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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3 views

About atomic transitions

I accomplished my homework but it opened a whole pandora's box of things I do not quite understand, so I would appreciate help. In x direction linearly polarized light excites atom transition from ...
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0answers
20 views

Do these symmetrical looking B field problems have the obvious answer?

I am struggling with two questions whose answers both appear to be trivially zero by symmetry arguments, when I was expecting more derivation to be required. Problem A), An infinitely long wire ...
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0answers
24 views

Variational principle perturbation

I am trying to learn 2D SHO and free particle variational principles. However, I came across a perturbation like so: $Ax^2y^2$. The particle is simple harmonic in $x$ and a free particle in $y$? I'm ...
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2answers
56 views

Is this symmetry factor in Peskin wrong?

I am trying to compute the symmetry factor of a Feynman diagram in $\phi^4$ but i do not get the result Peskin Claims. This is the diagram I am considering ...
-2
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0answers
18 views

Finding current in the circuit 4 [on hold]

When switch is at position 1, current occur in circuit. After a while current increases and time=t, switch is at position 2. What is the current at time t?
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0answers
12 views

Calculation of the potential of 2 spherical perfect conductors with the image method

I am searching for a way to calculate the potential on the surface of two perfect conductors that are spheres. I am not sure my method is correct. Here is a diagram of what I am studying : They ...
-3
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0answers
22 views

How to calculate speed [on hold]

Please Help I need to calculate the speed of an object, in this case a ice ball with a diameter of 38 mm, propelled vertical through a 38 mm diameter pipe of 300 mm long , and by means of pressurized ...
1
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0answers
15 views

Temperature change in adiabatic free expansion

Consider a gas with fixed number of particles $N$ undergoing adiabatic free expansion from $V_1$ to $V_2$. Apparently we have the following relation $$\Delta ...
-1
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1answer
35 views

Drag force on ceiling fan [on hold]

Ceiling fan consumes 80 Watt. if you re track the where all the power go. Then here it is core losses, rotational losses due to bearing, drag force and moment of inertia of ceiling fan. Now how to ...
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1answer
25 views

Are there any units involved in the partition function for a classical particle system?

Is the output of a partition function dimensionless or are there units involved? The question as it is: $$E_1\text{=0}K_B\text{,g=1}$$ $$E_2\text{=0}K_B\text{,g=3}$$ $$E_3\text{=0}K_B\text{,g=5}$$ ...
-8
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0answers
50 views

my car is overheating after I just fixed the water pump? [on hold]

Steam coming out the Cool lit jug.. my engine shaking. Also my exhaust pipe throwing Water vapor out. I just changed the water pump..
-4
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1answer
20 views

How does a force get distributed over 6 points? [on hold]

I'm building a truss bridge that needs to support a live load of 16N distributed over a distance of 12.5cm, supported by 6 points, 3 on each truss with equal spacing between them. can anyone help me ...
-5
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0answers
15 views

If a motor cyclist was travelling at 12 ms-1, then slows down at 3 ms-2. how long would it take to stop? [on hold]

help me please and make me get to go to my bed because i've school exactly 7 :45 a.m and please don't keep me up waiting for an answer.
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1answer
35 views

Cart speed and wheel rotation

Say you have a horse drawn cart. Does the outside of the wheel spin at the same velocity that the cart moves forward? The reason I ask is because I am working on a problem where a piece of mud ...
1
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0answers
32 views

How to calculate linear momentum in this elastic collision? [on hold]

A small mass m = 5.0 kg approaches from the left with velocity v and a greater mass M = 40.0 kg approaches from the right with the same velocity. If the collision is elastic and the value of v is v = ...
1
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1answer
37 views

Green's function for 1 D hubbard model?

Consider the 1D two-site Hubbard model at half filling $H=-t\sum _{\sigma} (c_{1\sigma} ^{\dagger}c_{2\sigma}+h.c.)+U\sum_i(n_{i\uparrow}-\frac{1}{2})(n_{i\downarrow}-\frac{1}{2})$ where the sum is ...
1
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1answer
37 views

What is the gravitational effect inside a electromagnetic shield due to an external electromagnetic field?

I am new in General Relativity. I know that electromagnetic field (or, the electromagnetic energy tensor, $T^{ik}=1/4\pi[1/4F_{mn}F^{mn}g^{ik}-F^i_lF^{lk}]$) can affect gravitation. Now if we take a ...
1
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0answers
11 views

Second-order correction in Quantum-Confined Stark effect

In the wikipedia article, there is a second-order correction in the Quantum-Confined Stark Effect. I could not understand how it was solved. I did not understand the meaning of 2(0) and 1(0) and how I ...
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0answers
33 views

How can I compute the average number of collisions of a particle in a spherical container?

The problem statement reads as follows: A spherical container of radius $R$ with a small hole is inmersed in an ideal gas at a tempereature T. When a gas particle enters into the spherical container, ...
2
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2answers
41 views

How do you find the projection operator onto an eigenspace if you don't know the eigenvector?

I was working on exercise 2.60 of Nielsen-Chuang which is as follows: Show that $\vec{v}\cdot\vec{\sigma}$ has eigenvalues $\pm 1$, and that the projectors onto the corresponding eigenspaces are ...
1
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1answer
35 views

Normalizing a wave function in a mixed well

So I got this potential and want to solve for the even wavefunctions http://imgur.com/GKAy4nD Since it's symmetric around the origin I need only to look at the interval [0,b] and solve for the ...
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0answers
20 views

What is happening kinematically to a car on an icy turn? [on hold]

I am stumped on a conceptual homework question that involves rotational kinematics and friction. Consider a car traveling along a road that encounters a turn covered in an unseen layer of ice. ...
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0answers
12 views

Mirror question [on hold]

A man stands in front of a plane mirror. The vertical distance between his eyes and the top of his head is 8 cm. Find the minimum height of the mirror so that the man can see all his body.
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0answers
4 views

Vertical wall at 3 different temperatures - do the same natural convection coefficient correlations apply?

I have an vertical enclosure that consists of 3 shelves, each with its own temperature and are thermally isolated from each other. We will call the shelves A, B, and C with A being the base shelf. The ...
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0answers
10 views

How to calculate the self diffusion of carbon in diamond? [on hold]

What is self diffusion and How to calculate the self diffusion of carbon in graphite and carbon in diamond ?
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2answers
29 views

Calculating velocity based on applied force

If I have a block sitting on a spring, ignoring air resistance, with 45N of stored elastic energy in the spring, and the block weighs 500g, what would be the process for calculating its maximum ...
2
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0answers
39 views

Two solutions, A+ and A-, for masses connected via springs

My prof gives an example of trying to solve the equations of motion for a series of gliders each connected by springs, with the same spring constant. From looking at the $j-1$ through $j+1$ glider, he ...
-1
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0answers
39 views

Beginner level: Is this a correct phase space plot? [on hold]

I am a beginner into chaos and nonlinear dynamics. I was trying to plot the phase space plot for Tent Map using Matlab. The outline of the plot resembles the Tent Map, but I am getting several cross ...
1
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1answer
39 views

Graph of energy stored in capacitor?

Question: In an experiment a capacitor is discharged through a constant current. Draw a graph of how the energy stored in the capacitor varies with time. The answer given is: But I seemed to get a ...
1
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1answer
39 views

Angular Momentum Expectation Values in Spherical Coordinates

I have a homework problem that asks: Using the spherical harmonics calculate $\langle J_x \rangle$, $\langle J_y \rangle$, $\langle J_z \rangle$ in the state $|l,m\rangle$. Use the derivative ...
-1
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0answers
15 views

Calculate angle to hit (x,y) with given, constant air drag [on hold]

Drag constantly decreases velocity by $3\%$ per second. Set $a=0.97$ for simplicity. Original velocity represented by $v$. Horizontal velocity is $f(x)=va^x$ And vertical velocity is ...
0
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0answers
18 views

Relative Kinetic Energy on Two Sides of a Spring [on hold]

There is a massless spring on a frictionless surface with two objects, $A$ with mass $m$ and $B$ with mass $M$, attached to the ends of the spring. Both objects are pulled and held at a position where ...
2
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2answers
64 views

Transition from 4-potential to E and B [on hold]

In my lecture notes there is a step that i cannot follow: $$\frac{i}{2}[\gamma^{\mu},\gamma^{\nu}] (\partial_{\mu}A_{\nu}-\partial_{\nu}A_{\mu})=: \sigma^{\mu\nu}F_{\mu\nu}=i\vec{\alpha} ...
2
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1answer
85 views

When will the Earth stop rotating?

I looked up leap second in Wikipedia. It is a second added (usually) to clocks to keep them in sync with the atomic clock. It said that 25 leap seconds were added in the last 43 years and that none ...
3
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1answer
66 views

Time Evolution of Position Operator

I am trying to understand why $$e^{-it\triangle}xe^{it\triangle}=x-2it\nabla$$ where $x$ is just multiplication operator by $x$. In particular, the text says this can be seen by differentiating with ...
-1
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0answers
23 views

work and energy with air rifle [on hold]

A $4 \text{ g}$ air-rifle pellet is fired into a target block attached to a glider. The total mass of the block and glider is $0.5 \text{ kg}$. The target slides along the track $1.455 \text{ m}$. The ...
-2
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0answers
19 views

Vertical Simple Harmonic Motion [on hold]

A body of mass 0.120 kg is placed in a horizontal plate. The plate oscillates vertically in SHM five oscillations per second. Determine the largest possible amplitude of oscillations such the body ...
-6
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0answers
16 views

Vector and motion in 2D [on hold]

Q. A particle is moving in a circle of radius r centre at O with constant speed v the change in velocity moving from A to B ( angle AOB = 40°) is : A. 2v cos40° B. 2v sin40° C. 2v cos20° D. 2v sin20° ...
-1
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0answers
23 views

Resistance of symmetric resistor network [on hold]

Just doing some general Physics questions and this came up: I'm confident that I could answer the effective resistance part of the question, but am unclear as to the reasoning required for the ...
1
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1answer
53 views

Prove $F_{\mu\rho} \tilde F_\nu^{\phantom{\nu}\rho} = \frac14 \eta_{\mu\nu} F_{\rho\sigma} \tilde{F}^{\rho \sigma} $ using Schouten identity [Done] [on hold]

How to prove \begin{align} F_{\mu\rho} \tilde{F}_{\nu}^{\phantom{\nu}\rho} = \frac{1}{4} \eta_{\mu\nu} F_{\rho\sigma} \tilde{F}^{\rho \sigma} \end{align} using Schouten identity \begin{align} 0 = ...
1
vote
1answer
44 views

Deriving the energy-momentum tensor conservation equation in complex coordinates, Polchinski 2.4.2

I am trying to derive equation (2.4.2) in Polchinski's string theory textbook, $$\overline \partial T_{zz}=\partial T_{\overline z \overline z} = 0 \tag{2.4.2}.$$ Using the conservation equation, ...
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1answer
37 views

Derivation of Pauli Hamiltonian

In my lecture notes there is a step that i cannot follow: $$\frac{i}{2}\epsilon_{ijk}\sigma_k [\pi_i,\pi_j] = -e\epsilon_{ijk}\partial_iA_j\sigma_k$$ with $\vec{\pi}=\vec{p}-e\vec{A}(x)$ When I ...
0
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0answers
40 views

How do I find the spatial modes on a circle? [on hold]

Hello, I have a wave on a circular rope and I have to find solution like $$u(t,\theta )= U(\theta )e^{iwt}$$ With the wave equation I find : $$U(\theta )=acos(\frac{w\theta }{c})+bsin(\frac{w\theta ...
-2
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1answer
28 views

Question on Projectiles and momentum conservation [on hold]

Firstly I apologize for bringing forth a high-school physics problem here, as some of you'll find this problem highly childish. But there are certain things i need to clarify about this. Q: An ...
0
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0answers
30 views

Initial position and velocity given, force given. Find the trajectory of the particle [on hold]

If a particle of mass m at time t=0 has velocity v = u j position r = -a i Force acting on the particle is F = $-(mv^2/a)$(R/R) (R is the position vector, v is the velocity vector) Find the ...
1
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2answers
52 views

Mathematics of simplified air resistance

I recently encountered a problem in which when throwing a ball upwards one has to determine whether it goes up or comes down faster when not only was gravity to be considered, but also air resistance ...
-2
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1answer
23 views

Acceleration of a system [on hold]

I worked out this problem that has two masses and and pulley with one mass hanging and the other sliding across a surface and I had to solve for the systems acceleration. Can someone tell me if I did ...
1
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1answer
30 views

Pendulum at an angle

A pendulum has a period of $T$ when swinging from a string. The pendulum is now placed on a frictionless incline at a 30 degree angle. What is the new period of the pendulum?
-1
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1answer
21 views

Faraday's Law: Current loop and proton

A single circular loop of wire with radius R carries a large clockwise current I(loop)=I0, which constrains a proton of mass M and charge e to travel in a small circle of radius r at constant speed ...