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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6
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3answers
82 views

Electric Field vs Distance (Contradiction)

There was a text book question in my physics class this morning on what happens to the strength of an electric field $E$, if its distance between two plates decreases by 2 fold. My initial thought ...
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1answer
59 views

EM-Wave: Calculate magnetic field $H$ from electric field $E$

In an exercise I am supposed to calculate the magnetic field from the electric field for a plane, harmonic wave in vacuum. $$\vec{E} = - E_0 \cdot \sin(\omega t - k z) \cdot \vec{e_y}$$ Using the law ...
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0answers
32 views

Gaussian unitary transformations

My reference (Quantum Continuous Variables / Serafini, Alessio) says "It should be intuitively clear that a unitary transformation sends all Gaussian states into Gaussian states if and only if it ...
2
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2answers
68 views

A question about a small block sliding down a larger one, involving conservation of momentum and energy [closed]

I have been working on a problem for some time but haven't found a solution yet. I am mainly looking for a hint that will help me find a breakthrough and not a full solution. Here is the statement of ...
0
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2answers
34 views

Direction of Vectors in Net Force Questions

While revising mechanics, I stumbled upon the following question: To facilitate comprehension of my question, I would like to present my working as follows: The net Force must be zero if the skier ...
2
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1answer
48 views

Trace distance expressed with maximization of probabilities

I'm studying chapter 9 of Nielsen and Chuang, where trace distance is defined by equation (9.1) $D(p_x,q_x)=\dfrac{1}{2}\sum_{x}|p_x-q_x|$ and later on the next page (also asked to be proven in ...
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0answers
15 views

Conformal mappings in electrostatics applied to an infinite conducting fence in a uniform electric field

A long straight fence made of conducting wire mesh separates two fields and stands one metre high. Sometimes, on fine days, there is a vertical electric field over flat open countryside. Well away ...
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2answers
77 views

Coefficient of friction in pulley system

EDIT: Picture doesn't show the mass of $\mathrm A$. It is $60\ \mathrm{kg}$. My attempt: I placed my coordinate system with $x$-axis parallel to the ramp surface and $y$-axis parallel to the normal ...
3
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1answer
76 views

BRST quantization: Explicit computation request

Following Green, Schwarz and Witten's book on Superstrings, the BRST charge is given by $$Q_B = c^i K_i-\frac{1}{2}f_{ij}^{~~~k}c^ic^jb_k\tag{3.2.4}$$ with $$[K_i, K_j] = f_{ij}^{~~~k}K_k,\tag{3.2.1}...
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0answers
21 views

Find electron-hole recombination rate? [closed]

In an n-type semiconductor, the thermal equilibrium electron concentration (n0=1015/cm3) and the intrinsic carrier concentration (ni=1010/cm3). The mean life time of minority carriers is 1us. The ...
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1answer
46 views

What's the difference between shock waves and acoustic waves?

What's the difference between shock waves and acoustic waves? I tried and searched around this subject, but I could not find any relative article about it. Please help me find a proper answer.
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0answers
44 views

What is the meaning of $\tau$=$Gb/l$

What is the physical meaning or intuition of the Shear Stress vs Shear Modulus and Burger Vector over length relation($\tau$=$Gb/l$). I have seen this formula being used to determine length in case ...
0
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1answer
22 views

Measuring small changes in length using the micrometer screw gauge

Which of the following method/instrument cannot be used to measure small changes of the order of a millimeter occurring in a length of about 50cm. 1)spherometer 2)travelling microscope 3)meter ...
0
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1answer
67 views

Free fall time after being accelerated [closed]

An elevator car whose floor-to-ceiling distance is equal to $2.7m$ starts ascending with constant acceleration $1.2 m/s^2$; $2.0 s$ after the start a bolt begins falling from the ceiling of the car. ...
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0answers
51 views

A charge accelerating while confined to an equipotential surface

Suppose I had the following in Cartesian $(x,y,z)$ space, where $x$ is horizontal, $y$ is vertical, and $z$ is depth: A negligibly thin conductive rod extending from $(0,1,0)$ to $(0,+\infty,0)$. An ...
0
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1answer
35 views

What is the relation between Earth's orbit and orbit of another planet around Sun? [closed]

Lately, I come across the following problem, "The maximum and minimum speeds of a comet that orbits the Sun are 80 and 10 km/s respectively. The ratio of the aphelion distance of the comet to the ...
0
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1answer
36 views

Scale factor for flat universe filled with radiation and cosmological constant [closed]

I am trying to solve problem 1.20 in the book 'Physical Foundations of Cosmology' from Mukhanov. In the problem, we should show that the scale factor $a$ for a flat universe filled with cosmological ...
1
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1answer
43 views

How can we see the microstructure of steel samples at room temperature?

When we intend to see the microstructure of a steel sample at a temperature say 950 $^\circ$C they say we quench it in order to freeze the microstructure. However, if we quench it are we not going to ...
0
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1answer
35 views

How to express the $Q$-value for alpha decay in terms of binding energy of the relevant species?

I have 2 questions regarding the solution to the following question: The solution states that: Mathematically, I understand how the end result $$Q=BE\left({}^{A-4}_{Z-2}\mathrm{Y}\right)+...
0
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1answer
25 views

Avarage Speed conventional formula VS Mean Value of Multiple Speeds [closed]

Here the basic problem: The motion of an extraordinarily jubilant bird flying straight up and down is given by the graph below, which shows the vertical position y as a function of time t. What was ...
-2
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3answers
31 views

Why we add acceleration of celestial body with acceleration of spaceship to determine force exerted by space ship on astronaut?

The acceleration due to gravity acts in the downward direction, and a spaceship accelerates in an upward direction. Therefore, why do we add the accelerations together instead of subtracting them?
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2answers
53 views

Solved problem on radioactive decay

I encountered a question that was given the disintegration rate of a radio active source measured at the interval of four minute. The source is consisted of two types of radio nucleus and the half ...
0
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0answers
39 views

Anticommutation relation of Grassmann numbers

Let $c,c^*$ be the fermion annihilation/creation operators and $\xi,\xi^*$ denote Grasssmann numbers where $$|\xi\rangle = \exp(-\xi a^*)|0\rangle$$ is the coherent state. Then why is it true that $$ \...
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2answers
26 views

Doppler shift moving parallel to wall [closed]

Suppose you are screaming and running in an area parallel to a wall. What would be the frequency you hear considering that you are getting away from the part you've passed and getting close the parts ...
1
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0answers
38 views

Relativistic Aberration Formula Derivation: looking for a hint

The literature on relativistic beaming / doppler beaming is scarce, so I hope the community can shed some "light" on this topic. I want to derive a formula for the phenomenon known as relativistic ...
0
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2answers
55 views

Contracting Riemann Tensor Troubles

It has been several years since I looked at General relativity, and I am trying to brush up on it because it was always interesting and I am in need of it for my research. Specifically, I am looking ...
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0answers
40 views

Finding the temperature of a gas using the $P$-$V$ diagram [closed]

I need to know a way to relate the P-V curve of an ideal gas to its temperature.I found a similar question on stackexchange but couldn't understand how the relationship between a point and the ...
0
votes
1answer
62 views

How to “transfer” indices from dot product to metric?

In this source, the author (Andrzej Pokraka, Solutions to problems from Peskin & Schroeder) is computing an integral related to scalar QED. In the step where the equation is labelled (29), the ...
0
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2answers
78 views

Why is $\theta=\beta/2?$ - Kinematics [closed]

An aircraft is flying horizontally in a circle of radius $b$ with constant speed $u$ at an altitude $h$. A radar tracking unit is located at $C$. Write expressions for the components of the ...
-1
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1answer
61 views

How to calculate $\langle0|S(b) S(b) |0\rangle $ where $S(b)$ is the squeeze operator? [closed]

How to calculate $\langle0|S(b) S(b) |0\rangle $ where $S(b)$ is the squeeze operator
1
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2answers
60 views

Question related to maximum static friction and minimum $\mu_s$

I think I have the right formulas the answer the following question, but there is something conceptual that is throwing me off (see end of question). A body with mass $m$ is located on a slope with ...
0
votes
1answer
45 views

Interaction between various forces, by which one is to calculate an angle? [closed]

I am new to Newtonian mechanics, and was wondering about the following question. I basically have the ingredients for the answer, but I cannot seem to find the way to put this together into one ...
2
votes
1answer
42 views

Fictitious forces in relation to normal force

I am new to Newtonian mechanics, and was wondering regarding the following: The system pictured below is moving at acceleration $a$ towards the left. The mass of the ball is $m$, and there is no ...
0
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1answer
39 views

How to solve this boundary-value problem in electrostatics? [closed]

I was reading The Feynman Lectures on Physics, Vol. 2 and came across the following on page 7-1. There are a few problems for which Eq. (7.1) can be solved directly. For example, the problem of a ...
0
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1answer
63 views

A Question About $4$-Spinor Contractions

Let $f_{abc}$ be a constant which is totally anti-symmetric with respect to indices $a$, $b$ and $c$. Let $\psi^{a}$, $\psi^{b}$, $\psi^{c}$ and $\epsilon$ be Grassmann-valued Majorana fermions. How ...
1
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1answer
68 views

Juggling a ball

I have the following exercise: Explain the progress of the force you need to juggle a $5\ \rm{kg}$ ball as a function of time. Now since the question is kind of imprecise to me, I have problems ...
-1
votes
2answers
52 views

Rate of change of area of a circular loop exiting a uniform magnetic field

The question is to find the variation in induced electromotive force w.r.t to time or put simply $\varepsilon(t)$ for a circular loop being removed from the region of the magnetic field at a constant ...
0
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3answers
49 views

Inclined plane with friction and a spring

I am given a body with a mass of 1 kg that slides at a constant speed down an inclined plane with an angle of 30 degrees. At the end of the incline there is a spring with a spring constant of 500 N/m. ...
1
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3answers
46 views

Why Didn't the Energy-Momentum Relation Work?

I am currently studying for the GRE Physics subject test by working through published past tests. My question is about problem 20 from the test GR8677: A positive kaon ($K^{{}+{}}$) has a rest ...
1
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1answer
77 views

Probability distribution of the overlap between two random quantum states in an $n$-dimensional Hilbert space? [closed]

Let two pure states $|\Psi\rangle$ and $|\Phi\rangle$ be drawn uniformly and independently from an n-dimensional Hilbert space $\mathbb{H}^n$. (see Note). What is the probability distribution of ...
1
vote
1answer
76 views

Perturbation expansion with path integrals

This is from Hugh Osborn's 'Advanced Quantum Field Theory' notes, Lent 2013, page 15. I want to evaluate the expression $$ Z = \exp\Big(\frac{1}{2} \frac{\partial}{\partial \underline{x}} . A^{-1} \...
0
votes
1answer
37 views

PDE from Hamiltonian density

For the wave equation Hamiltonian density is $2H=\phi_t^2+\phi_x^2$ while the Lagrangian density is $2L=\phi_t^2-\phi_x^2$. I can easily compute the pde from the Lagrangian density but how does one do ...
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0answers
11 views

How are the two Stacking Faults namely intrinsic(formed by vacancy agglomeration) and extrinsic(formed by interstitial agglomeration) different?

Stacking Faults and its association with partial dislocations seem to bother me day in and day out.With regard to this I do not understand how Partial dislocations lead to stacking faults and not the ...
0
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1answer
61 views

What is the speed of air molecule?

Given the minimal temperature measured in the Earth's atmosphere is not $0\space K$, air molecules are moving. I see no reason for air molecule to move all at the same speed. The question is ...
1
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2answers
47 views

Minimum and Maximum values of Effective Resistance [closed]

Assume we have resistors of resistances $R_1$, $R_2$, $\ldots$ , $R_n$. Let these resistances be connected in a network. Are the following statements true? $(i)$ Effective Resistance is maximum when ...
1
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0answers
30 views

Current associated with the orbital angular momentum

Problem: For a 3d electron in an external magnetic field of 2.50×10−3 T, find (a) the current associated with the orbital angular momentum, and (b) the maximum torque. Attempt: because ...
0
votes
1answer
75 views

An exploding projectile [closed]

A mass $3m$ is thrown with a velocity of $20\sqrt3$ m/s at an angle of $30^{\circ}$ to the horizontal from a point, say $A$, on level ground. After reaching a certain height, $h$, it explodes into two ...
1
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0answers
29 views

MCS Lagrangian and Euler-Lagrange

I'm trying to solve the Euler-Lagrange equation for the MCS Lagrangian density as given by Kharzeev in this article (Eqn. 7): $$ \mathcal{L}_{\textrm{MCS}} = -\frac{1}{4}F^{\mu\nu}F_{\mu\nu}-A_\mu J^{...
0
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2answers
41 views

Static friction in a slope and two boxes problem

The context: Calculate the string tension in the situation where two boxes are stacked on a slope and moving with uniform velocity, given that the kinetic friction coefficient between the heavier (...
1
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2answers
95 views

Chain slipping off a sphere - Mechanics problem

A uniform flexible chain of length l rests on a fixed smooth sphere of radius R such that one end A of the chain is at top of the sphere while the other end B is hanging freely. The chain is held ...