# 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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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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
1answer
76 views

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 ...
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?
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 ...
0answers
39 views

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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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^{...
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 (...
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 ...