Applies to questions of primarily educational value - not only questions that arise from actual homework assignments, but any question where it is preferable to guide the asker to the answer rather than giving it away outright. Please READ THE GUIDANCE IN META before asking homework-like questions.

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0
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1answer
27 views

Proofs on operator algebra

I'd like to ask the community to please verify the first two proofs below and help me get through the last one since I seem to be stuck. Thank you in advance. Proof 1: Given two noncommutting ...
-1
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1answer
31 views

Prove that any vector can be written as the sum of any three non-coplanar vectors

I'm trying to prove the above in the following form: $ \boldsymbol{V} = (\boldsymbol{V} \centerdot \boldsymbol{a}^1)\boldsymbol{a}_1 + (\boldsymbol{V} \centerdot \boldsymbol{a}^2)\boldsymbol{a}_2 + ...
0
votes
0answers
20 views

Finding Electronic Energy Levels of XeOF$_4$ by Representation Theory

XeOF$_4$ molecule with $C_{4v}$ symmetry. Fluor has both s and p orbitals on its valence shell. Let us suppose that the fluor atom has only a single s orbital in the valence shell. Let $\phi_i$ ...
0
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0answers
13 views

Radio galaxy lobes

A radio galaxy has giant radio lobes, each of the approximate diameter 20kpc. Protons of up to 9$\times$1019 eV remain trapped inside these lobes. What is the magnetic field that confides them? I ...
2
votes
0answers
20 views

Gravitational Collapse

Consider a spherically symmetric cloud of mass $M$ with radius $R$ and uniform density $\rho$. The gravitational collapse is governed by Newton's second law \begin{equation} \ddot{r} = - ...
-1
votes
3answers
30 views

Effect on capacitance when capacitor number increased [on hold]

What happens to total charge and potential difference when number of capacitors increased in series and parallel combination? My question in text book is: Initially, a single capacitance C1 is wired ...
1
vote
1answer
35 views

Standardising shadow length on sundials

The sundial is fundamentally flawed in that the length of each hourly shadow changes with the seasons. If the base of the sundial was engineered to move cyclically on an anual basis however, the ...
0
votes
0answers
34 views

How sketch the renormalization group flows in the $(\lambda, e^2)$ plan for scalar QED?

I have obtained beta functions of renormalization group for scalar QED: $$ \beta (e) = \frac{e^3}{48 \pi^2} $$ $$ \beta(\lambda) = \frac{1}{24\pi^2}(5\lambda^2 - 18e^2\lambda + 54 e^4) $$ which give ...
-1
votes
1answer
42 views

Classical Mechanics, The Theoretical Minimum: error in answer to partial derivatives exercise?

I'm reading Leonard Susskind's Classical Mechanics, The Theoretical Minimum, and I'm on the interlude on partial derivatives. There is an exercise that asks you to find all of the first and second ...
0
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0answers
20 views

Apparent depth and real depth in terms of viewing angle [on hold]

What is the apparent depth of an object in water in terms of : depth of object( $h$) refractive index of water ($\mu$) angle with which it is viewed from above ($\theta$) What I know : 1. ...
0
votes
2answers
36 views

Why does the ball lose contact if it has velocity great or less than $\sqrt{rg}$, while moving over a smooth sphere of radius $r$?

Let a ball of mass $m$ is given a velocity over the the top of a smooth sphere of radius $r$. The equation of motion at the topmost point will be $$mg - N = m\dfrac{v^2}{r}.$$ As $v$ increases, $N$ ...
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0answers
16 views

Non-trivial integral with the Bose-Einstein distribution and Cosine function

When I consider the Casimir interaction between an atom and a perfect conducting slab I find the following non-trivial integral: $$\int\limits_0^\infty {\frac{{\cos \left( mx \right)}}{{x + ...
5
votes
3answers
219 views

Is there any tension in a massless spring that connects two free falling bodies in different horizontal planes?

Two bodies A and B of same mass $m$ are attached with a massless spring and are hanging from a ceiling with a massless rope. They are in same vertical plane but not in same horizontal plane. Now the ...
0
votes
1answer
23 views

How does one decide if the process if beta decay positive or beta decay negative?

I've been given a sample and a daughter product and I'll have to write down the other products of this decay process. This is a beta decay due to the mass number A being constant in the sample and the ...
-1
votes
1answer
18 views

Electrostatics Problem - A non-conducting cylinder [on hold]

A nonconducting cylinder of radius 1.20 meters and length 2.00 meters carries a charge of 50 micro-coulombs uniformly distributed throughout the cylinder.Find the electric field on the cylinder axis ...
0
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0answers
37 views

How to go from Lorentz gauge to Coulomb gauge?

I want to know that if I find some solution, say a polarization vector in Lorenz gauge. Now I want to go to Coulomb gauge. There must be a transformation that links both the gauges. How would I find ...
-1
votes
0answers
25 views

Using the spin lowering and spin raising operator on a two-particle wave function, how do I do it? [on hold]

You have two spin 1/2 particles in the ground state of the harmonic oscillator potential, they do not interact with each other, and the Sz projection was measured to be zero. I am supposed to use the ...
-5
votes
0answers
50 views

Please explain the integration [on hold]

the integration of (6 - 0.06s) should be (6s - 0.03s^2). Why is the solution (12s - 0.06s^2)? It seems its factored by 2 but I don't know the explanation to it. Cheers
1
vote
1answer
31 views

Special OPE in $\beta\gamma$ system

I would like to find the OPE $$\beta(z)\gamma(w)^{-1}$$ given $$\beta(z)\gamma(w)~\sim~\frac{1}{z-w}$$ from the $\beta\gamma$-system in CFT. Is it possible?
2
votes
1answer
32 views

Would a non-conducting body ever acquire a uniform static charge throughout it's volume?

I'm studying electromagnetism and optics in first year and solving a lot of problems involving conveniently symmetrical conducting and non-conducting bodies having various uniformly distributed ...
0
votes
1answer
34 views

The minimum possible energy for five noniteracting spin $\frac{1}{2}$ particles

Question What is the minimum possible energy for five (noniteracting) spin $\frac{1}{2}$ particles of mass $m$ in a one-dimensional box of length $L$? Answer: $$ 2 \times ...
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0answers
34 views

Coordinates of a closed string in 5 dimensional flat spacetime

In the paper http://arxiv.org/abs/hep-th/0304255 when the authors are describing the motion of a closed string in the 5 dimensional flat space they use the coordinates (2.1) $X_{0} = κ \tau$ , $X = ...
-1
votes
0answers
64 views

Sum of powers of complex exponentials [on hold]

EDIT: Amusingly enough, this same question was asked (and answered) 4 days ago by someone else. The link is here I encountered the identity $\sum_{j=1}^{N} {e^{ikja}} = N\delta_{k,0}$ when reading ...
1
vote
1answer
53 views

Elementary question about global supersymmetry of a worldsheet [on hold]

I'm reading chapter 4 of the book by Green, Schwarz and Witten. They consider an action $$ S = -\frac{1}{2\pi} \int d^2 \sigma \left( \partial_\alpha X^\mu \partial^\alpha X_\mu - i \bar \psi^\mu ...
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votes
0answers
34 views

Facing a problem in Katrin Becker, Melanie Becker, John Schwarz's String Theory

In BBS's string theory book, in the equation (2.143), it says that $$\text{tr} \omega ^N=\prod _{n=1}^{\infty } \left(\prod _{i=1}^{24} \text{tr} \omega ^{\alpha _{-n}^i \:\alpha _n^i}\right)=\prod ...
-1
votes
1answer
45 views

Time dependent solution to infinite well

A particle of mass $m$ is confined within an infinite, one-dimensional potential well, $U(x)$, of width $a$. $$ U\left(x\right) = \left\{ \begin{array}{lr} \infty &\: x \leq ...
-3
votes
1answer
51 views

How do I normalize this wavefunction? [on hold]

I need to find the normalisation constant $A$ for the wave function: $$ \psi\left(x\right) = \left\{ \begin{array}{lr} A &\: \frac{-a}{4} \leq x \leq \frac{a}{4}\\ 0 &\: ...
1
vote
3answers
41 views

Calculating the probability of a given energy

Given a normalised wavefunction say $$\psi(x) = A\sin(n\pi x),$$ (where $A$ is a normalisation constant) I can calculate the probability of finding the particle being between a position $x$ and $x + ...
-3
votes
0answers
11 views

What is the energy levels of 1d hydrogen by using WKB? [on hold]

I need to find the energy levels of 1d hydrogen using WKB and $V(x)=-1/x$? can you help me please?
1
vote
0answers
18 views

Polchinski equation 11.2.7

In Polchinski's string theory volume 2, when discussing the GSO projection for the heterotic string he says: In the IIA and IIB superstrings the GSO projection acted separately on the left- and ...
-1
votes
1answer
24 views

Central Force - Trajectory [on hold]

There is a central force $\vec{F}(\vec{r})=f(r)\vec{e_r}$. 1.1.: The trajectory/flight path is defined by $r=r(\phi)$, compute $f(r)$. 1.2.: The path is defined by $u=u(\phi)$ with ...
3
votes
2answers
79 views

Minimizing the Lagrangian action of an impossible problem

I'm working my way through Structure and Interpretation of Classical Mechanics (SICM), and am stuck on an exercise in Section 1.4: Exercise 1.6. Minimizing action: Suppose we try to obtain a ...
-1
votes
1answer
35 views

Time dilation: reference frame [on hold]

So I'm calculating how long it will take for half of some muons to decay for in a stationary observers frame of reference. They have a half life of 2.2 * 10^-6 and are moving at a speed of .98c ...
-1
votes
0answers
11 views

What voltage is induced between the endpoints of bar, if we swing it by an amplitude [on hold]

After a long horizontal conductor carrying current of 10 A is vertically suspended 2m long insulated conductive rod. We swing it by an amplitude of 10 cm. (a) in the direction of the wire; (b) in ...
-1
votes
0answers
6 views

With what frequency triangle swings if it shuns from equilibrium? [on hold]

Guide V-shaped is with no friction in the swivel bearings A and B (see Fig.) Of the magnet with a vertical magnetic field of 0.8 T. The weight of the guide is 50 g, the length AB 4 cm, and the length ...
0
votes
0answers
34 views

How to count completely symmetric wavefunctions can be made by 6 (u, d, s, $\bar{u},\bar{d},\bar{s}$) quarks to form Baryons (i.e., 3 particle state)?

(Excluding antisymmetric part i.e., colour part) Peskin said 6 (particles + Anti-Particles) "guys" make (6(i.e., total "guys") x7x8)/(3x2)= 56 total symmetric state. How? There are 10 (decuplate) ...
0
votes
1answer
62 views

Question about scalar product of 2 four-vectors [on hold]

I'm looking for help to prove that $\mathbf{a\cdot b}=-ab\ cosh \ \theta$, where: $a=\sqrt{\mathbf{-a\cdot a}}$, $b=\sqrt{\mathbf{-b\cdot b}}$, and $cosh \ \theta$ is $\gamma$, for a Lorentz ...
1
vote
2answers
48 views

Why can velocity and acceleration be negative?

Why is speed and acceleration negative when $V_1$ of an object is say 150m/s, $V_2$ is 0 m/s and $\Delta d=0.50\,\rm m$? I found the time it takes which is 0.0033s and the acceleration to be 90909.09 ...
0
votes
2answers
73 views

Conceptual question regarding work

Here is a problem I am having some trouble with: The solution is below, my question is, why is conservation of energy valid in this situation. From my understanding, just before the box hits the ...
0
votes
2answers
33 views

Collsion in 2D, find angle and final momentum

Suppose that the collision is completely inelastic. Disregard friction as the surface is smooth. Suppose that one is in the direction of $(1,0)$ and the other $(0,1)$. Is the final momentum the ...
0
votes
0answers
55 views

momentum equation to find velocity of solid particle after collision [on hold]

If a particle collides with a liquid drop, particle might penetrate into drop and so the velocity of particle reduces. i.e. a sudden change in particle velocity happens right at the collision moment, ...
1
vote
2answers
30 views

Non-linear relationship between power supplied and perceived brightness in a filament lamp?

I was expecting the relationship between these two to be linear, but after conducting an experiment my data fits almost exactly on the curve, $y = 2800.2 x^{2.0173}$, where $y$ is the brightness in ...
0
votes
1answer
33 views

Athletics on the moon [on hold]

The moon is a lot smaller than the earth ($\frac{1}{81}$ of the mass and $\frac{3}{11}$ of the diameter). a)How high would a professional vaulter jump on the moon? Assume that he would jump ...
0
votes
1answer
55 views

Halley's comet. (Questions from a visiting astronomer) [on hold]

Okay, so we had an apparently famous astronomer in class today. He is giving lectures at our university this week and my class is one of them. So he just comes in on his first day and ends his ...
-1
votes
0answers
28 views

Central Force - Trajectory [duplicate]

There is a central force $\vec{F}(\vec{r})=f(r)\vec{e_r}$. 1.1.: The trajectory/flight path is defined by $r=r(\phi)$, compute $f(r)$. 1.2.: The path is defined by $u=u(\phi)$ with ...
-1
votes
0answers
23 views

Potential electrostatic energy of a Capacitor [on hold]

They give me the area ($0.06m^2$), the separation between the plates ($0.001m$) and the capacitor is charged at $150V$. At this point is really easy to find Q $\frac{V}{d} \rightarrow ...
0
votes
1answer
42 views

Combination of two concave lenses

When we have two convex lenses in contact we have the combination behaving as a converging lens. Can someone please give an analytical description for the combination of two concave lenses in contact ...
1
vote
1answer
50 views

How might I show that an operator is, by definition, an 'observable'? [on hold]

Here is my problem: I understand what is meant by 'observable' but don't have a formal definition at hand. How do I 'show' it?
3
votes
1answer
284 views

Calculating Kryptonian speed in the movie “Man of Steel”

This is my first question, so I'm not sure if this is the right place to ask but: How would you go about calculating the speed of Kryptonians in the movie Man of Steel (2013)? Specifically, I'm ...
2
votes
1answer
28 views

Force needed to roll wheel over edge [on hold]

I'm looking to find F. See image below. What we know: Mass of the wheel: $ m = 14 $ kg Wheel radius: $ r = 0.32 $ m What I've tried so far: $$ mg + N + \sin(38.7)k = 0 $$ Once the wheel ...