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 questions.

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1
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2answers
62 views

Rod sliding on a frictionless surface

A uniform rod$(m,l)$ is standing vertically on a horizontal frictionless surface. Gravity is downwards and uniform. I give its upper end a little push and off it goes. I want to find the Normal ...
0
votes
0answers
390 views

Nuclear reaction: creation of deuterium

A proton p collides with a neutron(at rest) n at relatively low-energies and creates a 'deuterium-core' d: $$p+n->d+\gamma$$ Find the wavelength for the photon as a function of the proton's ...
3
votes
1answer
134 views

Equations of motion of displacement field

We have an action: $$S[\boldsymbol{u}] = \frac{1}{2} \int dt \int d^3x \left\{ \mu (\frac{\partial u_{i}}{\partial t})^{2} - \nu (u_{ii})^{2} - \rho(u_{ij})^{2}\right\} $$ Where $u_{ij} = ...
1
vote
1answer
35 views

Circuits and Power

"The diagram below represents a simple circuit composed of 5 identical light bulbs and 2 flashlight batteries. Which bulb (or bulbs) would you expect to be the brightest? a) V only b) V and W ...
0
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0answers
38 views

Elongation of a beam [on hold]

I am trying to figure out the following question while I am studying for an exam that I have tomorrow. Given is a hanging aluminium beam with a cross section $A = 100 mm^2, \rho = 2780 kg/m^3, E = 72 ...
2
votes
1answer
175 views

Apparent size of objects under water

A friend of mine has a homework question and we're having some trouble figuring out what physical mechanisms come into play for this. An underwater swimmer sees a spherical air bubble that appears ...
3
votes
1answer
184 views

Homopolar motor and Lorentz force

My second grader thought making a homopolar motor for her science experiment would be fun. And, it was. Now I am trying to explain how it works and the Lorentz force. Please help me by giving me a ...
1
vote
1answer
58 views

Line of action force

The task is to reduce the two forces into a single net force and then find the equation of its line of action. I chose to find the net force and momentum with respect to the origin. The magnitude of ...
0
votes
1answer
30 views

Latent heat of vaporization

The molecular weight of water is 18.015 gram. The number of moles of water in one liter (1000 gram) will be: $3.34\times 10^{25}$ molecules (in 1kg). We know that latent heat of vaporization of water ...
2
votes
1answer
237 views

Spring system - 3 DoF system and its properties while changing stiffness

I was given the attached 3 degree of freedom spring system with the purpose of analyzing it. I came up with the following equation of motion and then I ran Matlab to calculate the corresponding ...
0
votes
0answers
21 views

What is the frictional coefficient between blocks? [closed]

1) 30 N 2) 27 N 3) 25 N 4) 48 N This question came in one exam. I have many confusions : 1) How the question is talking about minimum force? If I apply very large force then obviously they won't ...
0
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2answers
57 views

How is $\text{PE} = qV$ derived using knowledge of $F = \frac{q_1q_2}{4\pi \epsilon_0 r^2}$

I recently started grade 11, and this concept confuses me, since if we place a charge $q$ on a potential $V$, I do not completely comprehend what values would need to be substituted which would give ...
0
votes
1answer
27 views

Expectation value of Hamiltonian on number state [closed]

Hamiltonian is defined by $H_I = \hbar \omega (\hat{a}^+ \hat{a} + 1/2)$ What is the expectation value of the energy on the number state $$\vert \psi \rangle = \frac{1}{\sqrt{2}} ( \vert 1 \rangle ...
0
votes
1answer
34 views

Ball thrown in a moving train [duplicate]

A ball is thrown upward in a train moving with a constant velocity. Where will it land? My intuition tells me that the ball will fall at my back. But my book says that it will return back to the ...
0
votes
1answer
64 views

How to derive the commutation relationship between $\hat{L}^2$ and $\hat{\textbf{p}}$ [closed]

How to prove that $$[\hat{L}^2,\hat{\textbf{p}}] = i\hbar(\hat{\textbf{p}}\times\hat{\textbf{L}} - \hat{\textbf{L}} \times \hat{\textbf{p}})$$ I tried to expand $\hat{L}^2$: ...
-5
votes
0answers
39 views

VERY VERY IMPORTANT [closed]

A proton moves perpendicular to a uniform magnetic field at 107ms-1 in the +X direction and experiences an acceleration 2x1013ms-2 when it velocity is in the +Z direction. Determine the magnitude and ...
0
votes
1answer
75 views

One problem on Classical Mechanics [closed]

I'm wondering why in this problem I can't apply newton second law? The mass $m$ of the moving part will be the mass solved in the problem.
0
votes
0answers
67 views

Vapor pressure formula [closed]

Can anyone please help me work out the formula for getting the vapor pressure given: air temp, air temp wet and air pressure (wet baro)? These where entered into a DOS program and know one remembers ...
0
votes
2answers
142 views

What is the Reduced Density Matrix?

The difference between pure and mixed states is the difference in their density matrix structure. For density matrix $\rho$ of mixed state the trace of $\rho^{2}$ should be less than 1. For pure ...
0
votes
0answers
12 views

Self-inductance of a toroidal inductor

I am trying to determine the self-inductance of a toroidal coil of mean radius $R$ with $N$ loops of radius $a$ with a current $I$ flowing within them. I have calculated the magnetic field by noting ...
-1
votes
0answers
17 views

kinematics projectile motion [closed]

A particle is projected vertically upwards from O with velocity v and a second particle is projected at the same instant from P(at a height h above O) with a velocity v at an angle of projection ...
0
votes
1answer
34 views

A pearl that moves in a smooth vertical hoop

I wanted to ask about the situation of a pearl that moves in a smooth vertical hoop in circular motion as described in the following sketch. According to a simulation found in the internet , a ...
-2
votes
1answer
41 views

Question on kinematics [closed]

A particle is projected vertically upwards from point $A$ on the ground. It takes $T_1$ time to reach point $B$ but it still continues to move up. If it takes further $T_2$ time to reach ground from ...
1
vote
2answers
112 views

Questions on redshift [closed]

What are different kinds of redshifts and are they related to each other? If yes, how? Is redshift a measure of distance or time or etc? For example when we say a supernova with a redshift of $z=0.9$ ...
3
votes
2answers
272 views

Restriction of a Lagrangian

I'm wondering if anyone could help me with the following questions. Let $M$ be the Minkowski spacetime, given $f\in C^{\infty}(M) ; f(m)=x^{0}(m)$, with $\{x^{\mu}\}$ being a global Cartesian ...
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votes
0answers
26 views

Electric Potential on the vertex of a triangle? [closed]

In the topic of electrostatic energy and Electric Potential, master asked us to solve the following problem. Problem: We have a right-angled triangle with charge density of ρs. Find the potential ...
0
votes
0answers
11 views

Do I ever not account for temperature variation from wall temperature in pipe flow?

Let's say I have internal pipe flow with a constant wall temperature. If I evaluate the properties at either the mean inlet temperature of the average of the inlet and outlet, do I always have to ...
1
vote
1answer
37 views

How to calculate distance travelled from velocity vector and angle?

I am trying to get the distance a projectile travels, so I can display it in my program. I'm sort of new to physics, but I've tried looking this up. I'm not getting the right results however, and me ...
1
vote
3answers
229 views

Two dimensional elastic collisions with varying angle of incident

If in an elastic collision I know all initial values and that mass for each object remains constant throughout the collision (but different from one another) how can I determine their final velocity ...
3
votes
0answers
28 views

How much weight is lifted from each side? [closed]

I don't know the math to do this, so I am asking here if someone can work this out with all of the details I'm providing. Total mass EST.: 2,400 lbs. Length from front to back: 14 feet 6 inches. ...
-1
votes
0answers
13 views

Falling rod on a frictional surface [duplicate]

Suppose there is a rod balancing on a table.the rod tips vertical and begins to fall. derive an equations of normal force and friction force on the bottom as a function of angle between the rod and ...
0
votes
0answers
21 views

Collision of two body [closed]

A body which had a mass M attached one end of the spring which had a spring constant k. After that, a ball with mass m collide M with vo velocity and theta angle. If coefficient of restitution was e , ...
0
votes
1answer
215 views

Coordinate system

Quoting from 'Nuclear Physics - Theory and Experiment' by RR Roy, BP Nigam 2005 edition Link to text How did the author arrive at equations (23a, 23b,23c)? Chapter 8 Nuclear model II, 8.7 ...
0
votes
1answer
34 views

A pearl that moves in a smooth vertical hoop (Circular motion) [closed]

I couldn't understand something about the situation of a pearl that moves in a smooth vertical hoop in circular motion. When the normal force equals 0 , the pearl didn't disconnect from the smooth ...
0
votes
1answer
36 views

Essay topic in classical electrodynamics [closed]

I am currently in a course of classical electrodynamics, and we have discussed Lorentz transformations and such. We need to write an essay on a free to choose topic, but I have no idea what an ...
0
votes
0answers
97 views

Why is momentum not conserved? [closed]

A uniform free disc in rotating in a horizontal plane ($m,R,\omega$). The surface is frictionless. A point mass of $2m$ is dropped on it from a height of $5m$ vertically at distance of $\frac R ...
2
votes
1answer
80 views

What the heck is negative effective mass?

I am reading this book:Solid State Electronic Devices by Ben G Streetman and Sanjay Kumar Banerjee. I have some doubts in the article 3.2.2 Effective mass. In this the aythors say that ...
2
votes
1answer
195 views

Given wave function at $t=0$, what is the process of deriving time dependent wave equation?

Suppose $$\Psi (x, t=0)=Ae^{i\alpha _1}\psi _1(x)+Be^{i\alpha _2}\psi_2(x)+Ce^{i\alpha _3}\psi_3(x).$$ If $\psi _n$ are the energy eigenfunctions how would I derive $\Psi (x,t)$? I am having trouble ...
1
vote
0answers
57 views

Restrained double pendulum

The equations of motion of a double pendulum are well-known. Usually you'd have the them expressed in the rotations $\theta_1(t)$ and $\theta_2(t)$. There are two degrees of freedom. Now consider the ...
0
votes
1answer
36 views

Calculating electric potential from a changing electric field

Assuming that I calculated the electric field in a single point between a uniform charged positive sphere and an infinite long wire charged positive uniformly. Now, I want to calculate the velocity of ...
1
vote
3answers
173 views

Distinguishable, Indistinguishable Paramagnetic Ideal Gas

In the canonical ensemble, the partition function for an ideal gas is given by: $$\frac{Z}{N!}$$ The factor $N!$ accounts for the indistinguishability of the particles of the ideal gas. What ...
0
votes
0answers
16 views

Grover search algorithm for more than one marked elements [closed]

Grover search algorithm is a powerful tool for unstructured database search purposes. The two operations (Phase inversion and Inversion about the mean ) join hands to give the marked needle. I was ...
0
votes
2answers
32 views

Molecular mean free path probability

Let's let $p(\xi)$ be the probability a molecule travels at least $\xi$ between collisions, lets say $\xi=0.01$. When I think of this statement, I think it it is the probability the molecule is able ...
0
votes
2answers
213 views

How do you calculate the change in temperature of an adiabatic system?

I really don't even know how to start doing this problem. I understand PV=nRT and I understand that if either T, V, or P is kept constant, then the other 2 change at the same rate, and I understand ...
0
votes
0answers
34 views

Trace of Spin Products Using Rotation operators

So I am working on this problem involving the trace of spin matrices. I am trying to solve for $Tr(S_iS_j S_kS_l)$ using rotation operators. I know how to go about solving for $Tr(S_iS_jS_k)$. It is ...
2
votes
0answers
95 views

How to understand the transmission coefficient from the following question?

I want to understand the transmission coefficient and construct a time-independent Schrodinger equation where $$ V(x)=\left\{ \begin{array}{c c} \delta(x), & |x| < 1 \\ + \infty, & ...
3
votes
1answer
512 views

Transmission and reflection

What is the transmission amplitude of a wavefunction $\phi(x)=e^{ikx}(\tanh x -ik)$? I would have thought that it is $\tanh x -ik$ since this is the factor associated with the forward travelling ...
0
votes
1answer
55 views

Total momentum in linear monoatomic chain

Context: Solid state physics. Monoatomic linear chain. Question: To prove that the total momentum of the chain is zero. Attempted solution: I consider the sum: \begin{align*} p = \sum_{n=1}^{N} m ...
-2
votes
0answers
19 views

Kinetic Energy and Force of Spring [closed]

Two blocks, of masses M = 2.3 kg and 2M are connected to a spring of spring constant k = 180 N/m that has one end fixed, as shown in the figure below. The horizontal surface and the pulley are ...
0
votes
3answers
110 views

Time dilation in special relativity

Suppose a star ship is moving with some velocity. Two light pulses one in direction similar to star ship another opposite to it is shot towards the space ship. Then how time inside space ship adjust ...