# All Questions

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### Very confused about effective spring constant

I know that for springs in parallel, the effective spring constant is $k_1+k_2$ and for springs in series the constant is $1/(1/k_1+1/k_2)$. But there are some weird problems where finding the ...
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### What is the work being done a girl carrying a bag walking down the street.?

What is the work being done a girl carrying a bag walking down the steet
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### Flow Rate Calculation from a drain tank through a pipe

I want to know how to calculate the flowrate (Q) of a drain tank flowing through a pipe (see the figure below) All I know is that Q=Velocity(V) x Area (A) V= sqrt(2*g*H)=sqrt(2*9.81*10)=14m/s. I ...
5k views

### Why don't two musical instruments sometimes generate destructive interference?

I'm an electrical engineer, and I understand wave propagation, interference patterns, and so on. But I'm missing something basic, so perhaps my understanding isn't as good as I believe. I'll show my ...
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### How to check if some term in the Lagrangian involving gauge bosons is gauge invariant without explicit computations?

Normally (for fermions and scalars) we can simply use the decomposition of tensor products of gauge group representations to find invariant terms that we can write into the Lagrangian. For example ...
2k views

### Entropy and the principle of least action

Is there any link between the law of maximum entropy and the principle of least action. Is it possible to derive one from the other ?
2k views

### Why do atomic clocks only use caesium?

Modern atomic clocks only use caesium atoms as oscillators. Why don't we use other atoms for this role?
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### Global cosmic strings evolution

Recently I've read about axion string. It can be shown that the energy per unit length of the string located along $z$ axis is $$\mu = 2 \pi f_{a}^{2}\ln\left( \frac{L}{\delta}\right),$$ where $L$ ...
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I asked the question in the MathOverflow, but didn't get any response. I thought maybe better luck here. I'm reading the following paper about Petrov type D space times called "Type D vacuum ...
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### Rigid body problem in 2d

I have some questions about this exercise: In an horizontal plane, a $OA$ bar with mass $m$ and length $a$ moves, with another bar $AB$ (same mass, double length) attached in the point A. In the ...
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### Doubts taking the second functional derivative of the Klein Gordon action

I have very little background with functional derivatives and I would like to clarify some issues. I am trying to compute the second functional derivative of the Klein Gordon action expressed in real ...
17 views

### Do gauge bosons really transform according to the adjoint representation of the gauge group?

Its commonly said that gauge bosons transform according to the adjoint representation of the corresponding gauge group. For example, for $SU(2)$ the gauge bosons live in the adjoint $3$ dimensional ...
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### Why are bare mass terms for W-bosons forbidden, but coupling terms to Higgs doublets allowed?

The $W$ bosons live in the adjoint rep of $SU(2)$, which is three dimensional. The standard model Higgs lives in a $SU(2)$ doublet, i.e. the two dimensional rep. The $W$ bosons get their mass ...
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### Is quantum entanglement functionally equivalent to a measurement?

I saw the following talk the other day: http://www.youtube.com/watch?v=dEaecUuEqfc&feature=share In it, Dr. Ron Garret posits that entanglement isn't really that "special" of a property. He ...
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### How to improve the power output of an AC motor without changing its characteristics

I have an AC motor at 230V/50Hz and 2.2 KW power that will power a drilling bit for cutting wood. The motor has 3000 RPM, but if I attach the drilling bit (8 cm wide, 15 cm long and with a step of 8 ...
26 views

### I tried to solve circuit analysis problem but it failed [on hold]

I tried to solve circuit analysis problem in Hayt's Circuit Analysis 7th. But my answer is wrong and I don't know why I failed to solve this. Here's the question. I have to find what the value of v3 ...
14 views

### Spinning rubber ball with equatorial ridge

I have this rubber ball with something like a very slight equatorial ridge (sort of like Saturn's moon Iapetus) which I often spin around on my desk. I keep noticing that no matter the inclination of ...
15 views

### Symmetry axis and products of inertia

So if we have an object that has a symmetry axis let us say $z$-axis is a symmetry axis does this mean that the product of inertia $I_{zx} = I_{zy} = 0$? And if that is true why is it true ...
145 views

### Difference between directional and omnidirectional transmission or reception?

It is suggested that the next generation wireless technology will be highly directional since this increases the signal gain (MIMO) and decreases interference. How will the receiver still be able to ...
29 views

### Does a free electron, one that's not either in an atom or a wire, have an associated wave-function?

Would a free electron, one that's not either in an atom or moving through a wire, but moving through empty space on its own, have an associated wave-function? Or, is an electron described as a ...
13 views

### Mass spectrum of field theory

How can I find the mass spectrum of a field theory given a Lagrangian made of a canonical kinetic term and a potential. I mean, I think I have to find the matrix of the quadratical terms in all the ...
19 views

### Free fall motion of a body [on hold]

A ball is projected vertically upwards with an initial velocity of 100m/s. Find the speed of the ball at its maximum height Take g=10metre per second square.
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### Average velocity of a clock [on hold]

The minute hand opf a clock is 4 cm long Find the average velocity of the tip of the minute hand between 11:00 a.m to 11:30a.m and 11:00 a.m to 11:30 p.m
16 views

### Shear flow in J section type beam

What does the distribution of shear flow look like in a J section type beam. I'm only interested in a qualitative picture of it. I'm not interested in the calculations themselves. It is the top of the ...
16 views

### Fun physics book for high school student [duplicate]

can anyone recommend me a physics book for a highschool student (not these typical school books) a book that will let you think mostly interested in theoretical /quantum physics done with the ...
27 views

### Is magnetic reconnection reconciliable with magnetic field lines neither starting nor ending?

According to Maxwell's equations, magnetic fields are divergence-free: $\nabla \cdot \mathbf{B} = 0$. If I understand this correctly, this means that magnetic field lines do not start or end. How can ...
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### Need help please [on hold]

An electron's speed is doubled from $0.2~c$ to $0.4~c$. By what ratio does its momentum increase? What happens to the momentum ratio when the electron's speed is doubled again from $0.4~c$ to $0.8~c$? ...
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### Bertrand's Theorem: Perturbative Methods Leading to $1/r^3$ Solution

My professor and I have been working on a proof of Bertrand's Theorem using perturbative methods. We have arrived at a solution yielding 1/r^3, which we had presumed to be an incorrect result. While ...
163 views

### Malleability in metals

I am trying to understand malleability in metal as a result of the micro structure. I am focusing on the metals of antiquity (in order of decreased malleability): Gold, Silver, Copper, and Iron. While ...
17 views

### its tricky circuit to given output voltage find without source [on hold]

solve it two quation in that pic,1) find Vout voltage and 2) no source given and find v1,v2,v3.
25 views

### Why Electron Does Not Radiate In Bohr Orbits? [duplicate]

Maxwell said that charged particles radiate when are in accelarating motion. I understand that $nλ=2πr$ must be fulfilled in order to create a sinusoidal standing wave and to satisfy the probability ...
15 views

### Hyper/parabolic kepler orbits and “mean anomaly”

In an elliptical kepler orbit there is an easy recipe to describe the motion/position of a satellite at time $t$. One just follows the following steps - an important detail for me is that the ...
92 views

### Entropy and Gibbs Free Energy

I've been struggling with the notion of entropy and gibbs free energy for almost three days now. Different sources on and off the internet say different things about entropy. Gibbs Free Energy is ...
26 views

### Arbitrary acceleration written as centripetal acceleration of approximating circle

Suppose I want to find the acceleration of a projectile given its trajectory, i.e. its space coordinates as functions of time. I have heard that the acceleration can always be written as the ...
36 views

### Quantum computing can be done via measurement alone, why is this significant?

I read in the Afterword section of Nielsen and Chuang's book Quantum Computation and Quantum Information that A second area of progress has been in understanding of what physical resources are ...
8 views

### Superconductor in a parallel vs perpendicular field

My question concern's the huge difference in critical fields regarding a thin superconductor(SC) which is surrounded by a magnetic field. lets imagine the SC is a thin film in the x-y Plane: Applying ...
164 views

### Hartree-Fock correction to $e$-$e$ interaction

The corrections to the energy per electron in a jellium model (uniform distribution of positive ion charge approximation to the regulated long range order ionic array) is given by (in units of Ry) ...
31 views

### Deriving Gravitational Potential Energy using Vectors

Here is my attempt at derivation: First you must find a vector function for the gravitational force. By the inverse square law, the magnitude of gravitational force between two bodies of mass $m$ ...
38 views

### Is it possible to go from the Master Equation formalism to Heisenberg-Langevin equations

If I have derived a master equation (e.g. in the Lindblad form) and solved for the density matrix, $\rho(t)$ I can get the mean value of an operator, A as: $<A> = \mathrm{Tr}A\rho$. But ...
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### Reaction of uranium and neutrons emits neutrons, or is it photons?

So we had this test in nuclears physics today, and part of one of the quesitons was the following: When $\rm U^{235}_{92}$ nuclides are bombarded with one neutron, they can decompose to krypton ...
10 views

### Modelling Fourier Transform Profilometry

Basically I want to simulate a surface profilometry technique through Matlab. For that I want to create a GUI with controls for generating a grating pattern of light at a particular angle with respect ...
33 views

### Heat equation with heat loss to surroundings

I am trying to model one dimensional heat diffusion within a uniform copper rod with insulated ends. In addition, I am including a term that models the heat loss to the surroundings from the lateral ...
### Motion of particle in $T$ seconds [on hold]
A particle starts moving from rest position ($v = 0$) under a constant acceleration $a$. If it travels a distance $\Delta{x}$ in time $T$, what distance will it travel in the next time $T$?