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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11 views

How to find the lagrangian? [closed]

I'm having a hard time finding the lagrangian for this The system is on a table. All fours springs are the same. The rod has mass $m$ and length $2l$ and doesn't move on direction $y$. The ...
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1answer
11 views

Block, pulley and an external force moving the whole system problem (classical physics)

I'm not sure in which blocks is $\vec{F}$ acting on, I know it is clearly acting on the block of mass $M$ and clearly acting on the block of mass $m_2$. I'm not certain about the effects of $\vec{F}$ ...
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1answer
22 views

Heaters in parallel and series

I have a heater rated at 13 kW with two identical heating elements connected in parallel. If I connect elements in series instead what should be my expectation: That the total resistance in the ...
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1answer
32 views

The 1-dimensional Kronig-Penney Model: Trying to understand the relation between hopping energy and effective mass

The 1-dimensional Kronig-Penney Model predicts a relationship between energy, $E$ and wavenumber, $k$ of the form: $$\cos(ka)=\cos(qa) - \frac{m_e\,A\,t_0\,\sin(qa)}{\hbar^2\,qa}$$ where $$q=\sqrt{\...
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2answers
40 views

Beat frequency when a bat flys towards a wall

A bat is flying towards a wall while emitting an ultrasound of frequency 25kHz. Emitted sound and the sound that bounces off a wall form a beat frequency 1.65kHz, that the bat detects. With what speed ...
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1answer
35 views

Spherical Capacitor [closed]

How would we derive the equation for the capacitance of a spherical capacitor when the inner sphere has been earthed? I am aware of the formula $\frac{4\pi \epsilon_0}{\frac{1}{R_1}-\frac{1}{R_2}}$ (...
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Doubt in commutation relation of position and momentum functions [duplicate]

I am confused in the commutation relation of position and momentum funtion. How do we prove the relation which I have attached in form of age with it,if functions of momentum and positions are not in ...
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1answer
91 views

Why are the lifetimes of the neutral and charged $\Xi$ hyperons so different?

According to the Particle Data Group, the lifetimes of the neutral and charged $\Xi$ baryons differ significantly: $\tau(\Xi^-) = (1.639 \pm 0.015) \times 10^{-10}$ s, while $\tau(\Xi^0) = (2.90 \pm 0....
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48 views

Newton's shell theorem [closed]

While deriving newton shell theorem for a hollow sphere of mass $M$, radius $R$ and a point particle of mass m at a distance r, i am getting wrong answer while taking r=R (ie particle on the surface ...
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31 views

Classical mechanical problem consisting of a cylinder rolling on top of a box and the box on top of a smooth surface [closed]

Consider the following classical system. The top surface where the cylinder $m_1$ with radius $R$ rolls is rough enough so that $m_1$ can roll without slipping. The walls sandwiching $m_2$ is also ...
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1answer
38 views

Correct sign of vertical displacement in projectile motion

I understand how to do this problem perfectly fine. I am posting here however because I have a disagreement with my professor and classmates in finding the final y-coordinate of the projectile. I ...
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1answer
14 views

For the system shown in the figure initially springs are in natural length (natural length of both are same) [closed]

For the system shown in the figure initially springs are in natural length (natural length of both are same). Two blocks 'A' and 'B' of equal mass 'M' are released from rest simultaneously. The ...
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0answers
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How do i find the maximum acceleration of a front wheel driven car? [closed]

Whats the maximum acceleration of a 1600kg car? The static friction μ = 0.8. Front wheel drive.
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1answer
10 views

Induced field on a solenoid - dependance on position [closed]

Consider a cilindric solenoid of lenght L, with N spires of radius a. Calculate the induced field in a point P, in function of the angles $\theta_1 $ and $\theta_2$. The magnetic field for a solenoid ...
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1answer
30 views

What is the correct way to write this uncertainty?

$$ Z = 23,8937 * 10^{-5} [u] $$ $$ u(Z) = 3,721 * 10^{-7} [u] $$ I have to write this uncertainty in complete form. Would this be correct? $$ 0.0002389 \pm 0.0003721 $$
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1answer
37 views

General Expression for capacitance of any type [closed]

The capacitance of a parallel plate capacitor is given by C=Aε/d. For concentric spherical capacitor the value of capacitance is given by 4πεba/(b-a) (where b is the radius of the outer sphere and a ...
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1answer
41 views

Solving Particle in a Box using Newtonian Mechanics

so I'm trying to solve the one dimensional particle in a box problem known from quantum physics in a classical context. So according to Wikipedia (see picture on the right) we would get a particle ...
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2answers
78 views

What does the Problem 14 from Goldstein's book on classical mechanics chapter-7 (special relativity) really mean?

I am having difficulty in understanding problem number 14 in Goldstein's Classical Mechanics, 3rd edition, chapter 7 on special relativity. Here is the problem --- A rocket of length $l_0$ in its ...
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1answer
23 views

How to calculate uncertainty for a simple pendulum formula? [closed]

$$ T = 2\pi \sqrt{\frac{l}{g}}$$ $$U_{c}(T) =?$$ That's the only thing my teacher left me with. I don't understand how am I supposed to solve this without any measurements.
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1answer
30 views

Working out the properties of the Special Unitary Lie algebra $su(n)$

Given the special unitary group, we can define $$ M = -iX = \begin{bmatrix} b_{11} & -ia_{12}+b_{12} & ... & -a_{1n} + b_{1n} \\ ia_{12}+b_{12} & b_{22} & ... & ... \\ ... &...
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1answer
37 views

How to show if a world line is null for a particular metric

For the Kerr metric, with line element $$ ds^2 = -\frac{\Delta-a^2\sin^2\theta}{\rho^2}dt^2 - \frac{4Mar\sin^2\theta}{\rho^2}dtd\phi +\frac{(r^2+a^2)^2-a^2\Delta\sin^2\theta}{\rho^2}\sin^2\theta d\phi^...
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1answer
42 views

Does this Question have a number solution, how do I do this? [closed]

What is the magnitude of force F (assumed horizontal) to push the block up the plane at a constant speed? Take the coefficient of kinetic friction to be 0.25.
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How to find the Hamiltonian matrix when taken inner product with 1quantum numbers [closed]

Let's say I have a Hamiltonian H acting on 4 electrons on the n = 2 shell. I need to find the vanishing terms in the matrix of <2,l,m|H|2,l',m'>
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About deriving bernoulli equation from particle model and its pressure difference

Here i added a page from my fluid dynamics book where it shows particle model for deriving the equation. My question is why pressure is more at stream side aka 'positive "s" direction'.I would ...
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47 views

Space Time diagrams, [closed]

I am working through Minkowski Space Time and Vectors in Relativity and I am given the following question set of questions. I am relatively new to physics and relativity: Consider the situation in ...
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29 views

How to get Airy differential equation?

A particle with mass m is under the following potential: $$V(x) = \begin{align} 0 |x|\leq L ; c(|x|-L) |x|>L \end{align}$$ and the schrodinger equation: $$ \psi (x)'' + \frac {2m}{\hbar^2} ...
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0answers
13 views

Null point for a infinite current carrying conductor [closed]

If a current carrying conductor of infinite length is placed along the magnetic meridian and current is flowing towards north pole, then is null point possible at any point in space?
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1answer
27 views

Schmidt Decomposition : Writing it in this form

The beginning of this paper (pg.no. 1) on generalised Schmidt decomposition of three qubit states mentions the following: The Schmidt decomposition allows one to write any pure state of a ...
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0answers
37 views

A mass attached to the end of a spring is pulled to a position $𝑥 = 𝑥_0$ [closed]

A mass attached to the end of a spring is pulled to a position $𝑥 = 𝑥_0$ and released from rest, so that it performs oscillations. The experiment is repeated, but now with the system immersed in a ...
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57 views

Explanation Needed . Non conservative energy question [closed]

A planet of mass m is orbiting a star of mass $M (M \gg m)$. Initially (at $t = 0$), the planet has a circular orbit, i.e. $r$ is a constant and equals $r_0$. It then enters a region where the star's ...
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Could someone tell me where I went wrong on this collision problem please? [closed]

Hi could someone please help me on this problem https://imgur.com/a/EQfo6zu I used the formula given, I think I have all the right values. I also plugged all of these numbers in and still got the ...
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1answer
22 views

Effective weight of lifting 1 side of an object?

An employee at our warehouse lifted one side of a wooden pallet and dragged it $\sim 2$ meters and put it back down so that it was in a place that could be more easily picked up by a forklift. The ...
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14 views

Is there any relation between the torque required to rotate shaft of the alternator and the load connected to it?

Suppose that a TV and a LED bulb both could be operated by the electricity generated by the alternator. In the initial phase the LED bulb is connected to the alternator and then it is replaced by the ...
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1answer
21 views

What is the period of a ball rolling in a circle?

Imagine a uniform spherical ball of mass $m$ and radius $r$ rolling without slipping in a spherical bowl of radius $R$ with $r<<R$. There are two special cases of small amplitude oscillations: ...
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0answers
16 views

Kaluza -Klein theory curvature [closed]

I calculated the curvature of the Kaluza-Klein theory according to 5D Ricci Curvature I couldn’t find the same answer to the above link. Did I do mistake in my calculation? Please check attached ...
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13 views

How does earth's magnetic field trap charged particles?

I am very new here and I know this question is probably very basic for this place but I thought I would ask any way. I am in year 12 physics and this question is for a research paper that makes up a ...
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29 views

Current induced in straight wire by a moving point charge [closed]

I have a straight wire that is grounded through a resistor. It initially has no current flowing through it. A point charge $q$ moves past the wire with velocity vector $v$ relative to the wire. How ...
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2answers
72 views

What happens if I change the integration limits of the Fourier transform of $1$?

The Fourier transform of $1$ is the (one-dimensional) Dirac delta function: $$\delta(x) = \frac{1}{2\pi} \int_{-\infty}^\infty dp\ e^{-i p x}. \tag{1}$$ Now I would like to replace the RHS with: $$\...
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2answers
37 views

Time-dependent state probability harmonic oscillator [closed]

For my homework i am considering a harmonic oscillator which´s wavefunction at $t=0$ is the superposition of the eigenstates $\psi_n$. $$ \psi(x,t=0) = \sum\nolimits_{n} c_n \cdot \psi_n(x) $$ Now i ...
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1answer
18 views

Problem calculating Work function in Photoelectric effect

I'm having trobules understanding the solution of the following problem. In the problem we have a solar cell, which is impacted by a green light of $\lambda = 560nm$ and, as a consequence, there is ...
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0answers
24 views

Parallel Resisitors Problem [closed]

Hi, Could someone please help me out with calculating the resistance between points A and B in the circuit above? The solution is R(ab)=80.54 ohms, but I cannot figure out how this is calculated. ...
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0answers
43 views

TISE for hydrogen atom [closed]

I am given the radial component of the time independent Schrödinger equation for spherically symmetric electron wavefunctions: $$- \frac{\hbar^2}{2mr^2} \frac d {dr} \left(r^2 \frac {d \psi}{dr} \...
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20 views

Force exerted by a muscle [closed]

You have a forearm in a horizontal position and from the forearm you pull a muscle in a vertical position 4 cm from the elbow. The hand holds a weight of 6 kg at 44 cm from the elbow and the weight of ...
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0answers
29 views

A person is in a room whose ceiling and two adjacent walls are mirrors. How many images are formed? [closed]

The answer is 7. But I observed that one pair of mirrors is parallel, so shouldn't the no. of images be infinite?
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0answers
37 views

What is the Hamiltonian of this quantum system? [closed]

Quantum system with two disengaged degrees of freedom. The first one can be described with a monodimensional harmonic oscillator model and the second one with a rigid rotor model. What is the ...
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0answers
19 views

How do I determine the flow rate in the picture below? [closed]

I´m a begginer in fluid dynamics. I need to determine the flowrate in the picture. My problem is that I don´t know how to determine the pressure at the begining of the Pitot tube. Oil of specific ...
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0answers
13 views

Automatic hatch [closed]

I would like to calculate the torque of the motor that will drive the automatic hatch. This is the side view of the hatch. The closing and opening mechanism can be seen in the second to fourth ...
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0answers
16 views

Why the position of object is positive in light and lens question? [closed]

If you try to find the answer to the question a, you will see a strange thing. When the mirror reflects back to the lens to see what the final distance is, that position is not -1.6 but +1.6, even ...
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0answers
35 views

Dimensional notation: lower/upper indices and Einstein summation convention. How to lower the first index? [closed]

Electromagnetism - dimensional notation lower/upper indices and eintein summation convention . How to lower the first index? The task at hand is the following: lower the first index of $g^{\alpha \...
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1answer
40 views

How do I obtain the Dirac equation from the Euler-Lagrange equation?

Knowing that the free Dirac Lagrangian is : $$\tag{1} \mathcal{L}= \bar{\psi} (i \gamma^\mu \partial_\mu -m ) \psi$$ and that the Euler-Lagrange equation is: $$\tag{2} \frac{\partial \mathcal{L}}{\...

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