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Questions tagged [homework-and-exercises]

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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7 views

Variation of the Palatini action with respect to the triad and spin conection

I am trying to understand how could we get these two equations of motion by varying the action with respect to the triad and spin connection. The lecturer gave a hint that we can use integration by ...
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0answers
25 views

Write a Lagrangian for a constrained particle

I have the following exercise, but I have no solutions, so I'd need a check, since I'm pretty new to Lagrangian mechanics. Let $P$ be a point particle of mass $m$ constrained to move on the upper ...
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0answers
21 views

Eletromagnetism

A uniform plane wave of 10 kHz travelling in free space strikes a large block of a material having ε=9ε_0,μ=4μ_0 and σ=0 normal to the surface.If the incident magnetic field vector is given by B→=10^(-...
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0answers
33 views

Resistors parallel to short circuit

I have a trouble solving this problem. (Actually, each resistor is not parallel with the bottom wire since the voltage across each of them is different with that of wire.) In the manual, the bottom ...
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0answers
10 views

Capacitance of coplanar capacitor

I wanted to calculate the capacitance of capacitor with coplanar plates. How to do that? The dimensions of plate are width w, length l and the gap between plates is a.
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1answer
25 views

Mean kinetic energy without full laplacian

I need to calculate by solving the integrals the expectation value of the kinetic energy $\hat{T}$ and potential energy $\hat{V}$ operators in the state $\Psi$, defined as $$ \Psi(r) = \sqrt{\frac{1}{\...
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1answer
37 views

Write a specific Lagrangian

I'm pretty new to Lagrangian mechanics, and after some thoery I'm facing the first exercises, as the following: Consider a p.particle $P$ of mass $m$ which is constrained to a semi-circle of radius ...
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2answers
37 views

Change in Energy when placing an object on the ground

It seems like a simple question but I was wondering where does the energy go when I place an object from a height on the floor. Initially it's all stored as potential energy, and as I'm moving the ...
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1answer
266 views

Show two Lagrangians are equivalent

I need to show that these two Lagrangians are equivalent: \begin{align} L(\dot{x},\dot{y},x,y)&=\dot x^2+\dot y + x^2-y ,\\ \tilde{L}(\dot x, \dot y, x, y)&=\dot x^2+\dot y -2y^3. \end{align} ...
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0answers
30 views

Moment of intertia of a trinagle pendulum [on hold]

Find the moment of inertia of 3 rods with mass m and length L connected to form a triangle. The axis of rotation is that of a pendulum. The moment of inertia of a single rod with the axis through its ...
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0answers
15 views

Do you know answer(solution) about this question? for evaporation, thin film deposition [on hold]

Evaporation 7) For Al evaporating at 1100oC, vH = 318 kJ/mole. Assuming atomic vapor, what fraction of this energy goes into pV, what fraction into kinetic energy, and what fraction into potential ...
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0answers
15 views

What happens to the heavy elements when a gas giant with no core or one with metallic hydrogen is formed?

I have been trying to visualize how a gas giant like Jupiter originally formed. I am assuming that in its early stages it formed by aggregation similar to how the rocky inner planets were with an ...
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0answers
28 views

How to dock a rocket to the ISS? [on hold]

A supply rocket, with its engines, shut down, is trying to dock with the ISS. Initially, it is moving in the same circular orbit above the earth and the same speed as the ISS. The two craft are ...
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0answers
20 views

Finding number of photons per second crossing an area with distance ${}d$ from a radiating point

Estimate the number of photon per second entering one eye of the observer from 10 Watt red light source at 50m away form point source.
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1answer
56 views

How to efficiently compute the commutator $[\hat{r},\nabla^2]$?

Given a system with Hamiltonian $ \hat{H} = \frac {\hat{p} ^2}{2m} + \hat{V}(r)$ in a certain state $|\psi \rangle$, I want to find if $\langle r \rangle$ varies with time. From $$ i \hbar\frac {d ...
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1answer
21 views

Reciprocal lattice points and diffraction peaks

I am having a issue with the concept of what this question is asking. Question For a FCC crystal describe all the reciprocal lattice points corresponding to the two diffraction peaks. Here is my ...
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0answers
18 views

Muon capture cross-section

I would like to understand how the cross-section of muon capture depends on the nuclear charge. My attempts are based on the following thinking. Muon is captured to "high" orbital and then muon goes ...
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2answers
44 views

The Official Equation for the relationship between the Volume of a balloon and time

I would like to ask a question on whether there is a proportionality between volume of a balloon, and the time it takes to deflate. I have conducted several balloon hovercraft experiments. I need to ...
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0answers
15 views

Turn Projectile Motion into Quadratic Equation for Distance Calculator [on hold]

I'm doing the common projectile motion catapult project for a physics class, but I want to go above and beyond and make a calculator (in java) so that given a height, I calculate the distance needed ...
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0answers
32 views

Rotational acceleration with weight

I am having some issues with a question from my Physics homework. Question is: A wheel is hung up on an axis through the center. Above/over the wheel we hang a rope with 2 weights attached to each ...
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2answers
56 views

Computation in QFT

I'm always a mess with the upstairs and downstairs notation. To be specific, say I want to calculate the Euler-Lagrange equations of \begin{equation} \mathcal{L} = \frac{1}{2}\partial^\mu\phi \...
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0answers
23 views

An easy dinamic problmem [on hold]

On a ladder of l=6m placed at the height of h=4.8m and with the mass of 50kg there is a man of mass 80kg at the point 0 in the mid of the ladder (l/2) and the point c where the gravity of the ladder ...
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0answers
21 views

How much time does it take a light beam of a lantern at 1 light hour away from earth to hit a moving rocket sent from earth? [on hold]

Assume a stationary satelite in outer space at a distance of 299,792,458*3,600 meters from the earth. So basically the distance is the speed of light times 3600 seconds (or 1,079,252,848,800 meters) ...
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1answer
81 views

Block sliding down a table

Disclaimer: I came up with the following problem on my own, so I don't know if there is a solution that makes sense our if my approach to solving it is correct... Any help is appreciated. Consider ...
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0answers
52 views

Why is the force of gravity the net force?

The first image is the diagram** in this FBD, use Fg on m1 as its net force (not concerning the friction). why do you not subtract the tension force in Mass 1?
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1answer
42 views

Electric Field for two identical charges

I have a task to find an electric field I don´t know how it should look like: We have 2 identical charges $q$ at a distance $d$, on x-axis at $(\frac{d}{2},0,0)$ and $(\frac{-d}{2},0,0)$. How can I ...
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0answers
34 views

An rod is submerged into water in a graduated cylinder. Derive the total mass of the system as a function of depth of the rod [on hold]

I need help with this derivation. I am an instructor who needs to conduct this lab soon, but regretfully, I can't derive the equation. Consider an open graduated cylinder filled with 15 cm of water ...
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0answers
20 views

Time Period of Oscillations ( A mechanical problem) [on hold]

So , basically I came across the following question : A massless rod (lying on a frictionless horizontal plane) is hinged at a point O. A string carrying a mass m at one end is attached to point A ...
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0answers
37 views

Exoplanets by the transit method [on hold]

Many exoplanets have been discovered by the transit method, wherein one monitors a dip in the intensity of the parent star as the exoplanet moves in front of it. The exoplanet has a radius R and the ...
1
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1answer
27 views

Interchanging $\epsilon_F$ and $\epsilon$ when deriving Density of States (DOS) of Free Electron Fermi Gas (FEFG)

When deriving the Density of States for a Free Electron Fermi Gas, Kittel seems to flamboyantly interchange $\epsilon$ and $\epsilon_F$ when going between equations. I am specifically puzzled as to ...
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0answers
19 views

Confusion on conservation of energy in the beta decay process

In a question,regarding the beta decay process, it is given that. Assuming the anti-neutrino to be massless and possessing negligible energy and the neutron to be at rest, momentum and energy are ...
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0answers
27 views

Inductance problem confusion [on hold]

I am preparing for my upcoming finals and I have no idea how to do these two inductance problems I am so confused. Its an MCQ, here are the problems: I am confused with problems 2 and 3, the rest I ...
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1answer
92 views

Problem on the inclined plane

Yesterday I was just thinking about the inclined plane and how an object placed on it moves with uniformly accelerated motion, which depends on the inclination angle of the plane itself. I thought I ...
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0answers
33 views

Spacetime relationship in SR & GR and metre definition

The metre is defined as the length of the path travelled by light in a vacuum in 1/299 792 458 second (Wikipedia metre definition). This definition seems to render the relationship between space and ...
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0answers
19 views

A body is launched at a height of $8R$. Which of the following are correct? [on hold]

a body is launched at a height of $8R$. which of the following are correct? check the image for detailed question and correct answer and provide detailed solution
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0answers
22 views

If one if the masses is released [on hold]

Figure shows 4 identical masses of mass m , arranged on a cube as shown If one of the masses is released (A) it will collide with one of the masses (B) it execute an oscillatory motion (not Shm) (...
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1answer
45 views

Finding the mass of a relativistic particle resulting from decay

I'm studying relativistic kinematics for the first time. I’m working with a problem from a book but I’m stuck: A relativistic particle of rest mass $m_1$ decays at rest into a particle of rest mass $...
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1answer
45 views

Newton's Third Law Slope [on hold]

I know Newton's Third Law says there is an equal and opposite reaction when you apply a force. Consider the diagram of an object on a slope above. What is the force the object is applying on the ...
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1answer
33 views

How Do I Calculate the Potential of System?

I was doing my homework when I came across this question: Three equal point charges, each with charge 1.40 μC , are placed at the vertices of an equilateral triangle whose sides are of length 0.250 ...
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1answer
44 views

how does a car move? and how do the brakes affect its velocity? [duplicate]

im doing a project that requires me to understand how cars move. from what I understand , its the friction that is creates in the tires that enables the tire to rotate, which then push the car forward....
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3answers
56 views

Kinematics and dynamics of a crashing satellite

A satellite in low, circular orbit around the Earth experiences drag (friction) and slowly spirals into Earth's atmosphere. It then enters the Earth's atmosphere, heats up catastrophically and burns ...
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1answer
39 views

A circular motion problem [on hold]

A car goes on a horizontal circular road of radius $R$, the speed increasing at a constant rate $\frac{dv}{dt}=a(constant)$. The friction coefficient of the tyre is $\mu$. Find the speed at which the ...
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1answer
23 views

Qualitative Argument in a Projectile Motion Question [on hold]

Essentially, my qualms are with Part (a). The argument I’ve come up with are as follows: The ball was launched perpendicular to the plane. That means that the launch angle, which is the angle with ...
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1answer
61 views

Proof for $\langle i[A,B]\rangle$

I have to prove the following equation: $$ \langle i[A,B]\rangle = 2\mathfrak{Im}\left[\int dV(\overline{B\psi)}(A\psi)\right]\,,$$ where A,B are hermitian operators. Here is my calculation, but I don'...
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2answers
38 views

Magnetic field of long cylindrical wire

i have been studying electromagnetism on my own and I got a question about an exercise in David Griffiths, Introduction to Electrodynamics book. So I'm asked to calculate the magnetic field both ...
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1answer
22 views

EM- waves in anisotropic medium

When ElectroMagnetic waves from vacuum enter the anisotropic medium. If the $\epsilon$(permittivity) matrix is diagonal (with not all diagonal entries being equal). There'll be two waves in the ...
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0answers
22 views

Dynamics - Masses on Inclined Planes [on hold]

Calculate the coefficient of friction between the block and the ramp if the block reaches a velocity of 0.45 m/s at the bottom of the ramp. Given values: Block has a mass of 60 kg The inclined plane ...
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0answers
22 views

Acceleration of a pulley [on hold]

In the figure shown, m1=1kg, m2=2kg, m3=0.5kg and m4=4kg. On solving, I observe that the pulley P accelerates downwards. Why does that happen when P has lesser mass hanging from it than Q does? The ...