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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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22 views

Question about units in photonics calculation

My textbook has a photonics example problem, where the absorbed light energy density is calculated as $$\Psi = 25 \times 9.55 = 238.75 \text{mJ/cm$^3$} = 2.38 \times 10^5 \text{J/m}^3$$ I'm new to ...
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2answers
85 views

Why will the balls experience same acceleration [closed]

Three identical balls , each of mass m , are pushed by a force F on a frictionless table as shown below. Find the net force on each ball.(The balls are not connected in any way). In the solution ...
2
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0answers
27 views

Forces acting on baseball question [closed]

I got a question wrong on my homework and am not sure why: You hit a baseball with a bat and it flies into left field. We are looking for forces that act on the baseball starting right before you ...
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2answers
65 views

Thermal Physics in a closed system [closed]

Body X with a temperature of 0°c is brought into contact with Body Y of 100°c, and equal mass. The specific heat capacity of X is greater than that of Y, and this is a closed system. Apparently the ...
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0answers
41 views

Mechanics Question - Thrust in a Rod between train and carriage [closed]

I am slightly confused regarding this mechanics question as the worked solution displays thrust as acting towards the centre of the rod whereas I have been taught to see thrust as acting towards the ...
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0answers
45 views

electric potential of electric dipole in 2D

I have the homework question as follows. What I did was first find that $$\phi = \frac{-\lambda}{2\pi\epsilon_0}(\ln(r_+)-\ln(r_-))=\frac{-\lambda}{2\pi\epsilon_0}\ln\left(\frac{r_+}{r_-}\right).$$ ...
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1answer
60 views

Question on laws of motion [closed]

Two bodies A and B of masses 5 kg and 10 kg in contact with each other rest on a table against a rigid wall . The coefficient of friction between the bodies and the table is 0.15. A force of 200 N ...
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0answers
23 views

Finding the time value of a simple harmonic motion given positional equation [closed]

How do I find the $t$ value in $x(t) = 2.5\cos(\pi t)$ given that $x = 1.5$ My thought is that if I put: \begin{align} 1.5 &= 2.5 \cos(\pi t) \\ \cos^{-1}(1.5/2.5) &= \pi t \\ t &= \...
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1answer
27 views

A body is projected from ground with some angle to the horizontal, how the angular momentum about the initial position of the motion increases?

A body is projected from ground with some angle to the horizontal, how the angular momentum about the initial position of the motion increases? I have tried to solve this problem using parallel axis ...
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1answer
27 views

Determine variable radii of a lens [closed]

How can I determine the radii of a thick lens if I only know their proportion? I can find a lot of examples determining the focal length from the radius but barely anything about the other way around. ...
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3answers
71 views

Normal reactions with moments

A uniform ladder, $AB$, is leaning against a smooth vertical wall on rough horizontal ground at an angle of $70°$ to the horizontal. The ladder has length $8\ \rm m$, and is held in equilibrium by a ...
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1answer
35 views

Free boson Equation motion from action

So in David tongs notes we have $$S=\frac{m}{8\pi}\int d^2x\partial_i\varphi\partial^i\varphi$$ and he finds that the equation of motion is $$[\partial_{t}^2-v^2\partial_{x}^2]\varphi=0$$ now my ...
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3answers
39 views

Friction on car tires

In tipler and mosca 5th edition the following passage occurs: I am a bit confused by the sentence starting "If the power delivered by the engine is small enough...". I am not seeing why the power ...
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1answer
34 views

Calculating final pressure in irreversible adiabatic compression

I am trying to solve the following question. A gas is enclosed in a cylindrical can fitted with a piston. The walls of the can and the piston are adiabatic. The initial pressure, volume and ...
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0answers
45 views

How to derive the gravitational potential on an infinitely thin disc of finite radius?

I am stuck on a problem that requires me to calculate the gravitational potential on an infinitely thin disc of finite radius $R$ with mass $M$. This infinitely thin disc models a galaxy. Once I have ...
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1answer
42 views

Moment of Inertia of a semi ellipsoid?

I understand that mass is simply found by taking the integral of $\rho$ and differential volume. I just do not understand how you can take the del of $I_x$ and where that equation $y^2\text{d}m/2$ ...
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2answers
29 views

Why is normal force at the bottom dependent on normal force on top?

Why does the normal force on bottom of the track have anything to do with the normal force on top of the track? Why isn't the normal force at the bottom simply $mg$?
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5answers
62 views

Is the normal force in the $x$-direction equal to $mg$ for a sliding object? [closed]

At point A, is the normal force in the x direction equal to mg? When the object is going down I imagine $N_x$ decreasing and $N_y$ increasing until point B where $N_x = 0$ and $N_y = mg$. That's why ...
3
votes
2answers
98 views

Can Potential Energy be found by Energy Density? [closed]

I wanted to find a creative solution to the problem: Find the electric potential energy of a uniformly charged sphere of radius $R$ possessing a charge $Q$. My attempt at a solution: Consider a ...
1
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0answers
26 views

Derivation of Feynman rules from generating functional for non-eq QFT

I consider Yukawa non-equilibrium theory with interaction $g\bar{\psi}\phi\psi$ with massive fermionic field $\psi$ with mass $m$ and massive scalar field $\phi$ with mass $M$. I would like to ...
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votes
1answer
60 views

Circular motion of a ball suspended from the rim of a rotating table [closed]

Susppose there is a ball with radius $r$ suspended to the rim of a circular spinning table with radius $R$ by a string with length $l$ which makes an incline angle $\theta$ with the vertical axis. If ...
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0answers
24 views

What is the gas molecule's velocity after hitting a thermally conducting disk? [closed]

A gas molecule hits a thermally conducting disk which is moving with speed $v_{disk}$. Assuming that the gas molecule attains the temperature of the disk $T_{disk}$ after the collision, what is the ...
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1answer
26 views

Transverse and Longitudinal waves in a wire

The question is : The speed of longitudinal wave is ten times the speed of transverse waves in a tight brass wire. If the Young's Modulus of the wire is Y, then strain in the wire is? I have read ...
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2answers
73 views

Showing the form of the covariant derivative of $\phi$, if $\phi$ transforms as the adjoint representation of $SU(n)$

I want to show that if $\phi$ transforms as the adjoint representation of SU(n), its covariant derivative is given by $\textbf{D}_\mu \phi = \partial_\mu \phi + i [\textbf{A}_\mu, \phi]$. (Exercise in ...
1
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1answer
75 views

Object on a banked curve

If an object is to move on a frictionless banked curve without skidding outwards or inwards, the object is provided centripetal force to maintain it in a circular track by the component of the normal ...
-1
votes
1answer
30 views

Homework problem on pulling a rotating cylinder wrapped with massless rope [closed]

I am trying to solve the following homework problem on mechanics. The solution I attempted is as follows. Equations of motion for cylinder: $F=ma $ $\rightarrow$ $100N=20kg \times a$ $\...
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1answer
33 views

Momentum and changed a changing mass

let there be a spaceship which is floating in space with a surface size of A. The spaceship floats in a constant speed of $v_0$, dust with density of $\rho$ gets stuck on the (only) surface of the ...
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2answers
43 views

Explain the effects of differences in the velocities of electromagnetic waves to different substances

It's a GCSE specification point and I cannot find notes or answers on the internet. Can someone help me out?
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1answer
26 views

Understanding spin & spatial components of wave-functions

I would like to understand the following statement: The $S$ and $P$ states can be expressed as products of the spin wavefunctions, $|+\rangle$ and $|-\rangle$, and the spatial wavefunctions, $|...
1
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0answers
63 views

If given $\overline {x^2}=0$, is $\bar x=0$? [migrated]

If $$\bar x=\frac{\int_V x\, \mathrm dv}{\int_V\, \mathrm dv} $$ and if given $\overline {x^2}=0$, is it necessary that $\bar x=0$?
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0answers
14 views

Finding Shear Force in a Beam

I have the following question: The question is asking for the shear force at point C. To work this out, I've determined the reaction force at point A. To find the shear force, the book then sums the ...
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0answers
26 views

Expand Helmholtz energy to fourth order in m (magnetisation)

The energy of a system of spins is given by: $$H=\frac{1}2\sum_{i,j=1}(J\sigma_i\sigma_j-h\sum\sigma_i)$$ I found that we can rewrite in terms of magnetisation, $m=\sum\sigma_i/N$: $$E=-N(\frac{Jm^2}...
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0answers
35 views

A. Zee Contour Integral

In A.Zee's book I have come a cross an integral which I found difficult to solve.
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1answer
41 views

Question regarding sound waves [closed]

An airplane is flying overhead at $150 \; \mathrm{m\,s^{-1}}$ whilst emitting an engine noise with frequency of 1 kHz. What frequency sound wave would a stationary observe hear once the airplane has ...
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3answers
41 views

Intensity of the resultant of two complex waves [on hold]

Suppose I have two waves: $Y_1= a_1e^{i(wt-kx1)}$ and $Y_2= a_2e^{i(wt-kx2)}$ I know by superposition $Y= Y_1+Y_2$ and intensity $(I) = |Y|² $ But how can I solve it. It seems hard for me to find the ...
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0answers
41 views

Please tell me I finally understand Gauss's Law (flustered for a couple days)

For the past few days I've been confused over using Gauss's Law to find the electric field of an infinite conducting sheet of charge. I believe I have figured out why my reasoning below is wrong, so ...
1
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1answer
31 views

Microcausality when quantizing the real scalar field with anticommutators

We know by the spin-statistics theorem that the real scalar field has to be canonically quantized by commutators. But if we try to use anticommutators, we would expand the field $$\phi(x)=\int\frac{d^...
3
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2answers
96 views

How to decelerate from velocity $v$ to stop time $t$ over distance $d$? [closed]

I'd be grateful for some help with this problem I am trying to solve. Let's say that I have an object travelling at a velocity $v$. I want that object to come to a halt in time $t$ AND travel exactly ...
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0answers
22 views

Magnification Factor

so I have to write a lab report about an experiment I did in class with using the equation for magnification factor = known length/measured length (this was used to find the length of a bridge ...
2
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1answer
57 views

LSZ reduction derivation Srednicki

In the derivation of the LSZ reduction formula equation (5.21) Srednicki claims that in case of an interaction term in the Lagrangian density $a^{\dagger}(\textbf{k})$ will no longer be time dependent....
3
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1answer
161 views

Testing the second type of equations of Lagrangian Mechanics

I'm new to classical mechanics and learned a new cool thing, second type equations of Lagrangian Mechanics. So, I was just testing if it really works or not. So, I made a question myself to test it, ...
3
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2answers
87 views

Spatial position and 3 momentum relation between particle states in canonical quantization (QFT)

In the beginning of the chapter on LSZ reduction, in Srednicki's book, he says that the operator $a_1 ^{\dagger}:=\int d^3k\: \text{exp}\Big[-(\textbf{k}-\textbf{k}_{1})^2/4 \sigma^2\Big] a^{\dagger}(\...
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0answers
32 views

Canonical ensemble: average value

I am having trouble with the following physics question. I have a quantum system of N distinguishable particles, whose energy is given by: $\varepsilon_{n}=n\Delta\qquad n=0,...,n_{0}$ I don't know ...
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0answers
35 views

A particle on a ring: orthogonality of eigenstates

Let us consider the quantum-mechanical problem---a particle on a ring of a circumference as $2\pi$ with a magnetic flux $A$ inserted through it: \begin{eqnarray} H=(-i\partial_\phi-A)^2, \end{...
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4answers
88 views

Rocket Leaving Earth's Gravitational Field

I stumbled across this seemingly simple question that really stumped me on further thought: A rocket is intended to leave the Earth's gravitational field. The fuel in its main engine is a little ...
2
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0answers
45 views

Time-independent and time-dependent perturbation theory yield different results

First, here's the problem statement. Suppose you have an infinite square well of length $a$, where the box extend from $x=0$ to $x=a$. At $t=0$, you add a perturbation $H'$ of the form: \begin{...
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0answers
42 views

Different clock rates near a black hole

We have two TVs on Earth that 2 users watch living in different time zones. For them, the TV shows happen at the same hour, they receive signal from the same source. they agree because their clocks ...
1
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1answer
44 views

Can the total energy-momentum of a rigidly accelerated extended body be expressed in terms of its mass, acceleration and velocity?

Suppose an extended body is rigidly accelerated, hence maintaining a constant mass and distance between its points in its rest frame. Is it possible to express its total energy in any frame in terms ...
1
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2answers
57 views

Parity in $\rho^0 \rightarrow \pi^0+\gamma$ decay

I am doing a homework question for $\rho^0 \rightarrow \pi^0+\gamma$ decay. It is given the $J^{PC}=1^{--}$ for the $\rho^0$ meson and that parity is conserved for this process. To calculate the ...
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votes
0answers
28 views

Schwinger oscillator model

How do we reach the equation 3.8 .10 I didnt understand how its second last step changes into equation 3.8.10