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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5 views

Scattering Matrix of a Given Circuit - Microwave

This might be an easy question but I couldn't find it in our course book[Microwave Engineering, Pozar, 4th ed] or on internet. I have a homework and one of the questions asks me to find the S-Matrix ...
1
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2answers
146 views

Throwing an object around the Earth. Is it possible?

Some of you might be familiar with a cartoon called Astérix. In one of the episodes, Obelix (the really strong fat guy in the stylish striped pants) enters a speer throwing competition. He beats the ...
0
votes
1answer
22 views

How to prove Bloch function is periodic in reciprocal lattice?

How to prove Bloch function is periodic in reciprocal lattice? I saw in some textbooks this formula: $$ \Psi_{\mathbf{k}} (\mathbf{r}) = \sum_{\mathbf{G}} ...
-3
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0answers
12 views

Angular acceleration in terms of magnetic field and current [on hold]

A rigid uniform horizontal wire PQ of mass $M$, pivoted at P carries a constant current $I$. It rotates with a constant angular speed in a uniform vertical magnetic field $B$. If the current is ...
-1
votes
3answers
43 views

Traveling to Mars in 24 Days

I am trying to calculate an electromagnetic rail launch from the Moon but I need to know if humans can survive the launch. Please see the specs below. 150 miles of track Proposed final launch speed ...
0
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1answer
16 views

Potential of 2 parallel plates

How can I write down a potential of 2 parallel plates (area $A$) with charge $+Q$, $-Q$ such that V(inf)=0 and V(in) is what I would expect, $Ed = Qd/A\epsilon_0$. Do I have to write a discontinuous ...
0
votes
1answer
30 views

Special Relativity - three body problem

I have a problem with solving three-body problem in special relativity. I know that in general this problem doesn't have good solution, but I have a specific case when it should be good answer. ...
-2
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0answers
16 views

Men on the pully palte to lift [on hold]

A plate has a fixed pulley. A men, standing on a plate pulls up with one end of the rope to try to lift the plate and himself; the other end of the rope is over the pulley to fixed on the ceiling, ...
-2
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0answers
26 views

classical-mechanics [on hold]

If the mass of person is less than the mass of elevator, the $\mathit{N}$ will be negative. What does that mean? For instance: person mass = $30~\text{kg}$, elevator mass = $60~\text{kg}$. At ...
1
vote
1answer
18 views

Mullins effect in the elastic material

I was thinking on a experimental question, but I couldn't get the answer: How can we measure how the Mullins effect influences in the pressure inside a balloon? (I mean that, if there is a formula, ...
0
votes
0answers
27 views

General two state system with general pertubation

I am trying to solve this two-level system with time independent perturbation problem Consider an atomic system with only two stationary states $|1\rangle$ and $|2\rangle$ , of respective energies ...
-4
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1answer
77 views

How to Solve this integral? [on hold]

I want to solve this integral. $$\int_{-\infty}^{\infty}\frac{e^{ik(x-x')}}{k^2-\mu^2}dk$$
0
votes
0answers
13 views

How to compare the resistance to penetration of two targets with different densities & strength to the same projectile at the same velocity [on hold]

An iron cannonball (density .284, diameter 5.04", weight 18lbs) traveling at 1000 feet per second strikes flat wall of live oak (density .0364 lb/in, janka hardness 2680lbf, 125.6 MPa), how far does ...
0
votes
0answers
19 views

Moment of inertia and simple harmonic motion [on hold]

A meter stick is suspended from a frictionless rod inserted through a small hole at the 39-cm mark. Find the period of its small-amplitude oscillations around its equilibrium position. I know the ...
0
votes
1answer
20 views

Equation for Figuring out the Displacement for a Projectile at an Angle

I'm wondering if anybody can simply tell me if there is a simple equation to solve for displacement when you've been given the launch angle and initial velocity? See; http://is.gd/LpT1EA To some, it ...
2
votes
3answers
51 views

Classical Limit of the Quantum Harmonic Oscillator

The classical harmonic oscillator obeys an arcsine law in that the distribution of positions of the particle over a single time cycle is proportional to $\frac{1}{\sqrt{A^2-x^2}}$, $A$ being the ...
1
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1answer
30 views

Magnitude of the average velocity vector (not the average speed)

Thank you ahead of time for taking to look at this. For this following problem we were given an answer however I am almost positive the given answer is wrong. It doesn't even make sense. So here is ...
1
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0answers
20 views

Norm of Killing vector field

Let us suppose we have a Killing vector field with $X^a = 1/2$ and $X^b = 1/3$ and $g_{ab}=1$ where the other $c$ and $d$ components are zero. Now we want to find its norm: The formula for finding ...
1
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1answer
37 views

Center of mass of a Soda Can [on hold]

I have the following problem and can't seem to get my head around it: The mass of the beverage in a full soda can is $m$. The empty can's height is $H$ and mass $M$. A small hole will be drilled ...
0
votes
1answer
29 views

Determining the number of electrons passing through a conductor [on hold]

How many electrons pass through a conductor in 1 minute, if the current is 1 A ? My book says the answer is $Q = 3.75 \times 10^{20}$, but I don't know how they got it.
1
vote
3answers
71 views

Calculation of $\langle p\rangle$ and $\langle p^2\rangle$ for wave function [on hold]

Given the wave function $$\psi(x)=A\exp\left[-a \left(\frac{mx^{2}}{\hbar}+it\right)\right]$$ I would like to calculate $\sigma_{p}$. \begin{align}\langle p\rangle &=\int ...
0
votes
1answer
96 views

How the Moon causes the tides?

I am considering the following question but I con't quite figure it out... I have looked up differential gravity but I cannot derive the equation for the effect on earth, and I haven't found any ...
0
votes
1answer
41 views

Field transformations

I'm reading Maggiore's book "A modern introduction to quantum field theory" and I'm very confused by what he did in chapter 2.6 page 31 eq. (2.80). He basically wants to find the generators of the ...
1
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0answers
45 views

One dimensional motion problem [on hold]

I kept almost 2 hours trying to solve this problem : Two trains are 150 km apart. Train A moves at 50.0 km/hr towards train B, which moves at 30.0 km/hr toward train A on the same track. How ...
0
votes
0answers
30 views

When using quantum regression theorem to get the correlation function,like in the below system, why it appears sin and cos in the function?

In this four-level single atom interact with classic field system, when using quantum regression theorem, the derivation of the density matrix and the annihilator operator are used: Then use ...
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2answers
60 views

Bra-ket of products

I was trying to solve the following problem. (Lifted from Modern Quantum Chemistry: Introduction to Advanced Electronic Structure Theory by Szabo and Ostlund) I came across across a solution for ...
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votes
1answer
54 views

How to find a metric of a general observer?

Yes, that's it. How to find a particular metric of an observer in general relativity? Let's say we have a static metric: ...
0
votes
1answer
30 views

How can I caculate the time to traverse a distance with the Tsiolkovsky equation?

Using the Tsiolkovsky equation I know what the change in velocity is after burning x amount of fuel. $$ \Delta V = V_e \dot\ log(\frac{m_1}{m_1-m_2}) $$ $m_1$ is my starting mass and $m_2$ is the ...
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votes
0answers
16 views

Power and energy physics [on hold]

A power plant based on a gas turbine produce 500mw of electrical energy. It has 48% effiency, and dumps its waste heat into the atmosphere at 20 degrees C. if you assume the plant functions near the ...
-1
votes
2answers
49 views

How to eliminate variable $t$ from the equations to solve this question? [on hold]

A ball rolls horizontally off the top of a stairway with a speed of $1.52~\text{m/s}$. The steps are $20.3~\text{cm}$ high and $20.3~\text{cm}$ wide. Which step does the ball hit first? I tried ...
0
votes
0answers
31 views

Under what circumstances will a particle in a paraboloid oscillate harmonically around the lowest point? [on hold]

A particle of mass $m$ is moving in the gravitational field $g$. It is confined to move frictionlessly on the surface of the paraboloid $z(r,\phi):=a\cdot r^2$. Here, $a>0$ is a constant. The ...
0
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0answers
25 views

Gas cloud gravitation problem [on hold]

Consider the following problem from the 2014 F=MA contest: A spherical cloud of dust in space has a uniform density $ρ_0$ and a radius $R_0$. The gravitational acceleration of free fall at the ...
0
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0answers
24 views

Simple harmonic motion and elasticity

Question: One end of a long metallic wire of length L is tied to the ceiling. The other end is tied to a massless spring of spring constant K. A mass m hangs freely from the free end of the ...
0
votes
1answer
78 views

Measured voltage on coil

I have a simple circuit of coil with inductance L $$u_c(t)= -L\frac{di}{dt}$$ and AC source with output voltage $u_s$ What is actually measured by the voltmeter ($u_c$, $u_s$ or $u_s - u_c$)? If ...
0
votes
0answers
21 views

Radioactive nuclei are put into a fluid and decay with time. Find the point where the density of the radioactive nuclei is a maximum [on hold]

I've found the following question: A steady 2D flow is described by $u_x = 2/x$, $u_y = 1$. Find and sketch the streamlines. Find also a general expression for the surface density of the flow ...
0
votes
1answer
47 views

Load resistance adding with minimized power and voltage constraint [on hold]

Consider the simple circuit below: Suppose $V$ and $R$ have set voltage and resistance values respectively, and you are to choose the resistance values for $R_1$ and $R_2$. However, you must choose ...
0
votes
0answers
28 views

Vector Physics Line Point Collision [on hold]

Question: What laws should I use to distribute the net velocity where: A Ball collides with a rope that always stays at high tension with two Balls on both sides which have arbitrary velocities. ...
2
votes
1answer
39 views

Calculation of the efficiency of a Carnot engine for photon gas

I am working on a problem considering the Carnot cycle for a photon gas. The cycle has two isothermic processes at $T_{H}$ and $T_{C}$ and two adiabatic processes. The entropy of the photon gas is ...
0
votes
2answers
40 views

Finding an initial velocity given the acceleration [on hold]

The acceleration of a particle is given by (1) $a_x(t)=− 1.95m/s^2 +( 2.90m/s^3 )t.$ I am required to find the initial velocity $V_{0,x}$ such that the particle will have the same $x$-coordinate ...
0
votes
0answers
28 views

How much time it takes for a car to pass a truck [on hold]

The driver of a car wishes to pass a truck that is traveling at a constant speed of $19.7~m/s$ . Initially, the car is also traveling at a speed $19.7~m/s$ and its front bumper is a distance $24.1~m$ ...
1
vote
1answer
62 views

Why does $\frac{d\tau}{d\sigma} = L$?

I am given a (3+1)-dimensional spacetime that has the line element \begin{equation} ds^2 = -\left(1-\frac{2M}{r}\right)dt^2 + \left(1-\frac{2M}{r}\right)^{-1} dr^2 + r^2 d\phi^2 \end{equation} ...
0
votes
2answers
61 views

An interesting problem on springs [on hold]

If I place two identical objects of mass $m$ at either end of a spring with spring constant $k$ and the whole system is placed on a horizontal frictionless surface, then what is the frequency of ...
5
votes
1answer
249 views

How do I solve this Gaussian path integral?

Suppose $$ Z = \int \mathcal D[\phi^*] \mathcal D[\phi] \exp(\phi^*A\phi + \phi B\phi) $$ where $A$ and $B$ are operators. I know how to solve a Gaussian path integral involving only $\phi^* A \phi$ ...
-2
votes
0answers
35 views

PHYSICS QUESTION OF CIRCULAR MOTION [on hold]

Numerical-A piece of stone is tied at one end of a 80cm long string and it is whirled in a horizontal plane with a uniform speed making 14 revolutions in 25 seconds. Calculate the magnitude and ...
-1
votes
0answers
22 views

Question: calculate deviation of light due to gravity by imagining it as a change in optic medium [on hold]

Q: light bends due to gravity. we assume that due to gravity of a point object of mass $M$ , it changes the refractive index of vaccum at distance $R$ from the object as $$n = 1 + \frac{2GM}{RC^2}$$ ...
1
vote
0answers
27 views

Legendre Transformation for multiple variables

I need to show that for $F(x_1, .., x_n)$, the Legendre transformation is, $$G(s_1, ..., s_n) = \sum_{i}^{N} x_i s_i - F$$ where $$s_i = \frac{\partial F}{\partial x_i}$$ and has the property that ...
3
votes
1answer
50 views

Springs at an angle [on hold]

I'm trying to find the equation of motion for the following system: This is how I proceeded: Let's call the length of the hypotenuse $s$. Then, $$F = 2 \sin{\theta}\cdot-k(s - l_o) = -2kx ...
0
votes
0answers
22 views

Finding the effective force constant in an accelerating frame [on hold]

Consider problem #16 here: http://www.aapt.org/physicsteam/2013/upload/exam1-2012-unlocked-solutions.pdf Let me share my thoughts, and hopefully someone can tell me where I am going wrong. ...
-1
votes
0answers
21 views

2012 F=MA Contest #12 on tension [on hold]

There is an interesting problem (#12) from the F=MA contest: My approach: Let the tension in the string supporting the hanging weight be $T$. Since the angles between all the strings are constant, ...
-1
votes
0answers
36 views

Proof of James-Guth stress-strain relation?

I'm working on a project to find the relation between a spherical shape ( a balloon ) radius and its internal pressure. I read an article about James-Guth stress-strain relation that can help me if ...