantum mechanics and I want to understand perfectly where the bands of the electronic sturcture come from. I've read that it is related with the periodic potential, Bloch waves and ...
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### How is 6W equal to 40W in LED bulbs adverts?

Every advert I come across for LED bulbs advertise them as the equivalent of a higher W regular bulbs. This makes no sense to me, if the room requires 40W to lighten it up then it'll always require ...
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### What kind of other light sources can disturb the detection of LASER

My project objective is to detect different lasers , for example Laser Range Finder: pulse width 10ns and repetition rate is ...
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### How 3D laser scanners find coordinates

I'm interested in 3d laser scanners. And I have a few questions about the way they work: 1.When the distance from the laser to a point is calculated how do they know what is the time the beam took to ...
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### Particle in a box, quantization of energy

I'm learning about how the energy of matter is quantized like how the energy of light is. My textbook illustrates the concept of quantization with the particle in a box: "A particle of mass $m$ ...
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### How is a bell shaped the way it is?

How is a bell shaped like a parabola? Is the shape significant to how the bell functions? Can a bell be only in a shape of a parabola to function properly? How does the size and shape of a bell affect ...
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### Time variable double slit single photon interference

Has the double slit, single photon interference experiment ever been performed where time is a variable, ie. one photon emitted per day? Would the interference pattern emerge?
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### Is diffuse reflection elastic or inelastic scattering of light and how about with spectral?

If images are lost in diffuse reflection does this mean the light will only scatter inelastically or can it be elastic as well?
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### Torque, need help understanding a solution

https://www.scribd.com/doc/100022472/Monbukagakusho-Physics-2010-Solution Problem B. This guy's answer just doesn't make any sense at all to me but he did arrive somehow at the correct solution. If ...
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### Work done on an object by the internal forces

How is the work done by the internal forces acting in a rigid body zero? Actually I read in a book an example for the same. Let me present that example here. Consider a rigid body having two ...
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### Two questions about the nature of induced emf

I have two questions about magnetic induction (I am in university level introductory E and M so maybe my questions will be answered over the next few years): In the linear generator (shown below), I ...
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### Field transformations

I'm reading Maggiore's book "A modern introduction to quantum field theory" and I'm very confused by what he did in chapter 2.6 page 31 eq. (2.80). He basically wants to find the generators of the ...
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### A question about the Fermi acceleration

In high school physics, we know that a charged particle cannot gain energy from a magnetic field. But, it seems that in the so-called Fermi acceleration, the particle is accelerated by the magnetic ...
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### What does it mean by “infinities” when dealing with QFT?

I found this PDF online here while browsing Nobel Prize winner contributions, which explains a bit about renormalization (a concept for which Kenneth G. Wilson won the Nobel). However I was somewhat ...
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### Physical explanation of Pascal’s Law

I have a problem with physical explanation of Pascal’s Law. For example, when I was teaching my sister (a high school student) about force transmitting by a rope, I said her: “In a very simplified ...
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### How does Einstein's curved space time produce acceleration in a free falling object?

I never really thought much about all of this before so I'm definitely a newbie. Please excuse my ignorance. If I understand what I have read so far: if spacetime is curved time would click by at a ...
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### Do electrons in a CRT emit radiation?

According to electrodynamics, accelerating charged particles would emit electromagnetic energy as radiation. Isn't it applicable to the accelerated electrons within a cathode ray tube? Do they give ...
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### Ampere's law and Stokes' Theorem for current density

In both Griffiths and Jackson, Ampere's Law (or conversely the curl of the magnetic field) is derived by applying Stokes' Theorem to the surface integral of the current density J. The argument relies ...
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### How do I find the relation between the accelerations of the ring and the disc (see image)?

What is the relation between the accelerations of the ring and the disc (see image)? Both the ring and the disc have mass $M$. The ring has a radius $R$ and disc has radius $2R$. They are connected ...
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### How can there be an infinite number of universes? [on hold]

Many proponents of the multiverse (Greene, Krauss, perhaps Hawking) have posited that there may be an infinite number of alternative universes. How could this be possible, or even understood? ...
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### Rolling dynamics of round objects down an incline

If a cylinder is rolling down an inclined plane with inclination $\theta=30^\circ$ (with coefficient of static friction between cylinder and the plane $\mu_s=0.8$) without slipping and it has mass ...
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### An example of a theory that respects the Weak Equivalence Principle but violates the Einstein Equivalence Principle

The Weak Equivalence Principle has any one of the following forms: the inertial mass is equal to the gravitational mass there exists a preferred class of trajectories through spacetime, known as ...
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### Is magnetism just electricity in a different FoR?

I am trying to understand magnetism and its relation to electricity: Suppose there are two electrons travelling side by side in deep space at 1 cm distance at .99 c In this article it is said that ...
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### Question about the apparent loophole in principle of least action

In Lagrangian formalism, given two points $(x_1,t_1)$ and $(x_2,t_2)$, we ask the question which paths $x(t)$ make the action $S=\displaystyle \int_{t_1}^{t_2}L\ \mathrm dt$ stationary and satisfy the ...
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### Spontaneous symmetry breaking of a spinor / vector field

Why does SSB deal only with scalar fields and not with fermion or vector fields? My professor told me that it's closely related to the Lorentz invariance of the theory, but I don't understand at all ...
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### EM: relative permittivity

After having defined the relative permittivity in lectures, I have problem understanding why it is a complex quantity. Thank you for your help,
### Why is $np$ always equal to $n_i^2$?
For you guys who studied semiconductor physics must be familiar with the equation: $$np=n_i^2$$ I can understand why this is true for the intrinsic case (the broken bonds would always provide ...
A Dad is pushing his daughter on a swing. The homogeneous push lasts for α = 10% of the period and is centered around the phase φ = 0. The Fourier series expansion for this is,  ...