# All Questions

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### Magnetic Fields and Surface Tension of Water

Here's a research paper exploring the effects of a magnetic field on the surface tension of water. http://iopscience.iop.org/article/10.1088/1742-6596/156/1/012028/meta The conclusions were that ...
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### Why is the coefficient of kinetic friction greater than the coefficient of static friction? [duplicate]

I am a physics major taking freshman year physics courses. We did a lab experiment where we were given three objects and inclined plane, and we were asked to find the coefficients of kinetic and ...
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### 2D Kinematics and Projectile Motion [closed]

In this page, I have finished most of the problems. Some of them I am unable to figure out. For 3 a-j, is it possible to solve this problem given parts a and b, that is, can I solve these problems ...
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### How strong a magnetic field would be needed to deflect cosmic rays?

How strong a magnetic field would be needed to deflect cosmic rays? For example, lets imagine we wanted to protect the occupants of the International Space Station from cosmic rays. I assume we would ...
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### $Q=mc\Delta t$ vs $Q=c\Delta t$

The thermal capacity of a $60 \;\mathrm{kg}$ human is $210 \;\mathrm{kJ/°C}$. How much heat is lost from a body if its temperature drops by $2\;\mathrm{ °C}$? My original working out was: ...
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### Where this relation for general non rigid motion comes from?

In Goldstein's Classical Mechanics book in the chapter about the dynamics of rigid bodies the equation $$\dfrac{dL_i}{dt}+\epsilon_{ijk}\omega_jL_k = N_i$$ is presented. Now, in one exercise, we are ...
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### Why wasn't the theory that light's velocity depended on its source very popular?

Before special relativity, it was believed that light having a specific velocity meant it had to travel through something. My question is, why wasn't the idea that light depended on its source very ...
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### Moments question

The image is a body free-falling with two forces acting on it. If we take moments about A, we clearly get anticlockwise moments, and that is what will happen if this were to take place in real life. ...
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### What does Addition of Angular Momenta tell us about Group Theory?

I've come across this a lot, but I've never understood it. I do know basic Group Theory including Lie Groups. In Introduction to Quantum Mechanics, Griffiths ends the chapter on spin with the remark ...
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### A Lie derivative $\mathcal{L}_{\alpha^A}$ with respect to a spinor $\alpha^A$?

Suppose we work with Minkowski flat space $M$ (just to make things easy). If $\textbf X$ is a Killing vector field it is possible to define the Lie derivative of an spinor $\alpha^A$ with respect to ...
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### Graphing $E_p = E_k + E_{rot}$

I am measuring the moment of inertia of a flywheel and using conservation of energy to work out the value (I am ignoring friction). So for the experiment I have to roll the flywheel down a slope (I ...
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### Does going downhill create cold air in the exhaust of a car?

Consider a car going down a large mountain. Instead of braking continuously, the driver shifts down and the car is able to maintain velocity on just engine braking. No fuel is injected into the ...
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### Valence bands question

I'm currently doing solid state physics and learning about semiconductors. During the course, I have seen a lot of energy/wavevector graphs, like this one (pic from Kittel): I did not have a ...
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### Can we predict throwing a dice?

What happens if we throw a dice from same position, with same force, by creating a vacuum environment on earth? Will it be predictable now i.e. will the dice have same results all the time? If answer ...
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### Evaluation of width function; thermal neutron scattering cross section

I am attempting to perform some molecular dynamics simulation, based off of this paper, and do not have the slightest clue how they evaluate the width function (Eq. 4). My attempt at figuring it ...
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### Single Slit Diffraction

I am trying to derive the intensity variation function for a single slit diffraction. Sorry for the poor diagram... So I decided to take the amplitudes of the waves originating from the slit on the ...
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### What is the curve that describes the Daytime line in a Day and Night World Map?

A Day and Night World Map shows which parts of the Earth are in daylight and which are in night at a given instant. At one side of the Daytime line they are in daylight and at the other side they are ...
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### Motion without matter

Suppose you have a vacuum spacetime with non zero cosmological constant then you can show that two test particles will move toward each other or apart depending on whether it is negative or positive ...
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### How the malus law will be changed for the averaged intensity

If a polarized beam of light passes through an polarizer then, the intensity of the output polarized light obey Malus law. $$I =I_0 \cos^2 \phi$$ Now if we rotate the the polarizer with an ...
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### Easy method to measure the NA of a lens system?

I have a lens system made up of some components of the Leica Z16 APO zoom system. This includes a built-in iris diaphragm to adjust the Numerical Aperture (NA) and therefore the depth-of-field (DOF). ...
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### Multipole expansion in cylindrical coordinates

I am seeking the general solution for the Laplace equation in cylindrical coordinates or $$\nabla^2 \omega = 0.$$ In several texts, the general solution can be found via separation of variables ...
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### Why are basic physics problems neglecting to include atmospheric pressure?

The value of atmospheric pressure is 101325 Pascal, which is a large value which implies the force exerted by atmosphere is 101325 N m$^{-2}$. Surely this can not be neglected, so why is atmospheric ...
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### How did planets start to spin and do all of them spin in same direction? [duplicate]

How did planets start to spin in the first place? Some say that gases which condense and form planets start spinning and hence planets acquire a spin. And some say that they start to spin to conserve ...
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### Error propagation with dependent variables

Based on Microdosimetry theory, trying to figure out error propagation for a lot of quantities that are produced from radiation spectra where each channel with $f(y)$ counts has error $\sqrt{f(y)}$. ...
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### Electric field vector in visualization of polarization of EM wave

When we were taught polarization at the high school level, we were told that during polarization, we should consider the EM wave being axially or planarly filtered (e.g. by a polarizing sheet with ...
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### Why do they think WT1190F is low density?

A piece of space debris WT1190F is in the news in part because it has been far from earth (farther than lunar distance) in the past but will re-enter earth's atmosphere in a few weeks. See this note ...
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### Relation of orbital radius with linear velocity of satellite [closed]

If a planet is going around the sun in a circular orbit , and if suddenly (impulsively) it's orbital radius was increased , what would happen to the linear velocity of planet (With reason)
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### Exothermic vs. endothermic reactions

An exothermic reaction is a chemical reaction that releases energy by light or heat. Does this mean that if I mix two chemicals in a test tube when the reaction between them is endothermic then the ...
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### Why is it desired to have an aperture of a spherical mirror to be less than the radius of curvature?

In many instances, I have noticed that it is given that the aperture is less than the radius of curvature for a spherical mirror. Why is this given like that?
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### Which waveguide mode will be excited by circularly polarized laser mode $\mathrm{TEM}_{00}$ entering hollow fiber?

Linearly polarized Gaussian mode $\mathrm{TEM}_{00}$ mode, would couple into HE modes of circular hollow fiber. Which modes would circular polarized beam excite entering hollow circular fiber ?
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### Speed of electricity in cable [duplicate]

Movement speed of electricity in cable, is same as speed of light in space? In other words, electricity moves in usual electro cable aproximatelly 300 000 km/second ?
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### What is the initial velocity of an object, given a change in mass, and an exerted force [closed]

An 8.0-g bullet is shot into a 4.0-kg block, at rest on a frictionless horizontal surface (see the figure). The bullet remains lodged in the block. The block moves into an ideal massless spring and ...
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### Modeling transient heat transfer between two 1-D materials

So I have been working on creating a 1-D model of a material with a layer of heat resistant material stuck to it. For this I have been trying to use the implicit finite-differential method. The ...
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### Alternative expression for Bethe vectors of su(2) XXX chain

As is well known, the eigenvectors of the transfer matrix for the su(2) XXX spin chain of length $L$ are given by acting on the ground state $|\uparrow^L\rangle$ with all spins point up with the ...