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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23 views

Rotational inertia

Consider two bodies, A and B, of equal mass set at a short distance. Body A is spinning and body B is at rest. Then, through some kind of electromagnetic device, a strong repulsive force is ...
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1answer
23 views

Reading energy Eigenvalues from a Hamiltonian matrix for 1D harmonic oscillator

After a perturbation $V(x)$ added to the system, a matrix element $H_{nn}$ calculated in unperturbed Eigenstates for one-dimensional harmonic oscillator is given as: $$\epsilon \hbar \omega_0\begin{...
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35 views

Finding the molar ratio of gases: what the heck am I doing wrong? [on hold]

Note: This is not a homework question or something like that, and I've been trying to solve this question over 2 days, but I always find a negative answer, for some reason. In the book of Kondepudi, ...
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18 views

How to determine the direction of acceleration for block A and man? [on hold]

My confusion is how to determine the direction of acceleration of block and man. I know that in the same direction. acceleration of Man wrt rope/block will be Acceleration of Man minus Acceleration ...
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40 views

Conserved quantities of a simple Lagrangian

Suppose the following Lagrangian with 2 degrees of freedom: $$L = \frac{3}{2}\dot{q}^2_{1} \ + \frac{3}{2}\dot{q}^2_{2} \ + \dot{q}_{1}\dot{q}_{2}$$ My aim is to find all the conserved quantities of ...
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1answer
44 views

Show for $r \in \mathbb R$, $e^{(a_1 ^\dagger a_2^\dagger - a_1a_2)r}|0,0 \rangle = \frac{1}{\cosh r} \sum_{j=0}^\infty (\tanh r)^j |j,j \rangle$

Let $ a_1, a_2 $ be annihilation operators for the first and second component in the product state $|m,n \rangle$ using Fock basis. Following "Quantum continuous variables, A primer of Theoretical ...
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25 views

Determining whether I hit the bed when jumping from a cliff [on hold]

For simplicity, assume a perfectly spherical object with uniform density falling from some height to a lake. How can I determine whether I hit the bed of the lake? Maybe this has something to do with ...
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14 views

Relation between current (not at saturation) and voltage in photoelectric effect

This is the question I got for an assignment. How do I determine stopping potential from this data? I can see that the relation between $V$ and $I$ seems to be linear, so can I just use the equation ...
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1answer
28 views

The exact location of normal force and friction vectors

I am struggling with a following question: "A cylinder of radius $r$ and height $h$ is standing on an inclined plane pitched at an angle $\theta$ to the ground. Given the coefficient of friction $\mu$ ...
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28 views

Please check my problem about SI prefixes [on hold]

2.2 A single logic gate in a prototype integrated circuit is found to be capable of switching from the “on” state to the “off” state in 12 ps. This corresponds to: (a) 1.2 ns; (b) 120 ns; (c) 1200 ns; ...
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1answer
30 views

Change of variable in 4D space-time

I was reading Sidney Coleman's article "Fate of the false vacuum: Samiclassical theory" and i stumbled upon a change of variables that i can't seem to prove. The problem is this: trying to solve the ...
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10 views

Why the sum of deformations in given figure is zero?

When a vertical block is kept between two fixed support and when the bottom support is removed, it has the possibility of facing a deformation downwards which was impeded by the upward reaction force ...
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22 views

Toilet Paper roll [on hold]

Why we don't replace R in formula (RT=(1/2)m(R2+r2)(a/R)) into (R,outer diameter +r,inner diameter)? Does the inner roll also rotate?
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1answer
39 views

Inverted pendulum, EOM

Consider the inverted pendulum shown below: where $F$ is an external force, $B$ is the CoM of the rod, $A$ is the position of the joint where the rod attaches to the car. Angles increase in the ...
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external pressure vs internal volume plot of 1. expanding and 2. contracting cylinder-piston-gas system that obeys Boyle's law [on hold]

A gas which follows Boyle's law, is enclosed inside a standard piston-cylinder system having volume V and pressure P. If an external pressure is applied to that massless piston to 1. slowly compress ...
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28 views

How to solve for stable/unstable equilibrium in this question [on hold]

A solid cube of side b is at rest on curved surface of a cylinder(horizontal axis) of radius R. Assume no slipping. We have to derive condition for stable equilibrium. My approach: Slightly we can ...
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2answers
41 views

Can someone explain these circled points…?

Why is direction of friction in the direction of motion and here it is the platform that moves....the movement is not caused by friction so how is friction doing work? In the second one there is no ...
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1answer
72 views

How can i obtain the correction to the eigenvalues as a result of adding the gravitational potential to the hydrogenlike atom?

In treating the hydrogen-like atoms, we normally neglect the gravitational potential for the system. I am able to justify the above statement by comparing the gravitational potential to the ...
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0answers
27 views

What will be the equation of the path of spider?

A cobweb thread of negligible relaxed length is traced between points A and B on two opposite walls as shown in the figure. In this state the thread makes an angle $\theta$ with the horizontal and ...
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0answers
11 views

Factors effecting amplitude of damped spring

Let say we have a spring under simple harmonic motion. What are the independent variables that effect the time taken to reach half the initial amplitude during damping? I am guessing a few would be ...
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0answers
37 views

What's the shortest “safe” distance from a neutron star merger?

Take GW170817 for example, the first neutron star collision picked up by LIGO. Given how much data we got from that event, can anyone figure out what the "blast radius" is, and how far away from the ...
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14 views

Analysing a lift moving up with acceleration from the inertial frame of the ground

A frictionless inclined plane,making angle $ θ$ with the horizontal, of length $ L $ (inclined plane is a right-angled triangle, $ L$ is length of hypotenuse) is kept in a lift which is moving up with ...
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0answers
23 views

A tug of war with different forces at each end of the rope [on hold]

If a man pulls a rope with a force of 150N towards him and the other end of the rope is pulled by a small girl with a force of 50N towards her. Assuming rope to be massless. Find tension in the string....
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38 views

How am I supposed to get this answer? [on hold]

This question is from the Oxford Physics book, and I didn't understand how to get the answer. I'm assuming that the equation that I'm supposed to use E=mc^2 to solve the equation.
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1answer
109 views

Which wave function should I adopt?

Suppose I have Hamiltonian $H_0(\hat{p},\hat{r})$, it satisfies $H_0\psi(p,r)=E(p)\psi(p,r)$. If I make a change from $\hat{p}\to\hat{p}+p_0$, what is the form of the wave function of the Hamiltonian $...
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2answers
73 views

Calculate escape velocity without using energy?

Is it practically doable to calculate the escape velocity on the earth not by using the principle of energy conservation, but by calculating the velocity of the particle as a function of time using ...
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0answers
25 views

The problem was found in the book of mechanics by Kleppner and Kolenkow [on hold]

If you start a coin rolling on a table with care, you can make it roll in a circle. The coin leans inward, with axis tilted. The radius of the coin is 'b' the radius of the circle it follows is R and ...
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0answers
24 views

Energy with dipole [on hold]

What is the energy required to rotate an electrical dipole of $q=2e$ and $l=1.5$ nm by 50 degree when it is placed in a uniform electric field that has a magnitude of $45.2$ N/C?
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2answers
85 views

Divergence of a Vector Field - Surprising Result [duplicate]

I'm following the text Introduction to Electrodynamics by Griffiths, and I came across the following in an in-text problem: Sketch the vector function v = $\frac{\boldsymbol{\hat{\mathbf{r}}}}{r^2}$...
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0answers
38 views

Energy Eigenvalues from a given Hamiltonian [on hold]

Consider the Hamiltonian, H=σX Where σ=(σx, σy, σz) is the Pauli Vector with the respective Pauli Matrices and X=(x,y,z) is the position vector. What will be the Energy Eigenvalues for the given ...
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23 views

Kinematics of a Rigid Body; Thin plate rotated about a point [on hold]

I need help with this setting up this equation. So far I have after taking the moment about $O$: $$100(1-e^{-0.4t})-0.75mg\sin{\theta}= \left[\left(\frac{1}{12}m(l^{2}+l^{2})\right)+md^{2}\right]\...
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29 views

Pipe pressure drop question [on hold]

Water at $20^\circ$C is flowing at $0.1$ cm s$^{-1}$ in a $5$ cm diameter pipe. If the pipe has an equivalent length of 10 m what pressure drop would be expected? Comment on any assumptions made. Is ...
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0answers
30 views

Find Pressure applied given compressibility and change in volume [on hold]

The volume of a body decreases by $0.001\%$. If the compressibility is $1 \times 10^ {-5}$, then find the pressure applied.
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0answers
20 views

Bernoulli (?) question? [on hold]

can you help me to solve this problem? I have a pipe with water, at a constant pressure of 10 bar. I have then 3 holes, each one of different size and on an orizontal level. Out of which hole does ...
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7answers
819 views

Physical Interpretation of an Overdamped Pendulum

Consider a damped pendulum whose equation of motion is given in general by $$m\ddot{x}=-\mu\dot{x}-kx$$ where $\mu,k>0$ Rewrite this equation as $$\ddot{x}+2\gamma\dot{x}+\omega^2x=0,$$ where $...
1
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1answer
41 views

Work Done and Potential Energy in Electrostatics

This is a question from class 12 Physics NCERT Part 1: I am having so much trouble with this, I can't figure out where I'm going wrong. I solved part (a) here: (a) U= -27.2 eV (b) Here, the kinetic ...
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0answers
25 views

mass m on a inclined plane [on hold]

If the figure shows a smooth inclined plane what should be the minimum magnitude of acceleration which the mass m dropped freely vertically to the ground.
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0answers
26 views

How to solve the following problem of particle dynamics involving Hooke's law? [on hold]

I cannot solve the problem below: A particle hangs at rest at the end of an elastic string whose unstretched length is $a$. In the position of equilibrium,the length of the string is $b$ and $2\pi/...
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0answers
37 views

Motion in Uniform Magnetic Field

Sorry for asking an exercise question here, but I'm really having difficulty figuring this one out. Quick summary of the problem: a charged particle moving in a uniform magnetic field will experience ...
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0answers
42 views

A Simple RC Circuit [on hold]

This is the most simple Rc circuit (i have just started learning) .My questions regarding this is- (This is a dc voltage supply) Will capacitor get charged by the potential drop of resistor or ...
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0answers
14 views

Kinetic energy of daughter nuclei after decay given the Q value [on hold]

Assume that we know the Q value of the decay ${}^{230}_{90}\rm{Th}\rightarrow {}^{226}_{88}\rm{Ra} + \alpha$, and we make the simplification that the total mass of a nucleus is the number of nucleons ...
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0answers
26 views

Vacuum restructuring for superconductivity

I have posted several questions about superconductivity recently and all of them are related to vertex function but these questions were incorrect. I have found the following statement in book ...
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0answers
57 views

How Do I solve Newton’s law of gravity problem? $F = G (M1 * M2) / r^2$ [on hold]

I have been give this problem: F=(6.67∗10−11 (Nm^2/kg^2)) ∗ ((5.64∗1024kg) (1.99∗1031kg)) / (1.51∗1015∗m)^2 I do not understand (Nm^2/kg^2) but I do understand everything else
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1answer
68 views

A doubt in a question of mechanics [on hold]

Question: A mass of 2 kg is when kept on a ground,the ground yields steadily and the block goes down with an acceleration of 1 $m/s^2$. Find the reaction force on the block by the ground. I know ...
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0answers
12 views

MOSFET I - Vg curve [on hold]

Below graph shows the I - Vg curve in the subthreshold region, Quadritica region, and linear region for 5 different transistors. I was asked to determine which I - Vg curve in the below graph is ...
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0answers
13 views

Circular Motion and Force [on hold]

The maximum breaking strength of a piece of cord is 250 N. What is the maximum rpm at which the line can retain a 3 kg mass swung in a 1.8 m radius circle?
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0answers
18 views

Speed of Transverse wave and frequency in sitar wire [on hold]

I have a problem to solve though i am not clear on concept. I know, $V = C (T/m)^{1/2}$ Where $C$ is constant, $T$ is tension , $m$ is mass per unit length. Here is the problem A sitar wire in ...
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0answers
17 views

MOSFET Band Diagram from surface to substrate [on hold]

The question below asks to identify the band diagram of MOSFET at different Vg. This band diagram is drawn from oxide-silicon surface to silicon substrate. my understanding is that the band diagram ...
-2
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0answers
27 views

What force per unit of length does each line charge exert on the other? [on hold]

What force per unit of length does each line charge exert on the other? I have Two identical uniform line charges that are $d=0.8\,\rm m$ apart. They are parallel to the z axis. They have an ...
-1
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0answers
10 views

$C$-$V$ characteristics of MOS capacitors for different material [on hold]

For C-V curve below for 3 different materials, (Si, InAs, InSb). What material will be corresponding to curve A/B/C? My understanding is that C is inversely proportional to the dielectric constant in ...