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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8 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 forms ...
-2
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0answers
12 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 ...
-1
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0answers
9 views

Relative Kinetic Energy on Two Sides of a Spring

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 ...
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2answers
47 views

Transition from 4-potential to E and B

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} ...
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0answers
42 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 ...
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1answer
42 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
19 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
16 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 ...
-5
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0answers
15 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
19 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 ...
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1answer
33 views

How to prove $F_{\mu\rho} \tilde F_\nu^{\phantom{\nu}\rho} = \frac14 \eta_{\mu\nu} F_{\rho\sigma} \tilde{F}^{\rho \sigma} $ using Schouten identity?

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 = ...
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votes
1answer
29 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
32 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 ...
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0answers
32 views

How do I find the spatial modes on a circle?

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
25 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 ...
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0answers
28 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 ...
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2answers
42 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 ...
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1answer
21 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?
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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 ...
1
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1answer
31 views

Where do forces point in an equilibrium system

I have the system above, with three identical balls of weight $W$ and radius $r$. The angle joining the centres is $\theta$, and the coefficient of friction between the balls and the plane is $\mu$ ...
1
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1answer
27 views

Electric potential at midpoint?

I understand that if you assume the field is uniform between R and S then you can use E=V/d to get E=(30-20)/(0.5)=20Vm^(-1), so the answer is C (which is correct according to the mark scheme). But ...
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0answers
37 views

Direction of emf and it's physical sense?

I have a situation where there exist a time varying magnetic field and a circular loop place perpendicular to it. Let us assume that the magnetic field is $\vec B=\frac{B_0 t}{\tau}$ . I tried to ...
-2
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0answers
22 views

AS Level Electricity Question - Cube of Resistors [on hold]

Twelve equal resistors are connected to make the edges of a cube. Thus each resistor is connected to two others at each end. A power supply giving $6.0\mathrm A$ is connected between two diagonally ...
1
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1answer
17 views

Movement and Rotation of a Inverted-Pendulum-like Object with External Forces

I'm struggling to model the movement of a complex object based on an external force. Let's start with a simple example of what I'm looking for. We have a block $b$ of mass $M_b$, moving ...
-1
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1answer
33 views

Calculation & derivation of de-Broglie wavelength [duplicate]

How to calculate the de Broglie wavelength of an $\alpha$ particle that is accelerated through a potential difference of $V$ from rest.
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0answers
36 views

Solutions to Dirac equation in 2+1 D [on hold]

Hello I solved the Dirac equation in 2+1 D taking two different gamma matrix representations. rep.A: $$\gamma^0=\sigma^3, \qquad\gamma^1=i\sigma^1, \qquad\gamma^2=i\sigma^2.$$ rep.B: ...
-1
votes
1answer
14 views

A problem based on Joule's law and specific heat [on hold]

If a wire of resistance $20\Omega$ is covered with ice and a voltage of $210V$ is applied across the wire, then rate of melting of ice will be? I calculated heat produced by the wire by using ...
1
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0answers
59 views

Christoffel Symbol for Vaidya metric [on hold]

Can any one help me in getting nonvanishing Christoffel symbols for the metric $$ ds^2 = −f(u, r)du^2 - 2du\,dr + r^2d\theta^2 + r^2(\sin\theta)^2d\phi^2, $$ where $$f(u, r) = (1 −2m(u))/r.$$ This ...
4
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0answers
47 views

Problem understanding the symmetry factor in a feynman diagram

I am trying to understand a $1/2$ in the symmetry factor of the "cactus" diagram that appears in the bottom of page 92 In Peskin's book. This is the diagram in question (notice that we are in $\phi^4$ ...
-1
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1answer
18 views

Finding average acceleration of objects sliding down an inclined plane [on hold]

A toy car of 500g rolls down 1 meter of a 30° plane in 2 seconds. a) The car is a rectangular prism of 10cm x 8cm x 16cm (Width/Height/Depth). What is its average acceleration? b) The car now has a ...
1
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0answers
27 views

How to calculate angular momentum in this scenario? [on hold]

The answer to part a) is 1080. However, for part b) I don't understand why the equation wouldn't simply be $I_i\omega_i = I_f\omega_f$. Why do we have to subtract $m_bv cos \phi$ from ...
0
votes
1answer
73 views

How can we use the distances? [on hold]

How can we calculate $F_{H}, F_{Z}, s$ at the following picture ?? I am confused because of the distances. How can we use them?? EDIT: Does the following stand?? $$s=(\text{Number of ropes})*h$$ ...
1
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0answers
26 views

How to solve an overconstrained static model? [on hold]

What is the easiest code-able way to solve an over-constrained static model? How does Force Effect http://forceeffect.autodesk.com do it? Given a 10m long bar angled as the hypotenuse of a 3, 4, 5 ...
0
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0answers
23 views

How to find the terminal angular velocity of the given figure? [on hold]

I have posted the picture of question, can someone please tell how to do it ?
3
votes
2answers
77 views

Perturbation theory in quantum harmonic oscillator [on hold]

This question concerns the quantum harmonic oscillator: (a)Express the operator $\hat B = \hat x \hat p + \hat p \hat x + \hbar$ in terms of $\hat a_{\pm}$ and $\hbar$ (b)Write the matrix ...
2
votes
1answer
52 views

To which side will it move? [on hold]

Children pull the carousel with diverse forces (as in the picture below). In which direction will the carousel move? Calculate the torque. I have done the following: The torque of the force that ...
0
votes
1answer
59 views

Force of Gravitational attraction between the ring and the sphere? [on hold]

Let us say a uniform ring of mass m and radius 3a is kept above a sphere of mass M and radius 3a at a distance of 4a such that line joining the centres of ring and sphere is perpendicular to the plane ...
1
vote
2answers
83 views

Why does a reaction force still allow the thing exerting the force to move?

A $5$ kg and a $10$ kg box are touching each other. A $45N$ horizontal force is applied to the $5$kg box in order to accelerate both boxes across the floor. Ignore friction forces and ...
-1
votes
0answers
31 views

How to calculate angle subtended on the sky given the distances [on hold]

Here is my question: The Earth orbits the Sun with a semi-major axis of 1 AU. The Sun has a radius of 700,000 km. Use these numbers to calculate the angle subtended on the sky by the Sun, as ...
-2
votes
0answers
12 views

Question from radiation physic, photons per second [on hold]

A beam of red light, wavelength 700nm, has 3X10^6 photons per second and when incident on a thermophile is able to maintain an increase in temperature of 0.2K. How many photons per second is required ...
1
vote
2answers
38 views

How to get these SUVAT formulae?

When learning kinematics, I have learned these three formulae $s=1/2(u+v)t$ $s=ut+1/2at^2$ $v^2=u^2+2as$ s = displacement u = initial velocity v = final velocity t = time a = acceleration My ...
1
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0answers
26 views

Confusion about Shankar 1D Scattering in a Single-Step Potential

Section 5.4 of Shankar studies the problem of a single-step potential where $$V(x) = \begin{cases}0&x<0\\V_0 &x>0\end{cases}.$$ It was shown that a incident Gaussian packet $$\psi_I(x,0) ...
2
votes
2answers
69 views

Covariant commutation relations in Mandl and Shaw

In page 47 of Mandl and Shaw, the $\Delta$-function can be written as $$ \Delta(x) = \frac{-1}{(2 \pi)^3} \int \frac{d^3k}{\omega_k} \sin(kx) \tag{3.43} $$ and as equation $$ \Delta(x) = ...
-1
votes
1answer
18 views

Description of the motion of a charged particle attached to a spring [on hold]

A particle with mass $m$ and charge $q$ move in the $xy$ plane and is attached to an ideal spring with neglectable mass, a spring constant $k$, rest length $r_0$ and a magnetic field $B = B_0z$ is ...
0
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0answers
19 views

Ground State energy as a function of $N$ and $B$, $E_0(N,B)$

The one-particle Hamiltonian is given by: $$\hat{H}=\frac{1}{2m}\left(p+\frac{e}{c}A\right)^2$$ where $p=\hbar\vec{k}$ with $e > 0$ and vector potential $A=(0,x,0)B$, such $B=\triangledown ...
0
votes
2answers
30 views

How to calculate centre of mass

How do I find the centre of mass with given coordinates? For example if we have four objects with mass $m$ at coordinates of a square $(0,0,0),(0,0,a),(0,a,0),(0,a,a)$ or another example with eight ...
0
votes
1answer
15 views

Compute the observed temporal changes in irradiance [on hold]

I got stuck on the following exercise: Consider an image $E$ with grey level distribution given by: $$ E(u_0, v_0) = A \sin(\omega u_0), $$ at $t = 0$ ($u_0$ and $v_0$ denote the position at ...
-2
votes
0answers
22 views

Projectile motion with mass, find the range [on hold]

a missile has the position vector P(t) = ht i + t(v − 5mt) j. where m is the mass (kg) of the missile, and h/v is the horizontal/vertical speed (respectively) the missile is launched with. Using the ...
-1
votes
0answers
17 views

Finding the maximal extension of a string on a slope [on hold]

Suppose we have a light string of natural length $l$ and modulus of elasticity $Y$ attached to a fixed point ,$A$. on a plane inclined at an angle $a$ to the horizontal. Let the other end of the ...