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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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0answers
29 views

Electric flux through y-z plane in a dipole [closed]

A dipole lies on the $x$-axis with positive charge $+q$ at $x=d/2$ and the negative charge $-q$ at $x=-d/2$.find the electric flux $Φ$ through y-z plane placed midway between between the charges. My ...
0
votes
1answer
12 views

Finding intensity of laser after round trip in cavity with single mirror [closed]

I came across a question in a text book today, and was wondering if someone could maybe check to see if my physics and workings are correct, as I am getting a intensity value greater the what going ...
2
votes
0answers
18 views

Velocity of fluid dividing to two different paths [closed]

Fluid of density $\rho$ is flowing through a pipe with area $A$ with velocity of $V$. The pipe divides into two different pipes of area $\frac{1}{2}A$ and $\frac{3}{2}A$ with the right end opened. All ...
1
vote
3answers
71 views

Divergence, gradient and differentiation - radial irrotational fluid flow

Given a fluid with the steady spherically symmetric flow with only radial velocity $\vec v(r)$. We need to evaluate $ \vec v \cdot \nabla \vec v $. From vector calculus $$ \vec v \cdot \nabla( \vec ...
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votes
1answer
23 views

Number of particles needed when equilibrium energy is given [closed]

N particles obeying classical statistics are distributed among three states having $\epsilon_0=0$, $\epsilon_1=k_BT$ and $\epsilon_2=2k_BT$. If the total equilibrium energy of the system is $1000k_BT$,...
0
votes
0answers
26 views

Designing a potential well [closed]

Design a potential well in which the probability that the particle in the first excited state can be found in $x \in [-\frac{x_0}{2}, \frac{x_0}{2}]$ is equal to $\frac{1}{2}$.$$$$ So the first ...
0
votes
0answers
24 views

How do I use the amplitude of two transmission pulses from different length cables to determine the attenuation of the cable? [closed]

I need to determine the attenuation of a cable. To do this I took two different measurements using two cables with the same characteristic imedance ($50 \Omega$). One was length $22.93$m and the other ...
0
votes
0answers
18 views

Jeans equation for a fully isotropic velocity dispersion tensor

Given a known spherically symmetric gravitational potential, $\Phi(r)$, I need to calculate the value of the velocity dispersion tensor $\sigma_{rr}$. To calculate $\sigma_{rr}$ I am using Jean's ...
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votes
0answers
15 views

How to find entropy $s_1$ and $s_2$? [closed]

Hey guys i am looking thermodynamics chart but i don't find entropy value.Help me guys. How did you find entropy of this question?
0
votes
0answers
27 views

Finding the wave function of a particle under varying potential (QM) [closed]

Given: a particle with mass m is under potential of $$V(x)= \begin{cases}-\frac{A}{x} & \text{for}\ \ x\geq 0 \\ 0 & \text{elsewhere} \end{cases} $$ Show that the wavefunction ...
0
votes
1answer
48 views

What is the wavelength of radio waves, if the radiation power is 1kW? [on hold]

The radio station emits every second photons. What is the wavelength of radio waves, if the radiation power is 1kW? So here is what I did: t=1s n= P=1kW= W After that I calculated E=? But ...
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votes
1answer
82 views

How do I solve this Laplace equation? [closed]

I'm stuck with the following problem: Suppose we have an infinitely long box whose cross-section has a rectangular shape of dimensions 2×3. Take the four last digits of your student ID number as a ...
-2
votes
0answers
21 views

Oscillations and Vibrations [closed]

Please someone help me solve this question.
0
votes
0answers
18 views

Photoelectric effect and emission

How would I calculate the number of electrons emitted from say a given intensity of light. For instance number of electrons emitted per unit time interval (second) in a $\text{mm}^2$ metal surface? ...
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votes
0answers
33 views

The question is given in the image please help me find the solution [closed]

I'm unable to understand how come the answer is in tan form please help!! The answer given is C. Thank you all in advance for your help
0
votes
1answer
11 views

Using RHR for Integration of Amperes Law

My professor breifly mentioned in passing that one can use the RHR for amperian loops such that the thumb is pointing current in any conductor and the fingers and curling in the integration of ...
0
votes
1answer
46 views

Is the addition of a Christoffel symbol and the partial derivative of a vector a tensor?

The partial derivative of a vector $V^\lambda , _\nu$ is not a tensor. Neither is a Christoffel symbol $\Gamma^\lambda _{\mu \nu}$. Is the addition of these two objects a tensor? If they were ...
0
votes
0answers
92 views

Torque vector: what's up with the signs? [closed]

I'm trying to calculate the torques produced by the following 4 force vectors: Left: rear view. Right: side view. The figure is not to scale. The positions vectors of the 4 force vectors are: $$\...
1
vote
1answer
43 views

How could I derive the Noether charge for a real scalar field?

I know for a (free) complex scalar field $\psi$ the Lagrangian is: $$ \mathcal{L} = \partial^\mu \psi^\ast\partial_\mu \psi$$ and that Noether's theorem from the $U(1)$ symmetry of the system gives a ...
0
votes
1answer
44 views

Force on trapezoidal tank

I'm stuck with this problem that I have almost solved but the last bit seems impenetrable. Please take a look at the problem on the image i provided. Plan: I started by calculating the momentum flow ...
2
votes
2answers
42 views

What is the significance of maximum compression in spring?

A block of mass $m$ sliding on a smooth horizontal surface with a velocity $v$ meets horizontal spring of spring constant $k$ fixed with wall. Find maximum compression of spring. Why is the word ...
0
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0answers
43 views

Pendulum struck by moving mass [closed]

One of the problems I was given as practice is as follows: I am curious as to how I would approach this problem if perhaps the mass m that strikes the mass M is not approaching at an angle orthogonal ...
0
votes
1answer
52 views

Pendulum general case [closed]

How to prove that the general moviment of a pendulum, considered as point mass suspended from a fixed point with an inextensible rope, is an ellipse? What is the relation between the pendulum's ...
0
votes
1answer
31 views

Elastic collision respect to a CM frame [closed]

Two particles, m1 and m2 collide one-dimensionally, if the collision is elastic. show that with respect to the cm frame, the particles reverse their velocity directions, maintaining the magnitudes.
0
votes
0answers
35 views

Density of state $g(e)$ Bose-Einstein Condensation system [closed]

Consider Bose-Einstein condensation of a hypothetical system consisting of N boson particles. The energy of single-particle states is e= eo n^2, where eo is the energy of the ground state and n = 1, ...
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votes
0answers
17 views

Using the flux of Bremsstrahlung to analyse medium [closed]

An HII region at a distance of 400 pc is observed to emit thermal Bremsstrahlung across a wide range of radio wavelengths. Independent analysis of emission lines suggests that the characteristic ...
0
votes
0answers
26 views

Boundary conditions of spun string

Problem: Consider a string with mass per unit length $\rho$ and length $L$. It is spun about one end, with angular velocity $\omega$ , such that the motion is in a plane (we neglect gravity). Let $x$...
2
votes
1answer
60 views

Magnitude of vector field [closed]

I think this is more of a mathematical question, but since it's for a physics problem I decided to ask it here. I have this complicated magnetic field in spherical coordinates $(r, \theta,\phi)$, $$ ...
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votes
0answers
28 views

AP Physics Multiple Choice Question [closed]

Three blocks of masses $m$, $3m$, and $2m$ resting on a frictonless horizontal surface are connected to identical ideal springs, as shown above. A force of magnitude $F$ directed to the left is then ...
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votes
0answers
64 views

A particle of water is travelling through a cloud [closed]

I remembered one old "Challenge" that my teacher once showed us. It asked us to prove that the acceleration of a particle of water travelling through a cloud is equal to g/7. The M and the V of the ...
7
votes
6answers
2k views

Homework question about an engine pulling a train

An 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal track. If the engine exerts a force of 40000N , calculate the acceleration of the train. To find the acceleration of ...
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votes
0answers
38 views

How can I find $E$-field from $H$-field? [closed]

I found the answer to this question a little bit confusing I have two doubts 1. What is the first E field [RED UNDERLINED] obtained in the solution 2. From where did [YELLOW HIGHLIGHTED EQUATION] ...
1
vote
2answers
66 views

What is the electric flux through the flat and curved surfaces? [closed]

My question is from Physics For Scientists And Engineers 7th: A particle with charge Q is located immediately above the center of the flat face of a hemisphere of radius R, as shown in the figure. ...
0
votes
0answers
40 views

A block and cylinder bodies on a rough inclined plane [closed]

On a rough inclined plane of inclination 45° a block of mass m and a cylinder of mass 2 m are released Coefficient of friction between all the surfaces of contact is 0.5. If the acceleration of the ...
-1
votes
0answers
36 views

Calculating Magnetic flux through a surface [closed]

A cylindrical cable (called shell) consists of an insulator up to a radius of $1 cm$ and then a non-magnetic conductive material up to a radius of $1.4cm$. , which carries a current of $50 A$ in the $\...
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votes
0answers
15 views

How to calculate shear stress in Cartesian coordinates for two dimension flow? [closed]

I want to calculate shear stress component in $x$ direction for two dimensional flow (Newtonian). Can anyone share and discuss a little bit?
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votes
0answers
12 views

Hiw to find net frictional force if the surface in contact are rotating w.r.t. each other [closed]

A disc of radius R is kept flat on a rough horizontal surface of friction coefficient μ with its axis vertical. It is given an angular velocity ω about its vertical axis and horizontal velocity v(v
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votes
0answers
33 views

Special relativity conceptual doubt [closed]

Suppose a beam of light is fired from an object to a planet while the object is moving away from the planet. Will an observer on the object measure the time taken for the light to reach the planet ...
0
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0answers
23 views

The precision of a gravitational wave interferometer [closed]

The latest design of the space-based interferometer LISA will search for gravitational waves using three telescopes separated by arm lengths of 2.5 million km in a triangular configuration. The LISA ...
0
votes
1answer
34 views

Kinetic Energy Comparison Marble Question

A uniform marble of mass M rolls down a symmetric bowl, starting from rest at the top of the left side. The top of each side is a distance h above the bottom of the bowl. The bowl is rough enough to ...
1
vote
1answer
45 views

Weight and potential energy for a spring pendulum

Consider a spring pendulum like in this figure suppose the spring is arranged to lie in a straight line and its equilibrium lenght is $l$. Consider the unit vectors $e_1, e_2$, $e_x, e_{\...
0
votes
1answer
38 views

Silly Question about Supersymmetric Gauge Theory

In $\mathcal{N}=1$ super electrodynamics, one has the following vector superfield $$V(x,\theta,\bar{\theta})=\bar{\theta}\bar{\sigma}^{\mu}\theta v_{\mu}(x)+\bar{\theta}^{2}\theta\lambda+\theta^{2}\...
2
votes
2answers
273 views

How do you calculate the black hole diameter of M87*?

The recent data from the EHT Consortium on the size and mass of the central black hole of M87, named M87*, are telling us that the diameter of the event horizon ...
0
votes
0answers
20 views

What is the total current in the circuit and what are the currents through the resistors? [closed]

How do you: (a) calculate the current thorough the middle (30 ohm resistor) branch (b) the potential difference across the 30 ohm resistor (this would be easy with (a)) (c) what happens to the ...
0
votes
3answers
44 views

Why is the normal reaction zero in this question?

A skier rolling down a frictionless hill where she should fly off tangentially and lose contact with ground. In answer of this question, solution states that $mg\cos x = mv^2 / R$ at that point (to ...
2
votes
0answers
27 views

Variational Method / Proof using bra-ket notation [closed]

I want to prove that the expectation value of the Hamiltonian $H$ in a state $\Psi$ is an upper bound to the ground state energy. The proof in my textbook makes sense but does it using integrals. So ...
0
votes
2answers
53 views

How to estimate the temperature required for the 2nd energy level of hydrogen to be occupied?

In my atstrophysics lecture today we were covering atomic spectra and we saw that at both the hot and cool end, the atomic spectra of stars showed a very weak balmer series. The reason being at the ...
0
votes
1answer
68 views

First-Order Perturbation of Energy Eigenfunction

I have a homework questions where I'm struggling to understand the methodology to use. We derive first the energy functional for the energy eigenfunction equation (this is fine, I used some vector ...
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votes
0answers
14 views

Ballistic Transport [closed]

This a question from my HW assignment, the question is as stated below. Both ends of a graphene nanoribbon with a width of d=6 nm and a length L (L>>d) are connected to electric leads made from Cu. ...
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votes
1answer
46 views

how will the bead move ( laws of motion ) [closed]

I did the following question. A thin ring having a radius R spins about its vertical axis at an angular frequency of ω. Show that a tiny pellet inside the ring will stay at its lowermost point ...