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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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How to expand $(D_\mu\Phi)^\dagger(D^\mu\Phi)$ in $SU(2)$

I would like to calculate the following expression: $(D_\mu\Phi)^\dagger(D^\mu\Phi)$ where $D_\mu\Phi = (\partial_\mu-\frac{ig}{2}\tau^aA_\mu^a)\Phi$ and $A_\mu^a$ are the components of a real $SU(2)$ ...
Hendriksdf5's user avatar
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45 views

Hamiltonian of a system? [closed]

I have a three level lambda system with two ground states $\lvert1\rangle$ and $\lvert3\rangle$ and an excited state $\lvert2\rangle$, interacting with a classical field and the SPP mode of the ...
MARYAM BIBI's user avatar
0 votes
1 answer
30 views

Buoyant force on a stick making an angle theta with the vessel

I was solving this question The answer of the question uses balancing of torques. It uses $L=0.5$m I have problem with the fact that they have used buoyant force $F = \rho V g$. Marking the net ...
Toshiv's user avatar
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1 answer
27 views

Understanding Symmetries and Invariances in Electrostatic Fields [closed]

I'm currently studying electrostatics and I'm having trouble understanding the concepts of symmetries and invariances of electrostatic fields. I understand the basic definitions of symmetry planes and ...
Boulahya Kaouthar's user avatar
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0 answers
41 views

Tensor product using Einstein notation [closed]

I have a summation such as $$\displaystyle\frac{1}{2}\sum_{i=1}^{N}\sum_{j=1}^{n}\frac{\partial}{\partial X_{i}}\frac{\partial}{\partial X_{j}}[B_{ij}(\textbf{X})P(\textbf{X},t)]$$ By just applying ...
Yeongkyu Lee's user avatar
0 votes
0 answers
12 views

What will be the Maximum Power in the circuit? [closed]

There are two identical batteries each of EMF 2V and internal resistance 1Ω available to produce herat in an external resistance 'R' by passing current through it. What will be the maximum power that ...
improvement dude's user avatar
-3 votes
0 answers
31 views

How do I find the resistance between points M and N? [closed]

This is a network of infinite resistors arranged in the shape of an infinite cuboid extending in the horizontal direction. The resistance of each side of the squares formed is R. We need to calculate ...
Hardik Mirg's user avatar
0 votes
1 answer
74 views

What does $v$ actually represent in pulley problems?

Let us consider the problem of two pulleys with boxes attached to their ends. One pulley is movable, and the other is not. The problem requires us to find a relationship between velocities of these ...
Alexander Djurovich's user avatar
-2 votes
0 answers
55 views

I need an explanation for the time derivative omissions in Landau’s Mechanics: Chapter 1 [closed]

So I have been self-studying Landau and Lifshitz’s Mechanics for a little bit now, and I have been working through the problems, but Problem 3 is giving me some trouble. I solved the Lagrangian ...
Justyn's user avatar
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3 answers
69 views

Question regarding gravitational force as external force

So, I was watching a lecture on YouTube for problems on conservation of energy and momentum and I don't quite understand this: In this question, mass $M$ is released from the peak of the smooth ...
Hrishikesh Pandey's user avatar
0 votes
1 answer
35 views

Calculating the speed of an ejected electron [closed]

I'm working on some physics exercises and came across this one. Sadly, there is no solution provided, so I was wondering if at least my line of reasoning is correct. ...
Rommelaar's user avatar
0 votes
3 answers
63 views

How is the change in internal energy zero for the following scenario:

in the problem, they are removing partitions one by one. when they remove the first partition they've mentioned in the solution that internal energy will remain constant in the process (and after ...
khan zaid's user avatar
0 votes
0 answers
15 views

Kinetic energy of a disc rotating about an inclined axis [closed]

What is the kinetic energy of a circular disc of mass M and radius R rotating with angular velocity w about an axis that is inclined to its surface by an angle of 45 degrees? (Please answer using the ...
Mansi Singh's user avatar
0 votes
0 answers
29 views

What is the charge distribution on a parallel plate capacitor when one plate is grounded? [closed]

Let's say a large parallel plate capacitor is connected to a DC power supply. Then, the charge on the two plates will be +CV and -CV. If I ground the negative terminal (both the capacitor's and the ...
Михаил Чиланнов.'s user avatar
-3 votes
0 answers
30 views

I am trying to derive an expression for the magnetic field of charged particles travelling at significant fractions of speed of light [closed]

I tried using lorentz transformation , which gives me correct answer for speed of light c but when i try to input any other values nearby c it becomes a constant B = 10-7 . q/r² ( (v - u)/ 1- uv/c²)
PhysicsEnjoyer31415's user avatar
2 votes
0 answers
41 views

Average force using Heisenberg equation on a bounded particle

My problem states: Prove that the average force that acts on a bounded particle is zero. My question is: How do we represent a bounded particle? I believe that a bounded particle represents a ...
Jorge A. Cortés Dávalos's user avatar
1 vote
0 answers
53 views

Orthogonal complement of null vector [closed]

I am trying to solve excercise 8.5 d) from Straumanns book on general relativity (here V is an n-dimensional Minkowski vector space): Prove that the orthogonal complement of a null vector is an $(n-1)$...
Lenti's user avatar
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0 votes
1 answer
46 views

Issue with simplifying the equation related to non-uniform acceleration

I am exploring how the cross sectional area of a drogue chute affects its velocity after travelling a set distance. I got this equation: $$m_1 a = m_2 g - \frac12 c_d \rho A v^2,$$ This covers the ...
Melon's user avatar
  • 1
0 votes
1 answer
31 views

How to calculate the relative speed when three bodies are involved? [closed]

Two trains A and B start from a station and move in parallel tracks with velocities 40 kmph and 60 kmph respectively. A man sitting in train A. watches a lady who walks inside the train B at a speed ...
Sandhra Baiju's user avatar
0 votes
2 answers
49 views

For an objective falling, reaching terminal velocity and then hitting the ground, how can I find the force it exerts on the ground? [closed]

For example object with v=tanh(0.1213)t/0.01236. m=69kg. I thought I had to use F=mv/t but that just gives the net force of the object while falling. I don't know how to find the force impacting on ...
Yifan YIN's user avatar
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0 answers
41 views

Thermodynamics exercise on transformations

An exercise in my book states the following: n = 0.42 moles of an ideal biatomic gas are stored in an adiabatic cylinder with volume $V = 10^{-2} \text{ m}^3$, with an initial pressure equal to one ...
NICOLA TROMBINI's user avatar
0 votes
0 answers
46 views

Extrinsic Curvature Calculation on the Sphere

Given the following 2+1 dimensional metric: $$ds^{2}=2k\left(dr^{2}+\left(1-\frac{2\sin\left(\chi\right)\sin\left(\chi-\psi\right)}{\Delta}\right)d\theta^{2}\right)-\frac{2\cos\left(\chi\right)\cos\...
Daniel Vainshtein's user avatar
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0 answers
45 views

Force acting on an infinite wire with current inside a magnetic medium

I've recently stumbled on an electromagnetics problem that I don't quite understand. There is an infinite wire carrying current I that is placed inside a medium with magnetic permittivity μ1. At a ...
Zuzia Woźniak's user avatar
0 votes
2 answers
42 views

Extra force in other frame paradox

Suppose there are two balls: one steel ball that cannot break with a mass of 10 kg, and another smaller wooden ball with a mass of 1 kg, which will break if a force of 10 N is applied to it. Consider ...
Paryag  's user avatar
-3 votes
0 answers
31 views

How do i solve this current electricity problem? [closed]

Question: An engineer has access to a tetrahedron building block with side length 10 cm. The body is made of a thermal insulator but the edges are wrapped with a thin copper wiring with cross ...
Shivam Paul's user avatar
0 votes
0 answers
42 views

Problem explanation from Estonian-Finish Physics 2003 olympiad [closed]

I am having trouble understanding problem 5.3 (Vibrations) from the Estonian-Finish 2003 Physics olympiad. Specifically in 5.3 they say "brick is kept in motion along(horizontal) $y$-axis by a ...
WilliamHarvey's user avatar
-4 votes
0 answers
60 views

Could you please answer my questions (I have four questions)? [closed]

1.must the positive direction is always upwards when we study systems that have springs in a vertical way and we cannot assume that the positive direction is downwards? If I understand correctly, how ...
DD AA's user avatar
  • 1
0 votes
0 answers
41 views

Minimum energy to be given to a swing for it to completely coil up around the main support bar [closed]

The question is the title. To find the minimum energy to be given to a swing for it to completely coil up around the main support bar. Generalization, swing can be thought of as a single pendulum. By ...
Kyathallous's user avatar
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0 answers
13 views

A question on modified flux function under perturbation [closed]

Given a magnetic field $\mathbf{B}=B_{0}\left(\dfrac{xz}{z_{s}^{2}},\dfrac{yz}{z_{s}^{2}}, 1-2\left( \dfrac{x^{2}}{r_{s}^{2}}+\dfrac{y^{2}}{r_{s}^{2}}\right) -\dfrac{z^{2}}{z_{s}^{2}}\right)$ with the ...
gamble addict's user avatar
-1 votes
0 answers
33 views

A 1m diameter sphere contains a Avagadro Number (mass 2.015g) of ideal gas particles of mean velocity 550m/s, what is the pressure in the sphere? [closed]

I get an answer of 1.552 kPa using surprisingly simple mathematics and wonder if my method is valid. I used P(sphere) = M(gas) * v(particles)^2 / r(adius sphere)^3 / 4 / Pi() Can it be that simple.
Bryan Major's user avatar
1 vote
1 answer
85 views

Electrostatic potential outside of a charged ball [closed]

To preface, I have never solved Maxwell's equation or his resulting Poisson's equation in any coordinate scheme, nor am I a physics major. I'm entirely teaching myself how to do this, so I don't know ...
Researcher R's user avatar
0 votes
0 answers
21 views

How does the moment of inertia work in this physics question? (Vibrations and Waves by George C. King) [closed]

Not sure if this is just me having a moment where I'm missing something totally obvious, but I'm going over some old undergraduate physics work to reinforce my knowledge, and I've encountered a step ...
H-QM-W's user avatar
  • 11
0 votes
0 answers
31 views

Centrifugal Governor Question [closed]

I've been working through Hand and Finch's Analytical Mechanics and have just attempted this question: My attempt at a solution is as follows: First, find the kinetic energy of the two masses $m$ by ...
Zachary Holloway's user avatar
6 votes
0 answers
92 views

Mahan's derivation of energy current of a free-particle system

On p.25 of the 3rd Ed. analogously to the polarisation operator $\textbf{P}$ for particle currents $$\textbf{P}=\int\textbf{r}\rho(\textbf{r})d^3r$$ he defines an operator $$\textbf{R}_E=\frac{1}{2}\...
Redcrazyguy's user avatar
1 vote
0 answers
47 views

Find curve minimizing energy loss due to friction [closed]

I am looking for an ansatz of the following problem: Given a mass $m$ moving in a constant gravitational field along curves $C$ connecting two fixed points I want to find the curve $C_0$ that ...
TomS's user avatar
  • 917
-1 votes
0 answers
61 views

What is this bar for? [closed]

There is a bar in front of the classroom’s blackboard. I wonder what it is for? It shouldn’t be for people to stand on it as then it should be a plank instead of a tube.
elang's user avatar
  • 17
0 votes
1 answer
48 views

How does an object with friction fly off of a disc with angular acceleration? [closed]

Consider the image below An object is resting on a rotating disc with angular velocity w and it is at rest with respect to the disc. Now if we increase the angular velocity of the disc(give it an ...
Gaurav Batra's user avatar
0 votes
0 answers
43 views

What are the operators here and how are these formulas derived? [closed]

In (23), are grad and div some kind of scalar operators comparing to $\nabla$ and $\nabla\times$? because tbh I dont know how $\text{curl}(\mu^{-1}\text{curl}\textbf{A})$ turns into $\text{div}\mu^{-1}...
user900476's user avatar
0 votes
1 answer
46 views

Doubts in circuit analysis [closed]

I am really confused here how is the potential at O found? Resistance is equal at all resistors this is an illustration in my book, they have not explained why voltage at O is 50V, just stated that ...
Anton Bert's user avatar
0 votes
1 answer
49 views

Calculating vehicle and tire acceleration with tire slipping

I'm attempting to simulate the motion of a vehicle with $N$ tires and I'm running into some trouble deriving forces/acceleration when adding friction and tire slipping into the picture. Here's an ...
Gumgo's user avatar
  • 103
-1 votes
1 answer
24 views

Deriving the equal time anti-commutator of the Dirac fields [closed]

I am trying to solve an exercise on deriving the equal-time anti-commutator of the Dirac fields. But I got stuck somewhere and couldn't get the desired result. I would like to show that $$ \{\psi(x), \...
user174967's user avatar
0 votes
2 answers
72 views

Forces accelerating a tire

I'm attempting to write a little vehicle simulator and I'm working on calculating vehicle linear acceleration and tire angular acceleration. I'm trying to add in tire slipping and I'm having trouble ...
Gumgo's user avatar
  • 103
-3 votes
1 answer
77 views

Strange calculus involved in work and power [closed]

If force is given by $$F(t)= 3t^2i + t^3j$$ and velocity is $$v(t)= 5t^2i + 2tj$$ then what is work done in time $t$. Now we can obtain power by their dot product $F.v$ and then integrate power to get ...
Harjot Dhillon's user avatar
0 votes
2 answers
127 views

Tension in Atwood machine revisited

In their book "Theoretical Mechanics of Particles and Continua", Fetter and Walecka ask to solve the problem (problem 1.3 of Chapter 1) of an Atwood machine in which a massless string is ...
Didier Chatenay's user avatar
5 votes
2 answers
493 views

Physical meaning of each term of the square modulus of a wave function

The expression below is the square modulus of the wave function of a harmonic potential ($V=\frac{1}{2}m\omega^2 x^2$) in which it's stated that the probability of finding the particle in the $\psi_0$ ...
zzzzzzzzz's user avatar
0 votes
0 answers
50 views

Rocket equation non-constant gravity

I am currently trying to teach myself differential equations and wondered about the following problem. You have an object (a rocket of constant mass $m_r$) and initial velocity $v_0$ that tries to ...
Imago's user avatar
  • 253
2 votes
1 answer
61 views

How to find a covariant gauge derivative from a field transformation

For reference: I'm self-studying from Peskin's Particle Physics 2019, which tries to sweep all QFT under the rug. Consider an SU(3) gauge theory; I am told a $3\times 3$ scalar field $\Phi$ transforms ...
spiderhouse's user avatar
0 votes
1 answer
165 views

Can you ever obtain a pure rotation from composing Lorentz transformations?

An exercise asks one to show that given $v, u$ speeds much smaller than $c$ and oriented orthagonally, the composition of the lorentz boosts $B(\mathbf{v})B(\mathbf{u})B(\mathbf{-v})B(\mathbf{-u})$ is ...
Y G's user avatar
  • 57
0 votes
1 answer
33 views

Spherical pendulum newtonian [closed]

I want to solve the equation of motion of mass $m$ in a Newtonian way. I don't know what kind of force makes $φ$ directional motion. In addition, if I solve this problem with Newton's equation of ...
SungJin Park's user avatar
0 votes
1 answer
91 views

Action-angle variables for three-dimensional harmonic oscillator using cylindrical coordinates

I am solving problem 19 of ch 10 of Goldstein mechanics. The problem is: A three-dimensional harmonic oscillator has the force constant k1 in the x- and y- directions and k3 in the z-direction. Using ...
SYD's user avatar
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