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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18 views

Magnetic field of cylinder equal to two current loops?

I was reading a text about the magnetic field of a uniformly magnetized cylinder when I came across this. The author states that since \begin{equation} A_{\phi}(\rho,z) = \frac{\mu_0 a M}{4\pi} ...
4
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2answers
64 views

What would it take to cause lightning to jump between the Moon and the Earth?

This question comes from @Floris' speculation at the end of his excellent answer about what it would take to kill everyone on the Earth with electricity. Doing all this in 1/10th of a second ...
-3
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0answers
14 views

plot of h(a function of redshift). vs angle of rotation of plane polarised light over cosmological distances [on hold]

um doing a project on cosmic optical activity,trying to explain the cosmic optical activity using U(1) chern simons term.Trying plot the data. as far as theoretical part is concerned,I've worked out ...
0
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1answer
25 views

How to calculate the expectation value of position vector?

$$\psi (\vec{x})=Ae^{-(1/4a^2)|\vec{x}-\vec{x}_0|^2}e^{i\vec{p}_0\cdot \vec{x}/\hbar}$$ The wave function is like this, then how is the expectation value of position vector (not position) calculated? ...
0
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1answer
29 views

Force and initial height of thrown object [on hold]

You stand at the earth's surface, and you throw a ball of mass m1 into the air perpendicular to the surface. Let g at the surface be 9.81m/s^2. You know two points (height:h1, h2 and time:t1,t2) of ...
1
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0answers
38 views

Equivalence of delta functions when calculating decay rate [on hold]

$\newcommand{\bs}{\boldsymbol}$ Hello, I'm currently working through the lecture notes of my Theoretical Particle Physics course, and there, we are calculating the decay rate of the following process ...
-1
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0answers
18 views

Size of upward force actin on the rocket! [on hold]

A 1500 kg rocket lifts off from the surface of the Moon by expelling exhaust from its lower end as shown. The force applied to the exhaust gas is 15 000 N downwards. Since the rocket is lifting off ...
0
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1answer
18 views

About environmental lapse rate [on hold]

On a spring day a middle-latitude city about latitude 40N and a temperature of 10°C has a surface (sea-level) environmental vertical soundings reveal a nearly constant lapse rate of 6.5°C per ...
3
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1answer
61 views

Force and lions

Alright. So me and some other people right now need an answer to this. If you have 1 trillion lions traveling at 81 kilometers (top speed of a lion) towards the Sun in a pretty dense ball. (As dense a ...
0
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0answers
20 views

What is the terminal velocity of a tungsten cylinder 20ft long and 2ft wide? (if it had a more aerodynamic end)

I read somewhere that the drag coefficient of a sphere is 0.5. so I tried calculating with that and got 4150 MPH. I don't know if 1 would be a flat end or not. However I would assume if you were ...
1
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1answer
47 views

How can we prove this system is in a stationary state?

I'm having trouble using the given hint to solve the problem. The problem statement is as follows: At instant $t=0$, the probability distribution of a particle under a potential $V(x)$ is ...
1
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2answers
133 views

$\nabla^{\mu}\nabla_{\mu}$ in general relativity [on hold]

I am trying to work out $\square=\nabla^{\mu}\nabla_{\mu}$ in the metric $ ds^{2}=-A(r)dt^{2}+B(r)^{-1}dr^{2}+r^{2}d\Omega^{2} $$ My work: when applying $\square$ to a scalar $\phi$, then $ ...
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1answer
30 views

How to solve infinite square well with exponential solution (of oscillatory type)?

Given a potential well of $V = 0$ on the interval $(0,L)$ and $V = \infty$ outside the well, I am working to solve the Time Independent Schrodinger Equation $$\dfrac{d^2}{dx^2} \psi= ...
0
votes
1answer
24 views

Final charges on Capacitors connected in a curcuit

Capacitor C1 is charged to a Potential Difference of 10V. It's capacitance is 2 microFarad. Capacitor C2 is charged to a Potential Difference of 15V. It's capacitance is 4 microFarad. Now the ...
1
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0answers
19 views

SR: vector field and change of reference [on hold]

If $U$ and $V$ are vector fields, then the derivative of $U$ along $V$ is the vector field $\nabla _V U$ with components $$\nabla _V U=V^b \frac{\partial U^a}{\partial x^b}.$$ I would like to verify ...
0
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0answers
44 views

Description of charged circle with Dirac delta function in Cartesian coordinates [on hold]

I need to describe density of charge of uniformly charged circle ($x^2+y^2=R^2$; $z=0$, total charge $Q$) with Dirac delta function in Cartesian coordinates. In cylindrical coordinates it is: $$ \rho ...
2
votes
2answers
81 views

What is the Hermitian adjoint of operator $\hat{K}\psi = \psi^*$?

For many operators, their adjoint can be expressed as a function of other known operators, for example $$\hat{T}_a^\dagger = \hat{T}_{-a} \\ \hat{p}_x^\dagger = \hat{p}_x$$ where $\hat{T}_a \psi (x) = ...
0
votes
3answers
57 views

Tension on a string

A string is attached at both extremities and put under tension $T_0$ at rest. We know that if we pull the string upwards from the middle, the tension will increase. But why is it that, admittedly, ...
1
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2answers
34 views

Error calculation in parallel resistances

This is the question: There are two resistors with resistance values $R_1=100\pm3$ ohm and $R_2=200\pm4$ ohm. Find the equivalent resistance of parallel combination. According to what I've ...
0
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1answer
66 views

How to derive this equation? [on hold]

How to derive this equation? It comes from a book named Seismic Wave Propagation in Stratified Media.
2
votes
1answer
194 views

Wave on a guitar string, differential equation

The mechanical energy of an element of the string is : $$dE(x,t) = \frac{1}{2}\left[T_0\left(\frac{\partial y}{\partial x}\right)^2 + \mu\left(\frac{\partial y}{\partial t}\right)^2\right]dx$$ Where ...
0
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1answer
23 views

Diffraction in Circle of light? [duplicate]

I'm working on a project that refers to optics. the question says when a laser beam is aimed to a wire ( perpendicular to the surface ), a circle of light then will be seen on a the surface. I somehow ...
0
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0answers
21 views

Diffraction and Young's Double Slit [on hold]

This may be a little too basic for this site, but I am facing problems in solving difficult problems on Single Slit pattern and Double Slit as Superposition of 2 single slits. Can you please help me ...
1
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1answer
28 views

Find out gradient of electric potential at ${\bf r}$ created by eletric dipole of moment ${\bf p}$ [on hold]

Suposing an electric dipole of moment ${\bf p}$ located at the origin which creates an electric potential at ${\bf r}$ given by ...
0
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0answers
31 views

Electricity and Magnetism — Calculating Electric Field — *Confusing distance in problem* [on hold]

The question is as follows: A point +2.00-nC charge is at the origin, and a second -5.00-nC point charge is on the -axis at (a) Find the electric field (magnitude and direction) at each of the ...
0
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3answers
50 views

How high could i jump if I weighed only half as much?

E.g. I weigh 150 lbs I had a thrust device that lifted 75 pounds. How high could I jump? What if that thrust lifted 2/3 of my weight (100 lbs)? Obviously, I cant fly, but I guarantee if I weighed ...
0
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1answer
21 views

Cancelling internal forces/moments term when deriving inertial matrix

I am attempting to derive the inertial matrix for a general rigid body of mass $m$ as shown in the following diagram: The green vectors indicate the key position vectors: Position of centroid ...
1
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1answer
35 views

Altarelli-Parisi Splitting Function $P_{qq}(z)$ vanishing integral if and only if quark flavour sum rules and equivalence

Question 1: I believe both questions are in the context of Deep Inelastic Scattering. I've been given the expression for $F_{2}^{ep}(x)=(\frac{2}{3})^2[u(x)+\bar{u}(x)] +(\frac{1}{3})^2[d(x) + ...
1
vote
1answer
32 views

Description of charged sphere with Heaviside function in cylindrical coordinates

I need to describe density of charge of uniformly charged sphere (radius R, total charge Q, position of centre (0,0,0)) with Dirac delta function and Heaviside step function. The hard part is to ...
0
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0answers
22 views

Help with a couple of materials science questions

My first post on this forum is asking for some assistance in answering some questions. I was hoping as well as being given an answer, you could offer a little explanation so that I can understand - ...
0
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2answers
32 views

How to calculate the resultant movement of a superposition of harmonic oscillators?

If $x_{1}(t) = \cos(\omega t - \frac{\pi}{6})$ and $x_{2}(t) = \sin(\omega t)$ are two simple harmonic oscillators in the same direction and with the same angular frequency $\omega$, how to ...
0
votes
1answer
14 views

squared position dependent electric field [on hold]

Here's an asignment problem that got me puzzled. The thing is that I have to solve a second order non linear differential euqation, the course is an urdergraduate mechanics course. A particle of mass ...
0
votes
1answer
35 views

Finding magnetic field inside rotating sphere [on hold]

A sphere with radius $R$ has constant charge density $\rho$ per unit volume. This sphere rotates as a rigid body with an angular velocity of $\omega$ around an axis that goes through the center. What ...
0
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0answers
22 views

Hadamard Gate application help

I need to prove that if you apply Hadamard gates on the input/output before and after a CNOT gate that you get the same answer as flipping the target and control qbits of a CNOT. So for example I'm ...
0
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0answers
13 views

Post-measurement state for measurement outcomes on Qutrit states [on hold]

I am trying to do problems in Barnett.S. - "Quantum Information" book but I am confused on what the question wants and how to proceed. The question is as follow, The qutrit (three state system) ...
0
votes
0answers
17 views

Force on charged particle in magnetic field [on hold]

I am having difficulties achieving the answers to a question in my physics textbook. The question goes: A charged particle $q = 3.0 × 10^{–19}\space C$ and of mass $5.65 × 10^{–27} kg$ moves ...
-1
votes
0answers
33 views

Force and laws of motion, friction [on hold]

The 12 kg block is put on the 3 kg platform which attached to the wire AB. The friction coefficient between 12 kg and 3 kg platform are ų(s)=0.6 and ų(k)=0.40. Determine the magnitude of acceleration ...
0
votes
0answers
22 views

Calculating tension at every point on rope around pulley [on hold]

Suppose a pulley is fixed over a wall and a rope is passing over it such that both the ends of the rope are connected to boxes having masses $m$ and $M$ ( $m<M$) Assume mass per unit length of rope ...
1
vote
1answer
51 views

Moment of inertia of cylinder [on hold]

How to calculate moment of inertia of a cylinder about an axis passing through its geometrical center and inclined at an angle (\theta) with vertical.
1
vote
0answers
21 views

Gears in contact?

I was doing a practice exam paper question that was along the following lines: A gear, $A$,and moment of inertia $I_A$ is spinning about its axis at angular velocity $\omega$. Another gear $B$ ...
0
votes
1answer
23 views

Finding the acceleration of a mass/pulley system without knowing the direction of the displacement

I have the following question: I noticed that if I solve for the acceleration assuming that the acceleration is occuring to the left, I get the equation: $\displaystyle a = ...
1
vote
0answers
21 views

A level 9702/11/o/n/14 question 26 (about interference of sound wave) [on hold]

The speakers are identical and they are connected it's series with power, so I thought there would be complete destructive interference. Therefore, at some point the amplitude would be zero and also ...
2
votes
1answer
39 views

Finding final velocity in inelastic collision [on hold]

Information: In a shipping company distribution center, an open cart of mass 49.0-kg is rolling to the left at a speed of 5.40-m/s (see the figure). You can ...
0
votes
1answer
20 views

Mass-Spring system on an accelerating jet

Imagine a perfect mass spring system. If it's put on an accelerating plane, how will the motion change? Is the plane's acceleration like a driving/damping force, where: $$F_{\text{driving}} = ...
0
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0answers
36 views

Need help understanding $\oint H \cdot dl = I_{f,enc}$. Homework question [on hold]

A cylinder of radius R and magnetization $M = kr^2 \hat{\phi}$. I already found the magnetic field due to bound charges to be, $B = \mu_0 k r^2 \hat{\phi}$ inside the cylinder and $B = 0$ outside ...
2
votes
1answer
41 views

Variable Gear System [on hold]

I have a variable speed gear system with 4 primary parts. I need to find the relationship between input rotational speed (wi) to output rotational speed (wo). Fig. 1 shows three of the primary ...
0
votes
0answers
36 views

Fermion commutation relations QFT question [on hold]

Consider left-handed fermions in two spacetime dimensions $(t,x)$: $\psi_L=\frac{1}{2}(1-\gamma_5)\psi_D$ with $J_0^\epsilon(t,x)=\psi_L^+(x+\epsilon)\psi_L(x-\epsilon)$. (a). Use canonical ...
0
votes
1answer
19 views

Field inside an endless cylinder, without using Gauss's law [on hold]

I need to prove that a field inside an infinitely long hollow cylinder, with density $\sigma$, is zero - without Gauss's law. I am clueless on what to do, how to start. Any clues, hints, guidelines? ...
1
vote
1answer
27 views

Potential Energy in modified Atwood Machine

The initial length of the spring is $l_0$. I need help understanding how the potential energy of this system comes to be. I know the answer: $$ U = ...
1
vote
2answers
65 views

What should we do If we wanted to increase the angular velocity of a planet? [duplicate]

We could hit it with really fast objects, but could we manipulate the orbit of a large satellite to speed up its rotation? What would be the easiest way?