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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1
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2answers
25 views

Problem in finding the moment of inertia of a circular disc with axis at the periphery using integration [on hold]

Q: Find out the moment of inertia of a uniform circular disc of radius $r$ & mass $M$ & the axis passes through a point on the circumference. My attempt: Let the axis passes through $O$ on ...
0
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0answers
6 views

How can I find the magnetic vector potential of an infinite cylindrical wire

I'm struggling with an exercise that asks me to find the magnetic vector potential $\vec{A}$ inside of an infinite cylindrical wire of radius $R$ given that the current per unit area ...
0
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0answers
18 views

Finding the magnetic field of an infinite cylindrical wire

An infinite cylindrical wire of radius $R$carries a current per unit area $\vec{J}$ which varies with the distance from the axis as $J(s)=ks^2\hat{z}$ for $0<s<R$ and zero otherwise where k is a ...
0
votes
1answer
30 views

Newton's law of resistance

Anyone can give me some clues on how this can be proved? What kind of drag force is used? A particle is projected vertically upward with an initial speed $v_0$ near the earth's surface. Show that ...
0
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1answer
23 views

Cosmic Calendar [on hold]

So for an assignment I was given a event/ occurrence and was asked to find its cosmic date/ time. Here are a few examples of ones it asked for, I was wondering if there was a formula to find the date ...
0
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0answers
11 views

Determination of Ricci tensor and Scalar curvature from vielbeins [migrated]

Consider the following metric: $ds^2=h(r)\bigg(dr^2+r^2\big(d\theta^2+\sin^2\theta ~d\phi^2+(d\psi+\cos\theta ~d\phi)^2\big)\bigg)$ We can try to compute the Ricci scalar of this metric by using ...
0
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0answers
20 views

Particle moving in response to central force

So I have this question: There is a particle moving in response to a central force per unit mass of $$F(r) = {\alpha\over r^2} + {\beta\over r^3}$$ where $\alpha$ and $\beta$ are constants. ...
1
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0answers
18 views

Calculating the age of the Universe at temperature 1 MeV [duplicate]

I've been told calculate the age of the Universe at $T = 1 \, \text{MeV}$, assuming that $a(t=0)=0$. Now, I've already calculated the value of $H$ at that temperature, which was around ...
0
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0answers
31 views

Stability of a system

What do we mean by "stability of a system " in a Lagrangian of a model? Let's say that we have a very simple model like this $$L=\frac{1}{2}K_{ij}\dot{q}_i\dot{q}_j-\frac{1}{2}F_{ij}q_iq_j,$$ where ...
0
votes
1answer
52 views

Why does accounting for direction give the wrong result for a bouncing ball?

This a conservation of momentum problem, gathered from an old textbook. I thought it would be simple, but I seem to be goofing up somewhere in my basic conceptual understanding. PROBLEM: A 1.0 kg ...
0
votes
1answer
31 views

Finding the potential energy of a geosynchronous satellite above earth [on hold]

I am trying to find the gravitational potential energy of a 1kg geostationary satellite above the earth. So far: $E=mgh$, and $g=GM/r^2$ so substitution of $g$ into the energy equation gives: ...
0
votes
1answer
35 views

Eigen-state in a coupled Boson system [on hold]

What's the eigenstate of the coupled system like $$ H=a^{\dagger}b+ab^{\dagger} $$ The system is Boson system and $a,b$ are annihilation operators of fock states and they are commute. In my opinion, ...
0
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0answers
26 views

3D harmonic oscillator problem [on hold]

I'm stuck with this problem, that says Find the angular moment $l $ that can be measured for the energy state $5/2 \hbar \omega$, providing the probability of each value. Also build the wave ...
-1
votes
0answers
14 views

Different force vectors applied on a pulley [on hold]

All the pulleys are ideal. String is massless. Rate of work done by gravity at the given instant is (-x * 10^2) W, then calculate 'x' (refer to the diagram). By 10^2, I mean 10-squared.
1
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0answers
37 views

Calculate Helmholtz Free Energy with Entropy, Work given [on hold]

it's my first time here and I hope the post complies with the general rules. My problem originates here: I'm doing a statistical physics task which unfortunately leaves me clueless atm. I keep my ...
0
votes
0answers
50 views

Show that there exists a Lorentz transformation such that two events are happening at the same time? [on hold]

I'm havin problems with the following exercise in a theoretical physics course: Show that there always exists a Lorentz transformation such that the two events $P$ and $P^0$ in the figure are ...
0
votes
0answers
32 views

How to find the Kinetic energy of a quarter of a wheel? [on hold]

A wheel of mass 'm' and radius 'R' is rolling on a level road at a linear speed 'V'. What is the kinetic energy of the upper right quarter part of the wheel , considering the wheel to be of the ...
0
votes
0answers
19 views

Calculating acceleration en velocity up a slope [on hold]

let's say you have a object at rest. This object weighs 300 gram. The object here needs to get up a slope of 30 degrees that's 1 meter long. It needs to get get to the top in 4 seconds. Thing is it ...
2
votes
2answers
48 views

Derivation of velocities in the Coriolis force

In Fitzpatrick's Newtonian Dynamics book on the Coriolis force, he states \begin{align} v_{x'}&\simeq V_0\cos\theta-2\Omega t V_0\sin\lambda~\sin\theta \tag{433}\\ ...
0
votes
0answers
43 views

Computing the Ricci Tensor for a Spherically Symmetric Spacetime

For a homework question, we are given the metric $$ds^2=dt^2-\frac{2m}{F}dr^2-F^2d\Omega^2\ ,$$ where F is some nasty function of $r$ and $t$. We're asked to then show that this satisfies the Field ...
0
votes
0answers
28 views

Variation of Lagrangian expression [on hold]

Can someone please show me how to vary the Lagrangian below with respect to $h_μν$? It is supposed to lead to the linearized Einstein tensor as shown in Sean Carroll's book on Spacetime Geometry 2004 ...
0
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1answer
26 views

Falling dominoes

I have been attempting to determine the maximum velocity a line of dominoes can reach. I have found that there are two forces which act upon it: Initial impulse and gravitational force. As kinetic ...
0
votes
2answers
23 views

Entropy change in irreversible heat flow

For an irreversible heat flow from an object $A$ at temperature $T_A$ and another object $B$ at temperature $T_B < T_A$ , I'd like to know how to evaluate the change in entropy using the following ...
1
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2answers
47 views

Deriving the particular solution for a damped driven harmonic oscillator [on hold]

Consider a damped driven harmonic oscillator, for which $\beta = \omega_0/4$ and the driving force is given by $F = F_0\cos\omega t$ ($\omega_0$ and $F_0$ represent initial condition of those ...
0
votes
0answers
31 views

Symmetry Group of system to a given Hamiltonian

I want to determine the symmetry group of the following system: I consider a charged particle in a spherically symmetric potential $V$ and a homogeneous electric field of magnitude $E$ in ...
-3
votes
0answers
23 views

Why, Brush may start moving when placed on a vibrating horizontal surface? [on hold]

A brush may start moving when placed on a vibrating horizontal surface. Investigate the motion.
-1
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0answers
20 views

Projectile motion with respect to integrating equation of motion [on hold]

Explain projectile motion with the help of "integrating equation of motion"
1
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4answers
90 views

Rotation systems. Problem interpreting an equation

In this equation: Why is $(\frac{d}{dt})_r \bf{\Omega} \times \bf{r}=\frac{d\bf{\Omega}}{dt}\times\bf{r}+\bf{\Omega}\times \bf{V_r}$ In particular, I have qualms with the term ...
0
votes
0answers
29 views

Probabilities with a qubit

A two-state quantum system has orthonormal energy eigenstates ψ1 and ψ2, with energy eigenvalues E1 and E2 = E1 + ∆E (∆E > 0). These energy eigenstates form a complete set of wavefunctions for the ...
1
vote
1answer
37 views

Why is cross section inversely proportional to wavelength for interstellar scattering?

The following problem was part of a homework for my Cosmology class: Compare the probability of interstellar scattering of photons of yellow light (5000 angstroms) and 50 micron infra-red light. ...
0
votes
1answer
44 views

Finding Lagrangian with Non-holonomic constraints

I am stuck working on a problem that involves finding the Lagrangian for a free particle constrained to move on the surface of a disk of radius $a$. The particle collides elastically with the edge of ...
0
votes
2answers
43 views

What unit would the answer be in?

So I have used this formula: $$W= PV_1 \times \ln\frac{V_2}{V_1}$$ and I have converted my values to cubic meters and pascals. So Work Done, $W$, what would be the unit for that answer? I already ...
0
votes
1answer
24 views

Scale factor and energy density

I'm looking at some solutions of the Friedmann equation at this website: Solutions to Friedmann equation If I look at the first problem (Problem 1: matter or radiation in a flat Universe) and click ...
0
votes
0answers
33 views

Radially free falling observer [on hold]

I am a student. i read in a paper for a radially free falling observer in a Schwarzschild metric, the relation between the Unruh vacuum state and the proper time when the observer crosses the event ...
-2
votes
0answers
20 views

How to write Laplace equation for this problem? [on hold]

How to write Laplace equation in below example. we have two plate with surface S,If we have charge surface density in boundary,obtain potential between two plate. explicitly explain for me how to ...
0
votes
0answers
15 views

Postion of a cylinder rolling down an inclined plane [on hold]

I know that the acceleration of a uniform cylinder of known mass rolling down an inclined plane WITHOUT slipping with an inclination of $\theta$ is: $\frac23 g \sin\theta$. Suppose this term is called ...
0
votes
0answers
38 views

Green function integration problem [on hold]

When I'm trying to find the Green Function of Helmholtz equation for a cube $0≤x,y,z≤L$ $$\nabla^2u+k^2u=\delta(\vec{x}-\vec{x}')$$ where $u=0$ on the surface. I set to find the green function where ...
1
vote
1answer
27 views

Finding the potential between two spherical shells [on hold]

How to find the potential in region $a<r<b$ I know that the general solution for Laplace's equation is $$V(r,\theta)=\sum_{l=0} \left[A_l r^l +\frac{B_l}{r^{l+1}} \right]P_l(\cos{\theta}).$$ ...
0
votes
0answers
21 views

Capillary action height

How to calculate height of the liquid (let's say it's water) in the capillary (so height of capillary action) if you only have r given, r = 0.3 mm - r is radius of capillary. The equation I would use ...
0
votes
0answers
18 views

Equal volumetric rate versus area open to flow [on hold]

A vertical water disposal well has been drilled and cased. There are two horizontal rock formations that the wellbore intersects and these formations will be targeted for water disposal. To allow ...
0
votes
0answers
45 views

Does the magnification equation help you determine the type of eyeglass lenses? [on hold]

While picking up a pair of eye glasses dropped by a friend, you notice that they form an inverted image of the background and that the image is stretched horizontally as well. Your friend suffers ...
0
votes
1answer
30 views

Comparing voltages of Parallel and Series circuits

There are two electric circuits. Resistor 1 ($R_1$) connected with resistor 2 ($R_2$) by series circuit $R_1$ connected with $R_2$ in parallel Each of the circuit has battery with emf and the ...
0
votes
0answers
35 views

Inverse of a 3x3 matrix! I need some help! [migrated]

I have this 3x3 matrix $$E_{ij} = g_{ij} + \bar{\epsilon}_{ijk}z^k$$ and want to derive its inverse. I know that its inverse is given by $$(E^{-1})^{ij} = \frac{1}{1+z^2}(g^{ij} + z^{ij} - ...
-2
votes
1answer
45 views

Defining the Hamiltonian and obtaining equations of motion for physical system [on hold]

I have a relative simple mechanical system, that is modeled with 3 point masses and 3 links. There is an input force F working in the vertical direction and 2 torques $\tau_1$ and $\tau_2$. All ...
0
votes
0answers
37 views

Cohen Tannoudji solutions to exercises

Does anyone know where to find the solutions to the exercises of Cohen-Tannoudji's Quantum Mechanics? I am gonna try to do all of them and would like to check.
0
votes
0answers
22 views

Diameter of aluminium ring after it expands [on hold]

An aluminium ring has diameter $200.00$ mm at $25.00^0C$. What would be its diameter at $325.00^0C$? $\alpha$ for aluminium is $7.500 \times 10^{-5}K^{-1}$. Thanks for your help.
3
votes
1answer
89 views

Proving $[a_k^\dagger, a_q^\dagger]=0$

I am trying to prove the commutation relations between the creation and annihilation operators in field theory. I was already able to show that $[a_k, a_q^\dagger]=i\delta(k-q)$. I want to show that ...
1
vote
2answers
20 views

Understanding a graph of energy conservation with bounded and unbounded motions?

This graph is from the physics undergraduate text "Classical Mechanics by Douglas Gregory". Above this graph was the statement: What I didn't understand is- as stated in the under [*paragraph], ...
1
vote
0answers
35 views

Calculating rotation using moment of inertia [on hold]

I have an object A. I know its theoretical moment of inertia. I have a string wrapped around object A. I then apply a certain tension to that string, causing the object A to spin for a certain ...
0
votes
0answers
34 views

Time derivative of Magnetic Field as Source for Electric Field [on hold]

Show the following directly, and not by analogy with Ampere's law: $$ \vec E ( \vec r, t) = \frac{1}{4\pi} \int d^3r' \frac{(\vec r - \vec r')\times \frac{\partial \vec B (\vec r, t)}{\partial t} ...