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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 do I find the answer to these energy questions?

I have tried and struggled to solve these problems. I tried KEi-KEf for the bullet problem with no luck and I feel lost with part B. Any guidance would be appreciated.
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$\dot{W}_{lost}$ due to irreversible processes in a refrigeration unit

The condenser of a refrigeration system steadily admits $\dot{m} = 50 g/s$ of saturated R134a with a pressure $P_1 = 650kPa$ and a quality $x_1 = 0.9$. The condenser rejects $\dot{Q} = 7kW$ of heat to ...
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18 views

Confusion with entropy change associated with thermal reservoirs in an isobaric process

On chapter 8, Entropy, Zemansky and Dittman's book Heat and Thermodynamics has a problem with following details: A body of constant heat capacity $C_p$ and at a temperature $T_i$ is put in contact ...
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0answers
14 views

Dirac field local observables

This is actually a continuation of calculation I've been working on. It is well known that, in the case of Dirac fields $\psi(x)$, they satisfy anticommutatation relationships since they're fermionic ...
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1answer
24 views

Finding initial speed $v_0$ of a projectile given horizontal ranges measured at various launch angles

A spring-gun projects a small rock from the ground with speed $v_0$ at an angle $\theta_0$ above the ground. You have been asked to determine $v_0$. From the way the spring-gun is constructed, you ...
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1answer
46 views

Event Horizon & Black Holes

A spaceship just outside of the event horizon of a black hole emits a beam of light in a direction away from the black hole. What will happen to the light as it moves away from the black hole? a.) ...
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26 views

clouds float in atmosphere because of what? [on hold]

Q1)what is the reason that clouds float in the atmosphere? Q2)what is the factor that causes the clouds to float in atmosphere due to (a)low density (b)low viscosity (c)low temperature (d)low ...
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1answer
20 views

To find the acceleration of three blocks and the tension of strings if they are horizontally placed and pulled from the Right side

Find the Acceleration of blocks A, B and C, and tensions in the strings. All the blocks are of equal masses of 10kg. Also F = 60N. Let's name the leftmost block as A, middle one as B and Rightmost ...
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0answers
28 views

Radioactivity Numerical [on hold]

The half life of radium is 1600 years. What fraction of a sample of radium will be disintegrated after 6400 years? I know the answer is 15/16 and it came by (H.F)(power 4) but I would like to solve ...
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1answer
40 views

Dirac propagator causality

I was studying the Dirac propagator and came across an excelent article which includes all the derivation, and interestingly we can conclude that the anticommutator is zero for space-like intervals. ...
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33 views

Finding coordinate uncertainty of a particle

I'm given a one-dimensional particle of mass $m$ with "random uncorrelated" $\Delta x_o$ and $\Delta p_o$ and told to find the coordinate uncertainty at time $t$, $\Delta x_t$, then to find the ...
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1answer
31 views

Does bag size and volume affect heat loss rate? [on hold]

Same size of ice cream been placed in small insulation and big insulation bag respectively. Whixh bags of ice cream melts first? How can we explain about the correlation of bag size and melting rate?
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problem of spherical refracting surfaces [on hold]

A glass sphere has radius R=5cm and index of refraction 1.6 When viewed through the paperweight, how far away does the tabletop appear to be to the observer?
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0answers
33 views

Projection of a stone from the tower [on hold]

A stone falls from the top of the tower in 8 s. How much time does it take to cover the first quarter of the distance starting from the top?
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0answers
11 views

Force acting on 2 objects [on hold]

2 objects p and q of masses m and M respectively exert forces on each other, and there are no other forces on each other, the force F cause p to accelerate with acceleration a. What is the magnitude ...
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0answers
13 views

Calculating the time taken for a oscillator to get to a specific point? [on hold]

There is a simple harmonic oscillator with a mass of 0.5 kg, and a spring constant of 10 N/m and an amplitude of 0.03 m. How do I find at what time does the oscillator get to x=-0.01 m position if ...
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2answers
51 views

Has any terrestrial experiment been carried out which proves whether light can be thrown sideways or not?

Let me stress, TERRESTRIAL EXPERIMENT, as distinct from astronomical observations of the transverse Doppler effect, stellar aberration and so on. To clarify what I mean by 'thrown sideways'. ...
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Dynamics of string coupled pendulum

I have read that the equations of motion for a string coupled pendulum, as in this image: has equations of motion identical to a system of 2 pendula coupled by a spring: The dynamics of the spring ...
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0answers
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How to find the moment of inertia from the center of a rod, with two balls attached to it's ends? [on hold]

I tried, but I could not get the correct answer. Here are my attempts: - The inertia from the center of the rod disregarding the mass of the balls: $\ I_{rod} = \frac{1}{12}(m_{rod})(L_{rod})^2=\frac{...
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0answers
17 views

The minimum force required to move the block is [on hold]

My question : A block of mass 4 kg is placed to a rough vertical wall with a normal force of 100N.The coefficient of static and kinetic friction are 0.5 and 0.3 respectively. The minimum force ...
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0answers
27 views

Newton's Third Law bulldozer and pulley system [on hold]

A 3.74×104 kg bulldozer pushes a 9.35×103 kg boulder. If the wheels of the bulldozer push against the ground with 66800 N of force, what is the magnitude of the force between the boulder and the ...
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1answer
46 views

Electric Potential of Non-Uniformly Charged Infinite Plane

A little background: I was tutoring an undergrad upperclassman when we came to a problem that he had been assigned which I couldn't make heads or tails of - at least in terms of what was being ...
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2answers
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Simple electronic circuit analysis

I have to find a substitute for this circuit. (The substitution is on the right) The arrows in one circle are perfect voltage sources. The arrows in two circles are perfect current sources. It ...
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0answers
43 views

RG approach for KT-transition

Recently I have studied renormalization group (RG) approach for Kosterlitz-Thouless-transition (KT-transition) with help of "Condensed Matter Field Theory" by Altland. There is one important moment ...
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0answers
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Speed of the particle motion [on hold]

If time measurement unit and speed and distance measurement units are formed by us for our understanding, then i say that if i through a stone in air in any direction with speed 5km/hr, so the 5km/hr ...
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1answer
23 views

Electric flux of a cube with charge at the center

I've known gauss' law that any closed surface with some net charge inside, the electric flux would be q/ε0 , but some professor told us that a cube cannot be a gaussian surface and that the electric ...
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0answers
14 views

Cauchy problem for Hamilton-Jacobi equation

In Arnol'd V.I, "Mathematical methods of classical mechanics" p.257, I was asked to find a solution for the Cauchy problem $$H=\frac{p^2}{2},\ \ \ S_0=\frac{q^2}{2}$$ of the Hamilton-Jacobi equation ...
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1answer
27 views

Problem regarding finding work done

This problem is given in my physics book. How much work will be done if anyone wants to stack up 12 bricks? Given that each brick is $10 cm$ high and each brick's mass is $2 kg$. ($g=9.8$) Now in ...
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2answers
56 views

Derivation of quantum scattering of 1D square well

The above is the question. In my book, I'm given this formula for calculating the transmission probability (when E>V_nought): Here, k1 = sqrt(2m(E-V_0))/h_bar However, the problem is that this ...
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1answer
45 views

Find the minimum speed of a yo-yo, revolving in a vertical circle, so that the cord does not slacken

A yo-yo is swung with a constant speed in a vertical circle. If the yo-yo has a mass of 80 g and the radius of the circle is 1.5 m, find the minimum speed that this yo-yo must have at the top of the ...
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1answer
45 views

How might I write an operational definition for Vertical and/or Horizontal? [on hold]

WARNING: PLEASE DO NOT GIVE ME A DEFINITION, JUST TELL ME HOW I CAN DO THIS. I really don't want to have to rely on a forum website to tell me how I can solve my physics homework, but I honestly have ...
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0answers
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D-Field between plates of capacitor and its relationship to B

I need a little help with the below question. I just quite see how everything relates together and how to solve the question. I guess I just don't understand the relationship between H and D and how ...
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0answers
31 views

How fast would the Earth have to spin to lose the atmosphere?

I have read these questions: How fast would the Earth need to spin for us to feel weightless? Why can't we feel the Earth (or being in any non-inertial frame) rotating? And it made me curious....
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Question on physics homework (motion along a straight line) [on hold]

A question on my physics homework has me stumped. If anyone could point in the right direction or give me a formula, it would be much appreciated. The question is: A ball is thrown straight upward ...
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0answers
22 views

How is the fraction of a particular neutrino species calculated?

Suppose we assume that a neutrino is formed in the $\left| \nu_2 \right>$ eigenstate. Then, \begin{align} \left| \psi(x, t) \right> &= \left| \nu_2 \right> e^{-i\phi_2} \\ &= \left(...
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1answer
24 views

Electric field due to an infinite plane slab

So the solution says that in the x z plane, E = 0 by symmetry that is my problem, how is E = 0, maybe I can't visualize it correctly, however if I assume that that's really the case, I can continue ...
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2answers
56 views

If the commutator $[x,p]=i$, why does $[x,p^2]=2ip$?

According to Arfken et al. Mathematical Methods p.277 $$[x,p^2]=xp^2 - pxp + pxp -p^2x =[x,p]p + p[x,p]= 2ip \, .$$ According to the text this follows solely from $[x,p]=i$. I'm not understanding ...
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0answers
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For the quantities $Q$ below, find $σ Q$ in terms of variances and covariances of $u$ and/or $v$. The variables $a$ and $b$ always represent constants [on hold]

Error propagation is my downfall in labs. Please help me figure out the variance of the following quantities. For the quantities $Q$ below, find $\sigma Q$ in terms of the variances and covariances ...
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1answer
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Rotating table problem and problem solving methodology [on hold]

Here is the question 'A table with smooth horizontal surface is turning at an angular speed ω about its axis. A groove is made on the surface along the radius and a particle is gently placed at a ...
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0answers
12 views

Recoil energy Derivation [duplicate]

I am trying to calculate the recoil energy of the target nucleus A after getting scattered by a neutron, n in the lab frame. I have reached up to the point where I got this expression: $$P _ { n } ^ {...
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0answers
15 views

Photoelectric current [on hold]

5% energy of a monochromatic light source is converted into light of wavelength 5600 Å. If the power of the source be 100W then how many photons of light will be emitted by it per second?
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28 views

Neutron scattering off a target nucleus [on hold]

I am trying to calculate the recoil energy of the target nucleus A after getting scattered by a neutron, n in the lab frame. I have reached up to the point where I got this expression: $$P _ { n } ^ {...
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2answers
25 views

Tension in atwood machine [on hold]

Pretend you have an Atwood machine with a frictionless pulley system. On one end you have a very very heavy mass and on the other end, you have a basically negligible mass such that the combination of ...
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0answers
14 views

Resistance in a Network [on hold]

Five resistors are connected as shown. What is the equivalent resistance between A and B? I am having a hard time figuring out which branches are in parallel and how to find their net resistance.
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1answer
89 views

Silly question about kinematics and Christoffel symbols

An interresting "method" that allows you to know the acceleration vector with respect to any coordinate system is just a matter of recognize some key formulas. 1) Given the metric of a particlar line ...
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0answers
21 views

Energy of Landau Levels in a 3D Rectangular Slab

I'm currently going through Weinberg's Lectures on Quantum Mechanics, and I have some confusion as to where to start in one of the problems. In Chapter 10, Section 10.3, Weinberg goes through the ...
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1answer
59 views

A rotating solid bar in space [on hold]

Say there is a solid bar in space. Imagine it vertically, with the top point labeled $A$, and the bottom point labeled $B$ (these aren't points on the bar, but points in space between which the bar ...
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1answer
43 views

Why is the number of excited vibrational modes $g(\nu)d\nu$ proportional to $x^2e^{-x}$ in Debye's theory?

I come across a problem in Terrell Hill's "Introduction to statistical thermodynamics" saying that: In the Debye theory, the number of excited vibrational modes in the frequency range $\nu$ to $\nu+...
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0answers
23 views

Using Kirchhoff's laws to find current [duplicate]

There are four resistors, R1=25Ω,R2=15Ω,R3=40Ω, R4=20Ω that are connected to an element 12V as shown in the figure. Find the current in each resistor. $I_1=I_2+I_3$, where $I_1$ is the current ...