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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-1
votes
1answer
14 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
votes
0answers
31 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
23 views

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

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
47 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
2
votes
1answer
31 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
32 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 ...
-1
votes
0answers
29 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
50 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
49 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 ...
0
votes
0answers
32 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
44 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
50 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
40 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
23 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 ...
2
votes
1answer
55 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
32 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
10 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
33 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
72 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
53 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
29 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
41 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
282 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
27 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 ...
0
votes
0answers
31 views

Relativistic Decay of Particles (Questions about Landau's derivations) [on hold]

I'm currently reading Classical Theory of Fields by Lifshitz & Landau. In §11 "Decay of Particles" I can't really understand the solution to Problem 1: I have several questions: 1) How can one ...
1
vote
0answers
17 views

Force acting on a wheel of a two-wheeled self balancing bot

Proceeding further with my work on a self-balancing bot as posted here: http://bit.ly/1FfI6LK I've gotten stuck at the following equation: n = Gear ratio $K_t$ = DC motor torque constant $i_l$ = DC ...
-3
votes
0answers
28 views

How to calculate the gravitational force from Earth to Moon? [on hold]

I know the $G$ constant and the distance. Also I tried with Mass of Earth.
0
votes
1answer
62 views

Problem understanding Lorentz invariance [duplicate]

So they usually started with "...This is obviously Lorentz invariant, because of the 4-vector character of the quantity,..., (and after a two page long derivation) another quantity is also obviously ...
0
votes
1answer
23 views

Greater momentum than initial?

The question is : Heavier object A, initially at rest, is struck by lighter object B. Is it possible for object A to have a larger final momentum than the initial momentum of object B? The ...
1
vote
0answers
24 views

Uniform field through a disc

Let us say that we have a magnetic field of the form $B_0 \sin(\omega t) \hat e_z$ in all space. Now let us say we put a conducting disc within this field. I want to find the electric field within ...
1
vote
0answers
14 views

what is the charge and current density in system K2? [on hold]

Consider there are two inertial reference of frames K1 and K2 in which K2 is moving with the boost velocity of V in direction of X2. If someone at the rest in system K1 finds the charge density $ \rho ...
2
votes
1answer
34 views

Finding velocity field of spherical bubble in incompressible fluid

I have the following question: A spherical bubble in an incompressible fluid of density $\rho$ has radius $a(t)$. Write down an expression for the velocity field at a radius $R > a$. The pressure ...
0
votes
0answers
45 views

A question about Undamped Vibration [on hold]

A small collar of mass 1-kg is rigidly attached to a 3-kg uniform rod length L = 1 m. Determine the period of small oscillations of the rod when (a) d = 1 m, (b) d = 0.7 m.
0
votes
0answers
26 views

Quark and anti-quark number density [on hold]

Im trying to solve this question but having little luck in doing so. Would appreciate if someone could help me out!
0
votes
0answers
18 views

What is the speed of the panel when the spacing between the plates increases [on hold]

Flat-panel capacitor WITH the surface area of $200 cm^2$ is connected to the voltage $1000 V$. One of the plates we start to move away from a constant force $5\times10^{-4}$ Newtons. the spacing ...
3
votes
1answer
49 views

Special relativity kinematics problem [on hold]

I have this problem in SR kinematics. a spaceship travels near earth at c/2. it shoots off a light-ray at 45deg to its direction of travel (measured in its own ref frame). What is this angle in the ...
0
votes
1answer
30 views

Finding the tension in a rope with mass defined per unit length [on hold]

A rope of length L has its mass per unit length λ varies according to the function λ(x)=e^x/L. The rope is pulled by a constant force of 1N on a smooth horizontal surface. Find tension in the rope at ...
0
votes
2answers
47 views

What should the length of my wire be in order to get a surface area of 10cm^2?

I would like to make a flat coil out of a steel wire that is $1.22 ~\text{mm}$ in thickness (or $1.22~\text{mm}$ in diameter). I wish to make the coil up the wire into a flat disk that has a surface ...
2
votes
1answer
40 views

Boltzmann equation in cosmology

I have a question about the Boltzmann equation in cosmology. Im trying to understand how this can hold? Where does the logarithmic terms come from? It is explained quite well here ...