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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0
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1answer
19 views

Conserved current in a complex relativistic scalar field

For my field theory class I have the following Lagrangian density $$\mathscr{L}=\frac{1}{2}\eta^{\mu\nu}\partial_\mu\phi^*\partial_\nu\phi-\frac{1}{2}m^2\phi^*\phi$$ Where $\eta^{\mu\nu}$ is the ...
1
vote
0answers
22 views

Potential difference between Earth's surface and 2 meters above

Assuming Earth is a charged sphere of radius $R = 6400.10^3 m$ with uniform surface charge density $\sigma = -10^{-9} C.m^{-2}$ and with $\epsilon_0 = 8.85.10^{-12}F.m^{-1}$ I find that ...
0
votes
3answers
35 views

Tension Problem: Finding an angle when only given the tension in two ropes

A crate is hanging from a rope which is attached to a metal ring through which a second rope runs, as shown to the right. What is the angle $\theta$ if the tension in rope 1 is $1.19$ times the ...
-2
votes
0answers
16 views

What would be the formula for the time to impact of an object travelling at a constant velocity? [on hold]

What would be the formula for the impact time of an object? Assuming that the object is falling at a constant velocity, for example, the object is falling at 5 m/s, and is 50 meters away from the ...
3
votes
1answer
28 views

Potential energies of charges and spring

I wonder if someone could sanity check this very simple calculation. Consider a pair of charges $+q$ at rest separated by a spring of length $d$ and stiffness $k$. The spring provides the force that ...
-2
votes
0answers
18 views

Moment of inertia question 11 [on hold]

A circular disc of radius $R$ and mass $M$ has non uniform surface mass density $ar^2$ where $r$ is the distance from the center of the disc and $a$ is a constant.what is the moment of inertia about ...
0
votes
1answer
26 views

Relativistic addition of velocities in the y-direction? [on hold]

I've been having some difficulty with the problem described below, please have a look: Two particles with identical masses (m) undergo a collision. Before the collision they move with velocities ...
0
votes
1answer
58 views

How to calculate the energy required to rotate a planet?

How to calculate the energy required to rotate a planet from non-rotating state? Say the planet is Venus with equally distributed mass of $4.8676 \times 10^{24}$ kg, and desired rate of 1 rotation per ...
0
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0answers
25 views

Identity for the product of propagators

Working in 1 spacial dimension, it has been written that $$\begin{align} \int K(x_f,x,t_2)K(x,x_0,t_1)dx &= \int \langle x_f|e^{-\frac{i}{\hbar}\hat{H}t_2}|x\rangle \langle ...
0
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0answers
32 views

How to write the energy for a system of beams?

Goodmorning to everybody. In my problem I would like to write the energy of a system of 3 beams (see the link below), clamped together in A, positioned at 120° one to each other. ...
1
vote
0answers
12 views

What is the minimum required length of mirror to view full object if the mirror is convex?

i want to find the Radius of curvature of a convex mirror. i have all the factors to calculate except for image height of an object and image distane. so i can find the image height of an object from ...
2
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0answers
33 views

Feynman Propagator in Position Space through Schwinger Parameter

So I am aware of a thread at Propagator of a scalar in position space but it does not answer my question, which is more about poles in position space. Starting from $$D_F(x_1-x_2) = \int \frac{d^4 ...
-1
votes
0answers
10 views

Finding charge on plastic spheres [on hold]

A piece of wool is used to charge two plastic spheres. When the spheres are held 300mm apart, they repel each other with 9.00N of force. If the wool ends up with a surplus positive charge of 29.5μC , ...
0
votes
0answers
18 views

Dipole moment of a specific charge distribution

A negative charge cloud distribute as a ellipsoid and a positive point charge situated at the centre of the ellipsoid.What should be the dipole moment of charge distribution.I mean, if the negative ...
0
votes
1answer
30 views

Solving for the density operator in the quantum Brownian motion master equation

I want to solve for the density operator in the quantum Brownian motion master equation, \begin{align} \begin{aligned} ...
0
votes
2answers
52 views

Orbital Velocity Question

I have a satellite in a stable trajectory around Earth, of a Mass $m$. I know that its velocity is: $$ v = \sqrt{mG/r}\, $$ But now it begins accelerating directly against the gravity vector (i.e. ...
0
votes
2answers
31 views

Convert cubic feet per sec to feet per sec

Is there a way to convert cubic feet per sec to feet per sec. Or in general volumetric flow to velocity? I want to know the time taken by water to travel from point A to point B. I have the distance ...
4
votes
1answer
58 views

Triangular barrier in infinite potential well

Suppose I am looking to solve the wavefunction for the following 1D potential: $$U(x) = \begin{cases}V_0\frac{a-|x|}{a}&\quad\text{for}\quad|x|<a ...
0
votes
0answers
36 views

How is tunneling probability? [on hold]

Let's say we have an alpha particle tunneling through a barrier of width 2 fm and height 30 MeV, and the alpha particle has an energy 1 MeV below that of the barrier. The tunneling probability is ...
-3
votes
0answers
13 views

Computational hydraulics [on hold]

Find the emptying time for reservoir that the water level drops from 35m to 25m using following data: - V=15000 h volume-elevation relationship for reservoir - D=2m diameter for outlet works - h' ...
-2
votes
1answer
27 views

Probability with expectational value [on hold]

How can I calculate: $$P(T)=T^2 b^2 |<n| \delta (x) |n>|^2~?$$ Where $P$ is the probability.
1
vote
2answers
36 views

Show that a function takes the following form using the definition for the function of an operator

If $f(z)$ is a function with a Taylor series expansion $$f(z)=\sum _{ n=0 }^{ \infty }{c_n z^n },$$ then we define $$f(M)=\sum _{ n=0 }^{ \infty }{c_n M^n }.$$ First consider ...
0
votes
0answers
34 views

Why is entropy in a heat flow process always positive? [on hold]

Entropy in a quasistatic heat flow process is given by $$\Delta S=C_1\ln(\frac{T_f}{T_{10}})+C_2\ln(\frac{T_f}{T_{20}})$$ Where $C_1, C_2$ are heat capacities at constant volume, $T_{10}<T_{20}$, ...
-2
votes
0answers
19 views

Thermal conductivity of oak wood [on hold]

How do I calculate the thermal conductivity of a natural material (oak wood) having only the data of two considered temperatures for the heat transfer and the time that it takes for the sample to ...
0
votes
1answer
26 views

Equations for a collision between two particles

Say I have two particles on a 2D plane, they have a x and y coordinate, a x and y velocity, a mass, a coefficient of restitution and a coefficient of friction. What formulae would I need to determine ...
-1
votes
0answers
18 views

Launching a Spring from a meter stick [on hold]

In class we are engaging in 'war games'. We receive a random spring and launch it from a meterstick at a target flat on a nearby table. we will have the variables for: ...
0
votes
1answer
21 views

Influence of a defect on the flow field

I have a long microchannel where flows some water. The reynolds number is much smaller than one. Within the structure of this microchannel there is a big defect. It looks like a bump of size ...
1
vote
0answers
20 views

Decomposition into symmetric and antisymmetric form [on hold]

(a) Given a second-rank tensor Tμν, often viewed as an $N \times N$ matrix (for a space of dimension $N$), show by explicit construction that one can always decompose $T_{\mu\nu}$ into a symmetric ...
0
votes
0answers
33 views

Computing And Finding The Trajectory From The Lagrangian [on hold]

The Euler–Lagrange equations come from extremization of the action. So we expect the "true", dynamical trajectory to minimize (in this case) the value of $S=\int L \,dt$ For free particle motion, the ...
-3
votes
0answers
37 views

Generate the equations of motion for the one-dimensional Lagrangian [on hold]

Generate the equations of motion for the one-dimensional Lagrangian: $$L=\frac {1}{2}m\dot{x}^2-(Ax+B)$$ From the equations of motion (and the implicit definition of the potential), provide a physical ...
-2
votes
1answer
39 views

Perturbation theory of states [on hold]

A particle in 1 dimension with mass $m$ is in potential well with $V = 0$ for $–a/2 < x < a/2$ and infinite potential elsewhere. The particle is initially in the state $n = 10^9 + 1$, with the ...
0
votes
1answer
42 views

Pipe in space outputs gas - what is its angular momentum at $t$ and $t+dt$? [on hold]

Given a pipe in space (neglect gravitational force): The speed of the gas is $v_0$ (in relative to the edge of the pipe) The length of the pipe is $l$ The pipe rests (not moving) at $t=0$ The gas ...
2
votes
1answer
61 views

How to remember the Electromagnetic Spectrum?

This may sound off-topic but I am in a severe need of remembering the following shown Electromagnetic Spectrum along with the frequencies and wavelengths. So far I have looked at several mnemonics but ...
-4
votes
0answers
35 views

finding the speed and distance [on hold]

Valerie and Jake are performing the 1-D collisions experiment. They measure the mass of glider A to be $255~\text{g}$ and its length to be $17.3~\text{cm}$. The same properties of glider B are ...
1
vote
1answer
27 views

Variation of infinite grid of ideal one-ohm resistors: what would be the equivalent resistance between 2 points in a 3D lattice?

I'm sure that many here are familiar with this famous problem that popped up on xkcd one day: On this infinite grid of ideal one-ohm resistors, what's the equivalent resistance between the two ...
-1
votes
2answers
55 views

Electric field 0 everywhere inside Gaussian surface

Gauss's Law shows that the electric field everywhere inside a spherical shell of uniform charge density is $0$. Suppose we have a surface which divides space into two disjoint regions (an interior and ...
0
votes
1answer
44 views

Lie derivative in this paper [on hold]

In this paper http://arxiv.org/abs/1210.2332 it says in (3.19) p. 8 that $$L_{V}z^A =0$$ but I don't know much about Lie derivatives except what I saw now through wikipedia ...
-1
votes
0answers
45 views

How to design this material? (in theory) [on hold]

My university task is to design an injectable biomaterial that fills a tissue void and doesn't diffuse away from the site of an injection. I need to describe how I would carry out this design in ...
-4
votes
1answer
43 views

Total Energy Stored in Seven Capacitors [on hold]

The capacitive network shown in the figure is assembled with initially uncharged capacitors. A potential difference, Vab = +100V, is applied across the network. The switch S in the network is kept ...
-1
votes
1answer
26 views

Calculating values related to angular momentum and then their uncertainties [on hold]

Here is the problem: And here is my work (sorry it is handwritten, it would take a while to type this out) My problem is that I'm not sure if it's right or if it makes sense to keep getting 0 ...
2
votes
3answers
133 views

Finding the appropriate coordinate transformation given two metrics

Given the two-dimensional metric $$ds^2=-r^2dt^2+dr^2$$ How can I find a coordinate transformation such that this metric reduces to the two-dimensional Minkowski metric? I know that ...
0
votes
2answers
47 views

Proving the Lorentz invariance of the Lorentz invariant phase space element

I have been looking around for a satisfactory answer to prove that $$\frac{d^3\vec{p}}{2E_{\vec{p}}}$$ where $E_{\vec{p}}=+\sqrt{(|\vec{p}|c)^2+(mc^2)^2}$, is Lorentz invariant. The standard answer ...
-1
votes
1answer
28 views

A 45.0 kg girl is standing on a 163 kg plank? [on hold]

The plank, originally at rest, is free to slide on a frozen lake, which is a flat, frictionless surface. The girl begins to walk along the plank at a constant velocity of 1.47 m/s relative to the ...
1
vote
2answers
147 views

Confused about Gauss's Law for parallel plates

I am having trouble understanding Gauss's Law. Suppose we wish to find the electric field strength between two parallel plates with charge density $\sigma$. I know it should be ...
0
votes
1answer
20 views

Squaring the matrix element and summing over the spins of outgoing particles? [on hold]

I found the matrix element in a scalar interaction $\nu_ee \rightarrow \nu_ee$ to be $$M=\bar{u}(\nu,q_2)u(e,p_1)\bar{u}(e,p_2)(1-\gamma_5)u(\nu,q_1)$$ But I do not know how to find the $|M|^2$ and ...
2
votes
0answers
28 views

How to solve a difficult equation describing large vacuum fluctuations?

Suppose that a Quantum System can be described by the wavefunction $\psi(\vec{x},t)$, but due to the occurence of chaotic noise within the Quantum System, only the "filtered" wavefunction ...
0
votes
0answers
37 views

Finding the Expectation Value of basis states [on hold]

I am a Mathematician and I am taking a Quantum Computing class. We have been asked to find the expectation value of $X$ tensor $Z$ and $H$ tensor $H$. X is the not operator and switches the state the ...
0
votes
1answer
42 views

Difference of potential between points [on hold]

I have the following question: What is the potential difference between point A - B? Could somebody please explain me how should I count it? The voltage U=18V. I found out that V1=8V, V2=10V, ...
1
vote
0answers
20 views

Laser travelling through a radially graded index of refraction

A laser beam propagates through a region whose refractive index varies as $\mu=\mu_0(\frac{r}{r_0})$. At a distance of R the beam makes an angle of 30° with the normal. Find the minimum seperation ...
0
votes
1answer
33 views

Galactic Rotation of gas clouds

I need to use the formula for the radial velocities of gas clouds due to Galactic rotation together with the assumption of a flat rotation curve ($V$ independent of $R$) to find the Galacto-centric ...