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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0answers
8 views

Evolution operator for “blending” a pair of eigenstates

Consider a measurement operator ("observable") $\hat O$ which has ("a spectrum of") only two distinct eigenvectors; formally $$\hat O |\bullet\rangle := r_{\bullet}~|\bullet\rangle, \qquad \hat O ...
0
votes
1answer
29 views

Neutron star: escape velocity

The textbook from which I teach physics at the end of secondary school calculates the escape velocity at the surface of a non-rotating neutron star Newton-style using $\frac{1}{2}\cdot m\cdot ...
-1
votes
1answer
26 views

Kinematics - When projectile lands at an angle

Question: You are watching an archery tournament when you start wondering how fast an arrow is shot from the bow. Remembering your physics, you ask one of the archers to shoot an arrow parallel to ...
-2
votes
0answers
20 views

Incline plane problem

A string carrying a $5\times10^{-2}$kg bob hangs from the roof of a vehicle. It moves down an incline at an angle of $30 °$ with the horizontal with acceleration of $3\mathrm{ms}^{-2}$. What angle ...
-1
votes
1answer
43 views

Where does this hyperbolic tangent in Nakahara's text come from?

I don't see why the term with $\tanh$ appears in the equation 1.164 The textbook is the second edition of Geometry, Topology and Physics
-3
votes
0answers
9 views

How to find the enthalpy if I have the pressure and temperature?

There is a refrigerator with R-134a as the working fluid. The R134a enter the evaporator at P=100kPa, quality =0.2 and it leaves at P=100kPa, T=-26 degree Celsius How can I find out the enthalpy? I ...
-1
votes
1answer
18 views

Movement of accelerating points with maximum speed (i.e. running away from tiger)

I'm walking through forrest and suddenly I see a tiger. And now I have a problem. I want to know how much time I have left but I don't know how to calculate it. Or more fomally: Let's say there are ...
0
votes
1answer
43 views

The general equation for a wave packet derivation?

On Wikipedia it gives the general equation for a wave packet (and therefore for a wave?) to be: $$u(x,t)=\frac{1}{\sqrt{2\pi}}\int^{\infty}_{-\infty}A(k)e^{i(kx-\omega t)} dk$$ I have been trying to ...
-4
votes
0answers
38 views

Jee Main 2015 Question. Inter-molecular Interaction [on hold]

The Intermolecular interaction that is dependent on the inverse cube of distance between the molecules is: Hydrogen Bond? ion-ion interaction? ion-dipole interaction? London Force?
-3
votes
0answers
94 views

JEE MAIN 2015 Question. Electromagnetism [on hold]

Two coaxial solenoids of different radii carry current I in the same direction. Let F1 be the magnetic force on the inner solenoid due to the outer one and F2 be the magnetic force on the outer ...
2
votes
2answers
60 views

Neutron star: free fall acceleration

The textbook from which I teach physics at the end of secondary school, has a question about a neutron star: $M_{star}=1.4\cdot M_{sun}$, radius 15km. "Calculate the free fall acceleration at the ...
0
votes
1answer
49 views

Perturbations in linear response theory

I've been working on applications of linear response theory to condensed matter systems, and I've got quite far into the literature on the subject. However, there is an identity which seems to be ...
0
votes
1answer
33 views

A question about velocity and acceleration [on hold]

I'm working on MasteringPhysics and came across this problem. I thought I knew how to solve it but the website says my answers are not quite correct. Please see the image below for the problem and ...
0
votes
2answers
46 views

Explanation on anticommutation relations

Setup Given two states: $|K\rangle=a_i^+a_j^+|\rangle$ and $|L\rangle=a_k^+a_l^+|\rangle$. Evaluating the overlap: $\langle K|L\rangle=\langle|a_ja_ia_k^+a_l^+|\rangle$ Introducing: ...
0
votes
0answers
49 views

Variation on the “Hole through the Earth” problem

In the news recently was the announcement that the answer to the classic Newtonian mechanics question of the time it takes for a body to fall all the way through the Earth was corrected to account for ...
0
votes
1answer
37 views

The correct integral for Newton's shell theorem

I am trying to prove Newton's Shell Theorem, and the natural integral I think of to use does not seem to give the right answer, so I am trying to find my mistake. For simplicity, I will work in one ...
-1
votes
0answers
18 views

Geometric and algebraic aspects of geometric vectors [on hold]

I'm writing some notes for a honors physics class and I am having some trouble with some proofs. Say $\vec{A}$ and $\vec{B}$ are some geometric vectors. Then we defined the dot product ...
0
votes
0answers
11 views

Problem with Parabolic motion [on hold]

Could someone help me out with this one: A tank will fire a missile with speed of 400m/sec to a target that is at 1500 meters from it and 400 m above it. Which are the angles that a thank will have ...
1
vote
1answer
24 views

Getting the ($hkl$) Miller indices from the angle of incidence of a light ray

I was given an X-ray diffraction lab this week. We measured the count rate for different angles $2\theta$ of the rotation of the detector. From these measurements we plotted a graph of 2theta vs the ...
-2
votes
0answers
34 views

Which quarks will I find in a neutron? [on hold]

A: Three up quarks. B: Two up quarks and a down quark. C: Two down quarks and an up quark. D: One charm quark, one up quark and one down quark. please expain why
2
votes
2answers
45 views

Homework: electrical circuit with 2 voltage sources & 1 switch

I can't figure out how to handle this circuit. I am asked to calculate the total resistance and the current over each resistance (R1-R4) when the switch is open. However I'm not getting anywhere ...
0
votes
2answers
73 views

How is the uniform gravitational field approximation $F_g\approx mg$ near Earth's surface derived from Newton's law $F_g=GMm/r^2$ of gravitation?

I am really bothered about how we can derive the equation of projectile motion. Suppose a point mass will move in the gravitational field of the Earth according to the equation $$\ddot R ...
0
votes
0answers
146 views

The location of a point of destructive interference [on hold]

Two speakers, one at the origin and the other facing it at x = 1.04 m, are driven by the same oscillator at a frequency of 642 Hz. If the speed of sound is 343 m/s, find the following. I know the ...
0
votes
0answers
31 views

What's the quantization of a Hamiltonian? [on hold]

Suppose Hamiltonian of a conservative system in classical mechanics is $$H~=~\omega xp,$$ where $\omega $ is a constant and $x$ and $p$ are the position and momentum respectively. What is the ...
0
votes
2answers
26 views

What the wave function looks of a particle in the infinite square well looks like after collapse for measurements of position and energy

Consider a particle in a the infinite square well from x=0 to x=L. At t=to, I make a measurement of position and get x=L/2. What is the resulting wave function at t=to? My understanding, from reading, ...
1
vote
1answer
54 views

Derivation of Jefimenko's Equation in Jackson's EMT book

I have been trying to understand the derivation of Jefimenko's equation in Jackson on p.246-247 which can be seen in the photographs attached. First of all I did not fully comprehend the ...
0
votes
0answers
13 views

Compute the temperature of the reservoirs between which the engine operates [on hold]

An ideal gas for which $c_v$ = 3R/2 is the working substance of a Carnot engine. During the isothermal expansion the volume doubles. The ratio of the final volume to the initial volume in the ...
1
vote
1answer
33 views

How to derive $r, \theta, \phi$ for the sperical coordinate gradient?

I'm trying to figure out how to get the gradient in spherical coordinates. I'm as far as the author writes in this answer: http://physics.stackexchange.com/a/78514 and I understand how and why to get ...
0
votes
0answers
29 views

Total Energy of Inverted Pendulum

I have to find the total energy $V$ of an inverted pendulum (rod). The following parameters and their values are given: Mass $m$ and length $l$ of the pendulum $\theta$ as the angle of the ...
0
votes
1answer
27 views

“Definition” of internal energy

Conversation of energy implies that if we have a thermally insulated system which goes from state 1 to state 2: $$\Delta E_{12}=E(2)-E(1)=\Delta W_{12}$$ and the 1st law of thermodynamics ...
-2
votes
0answers
16 views

How would I prove the gamma normalization relation without Clifford Algebra? [duplicate]

The equation in question is $$ \left ( \gamma ^{\mu } \right )^{\dagger }=\gamma ^{0}\gamma ^{\mu }\gamma ^{0}$$
0
votes
0answers
22 views

Solving IVP $\frac{dE}{dx}= -\gamma\frac{dx}{dt}-\alpha$ [on hold]

I need to solve the differential equation $$\frac{dE}{dx}= -\gamma\frac{dx}{dt}-\alpha$$ with the initial value $$v(t=0)=v_O$$ I tried writing everything in terms of $v$ which gives me: ...
0
votes
2answers
36 views

Approximating energy loss caused by drag force

We know the usual aerodynamic drag equation is given by: $$F_d = -bv^2$$ Where $b$ here is just some constants combination related to the property of the fluid and the material passing through. My ...
0
votes
0answers
18 views

Clocks in moving inertial frames [on hold]

A clock $C$ is at rest at the spatial origin of an inertial frame $S$. A second clock $C'$ is at rest at the spatial origin of an inertial frame $S'$ moving with constant speed $u$ relative to $S$. ...
0
votes
0answers
31 views

Sinusoidal Wave Problem [on hold]

A sinusoidal wave of frequency $650 \mathrm{Hz}$ has a velocity of $450 \frac{\mathrm{m}}{\mathrm{s}}$. Determine the distance between two points on the wave that differ in phase by 1.5 ...
4
votes
3answers
73 views

In an elastic collision, can we choose between cons. of energy and cons. of momentum?

Suppose we have two solid spheres with masses $m$ and $M$, respectively, and that $m$ is significantly less than $M$. The lighter sphere is placed directly on top of the heavier one, and the two are ...
-1
votes
1answer
23 views

Wave Equation - Vibrations & Waves [on hold]

My question is basically how to do part ii) of this question even with the hint I am confused; the first part was simple; There's no dispersion & vp = vg = sqrt(hg) as required. But how would I ...
0
votes
1answer
40 views

How do you derive the Dirac equation for momentum space?

$\require{cancel}$ \begin{align} 0 &= i \gamma^\mu \partial_\mu \psi(x) - m \psi(x) \\ &= \int \frac{d^4 k}{(2\pi)^4}e^{-i k x}\left( \gamma^\mu k_m \tilde{\psi}(k) - m \tilde{\psi}(k) ...
0
votes
0answers
24 views

Gas Pressure in Industrial processes [on hold]

Can anybody please help with the following? Ammonia is produced by the Haber process where nitrogen and hydrogen combine to form ammonia according to the equation: N2 + 3H2 == 2NH3 If 1 mole of N2 is ...
-2
votes
0answers
31 views

Thermodynamics - Gas Laws [on hold]

Can anybody please help with the following problem? The compression ratio of an engine is 16:1. The volume of air in the cylinder is $0.5 \,\mathrm{litres}$ at a temperature of $30 ...
2
votes
1answer
43 views

What are the tidal effects of Io on Jupiter?

I recall reading an essay by Asimov (I think) around 1980 stating that the tides are a function of a power of the diameter of the primary, so (surprisingly) small close moons of Jupiter raise large ...
-1
votes
1answer
43 views

Determine the equation of motion [on hold]

The problem is the following. A ring of mass $m=1$ is moving along a circle of radius $R$ without friction. It's tied to a spring (coefficient $k$) of natural length $0$. The other end of the spring ...
1
vote
0answers
26 views

Hydrogen atom Ionization by Magnetic Field [on hold]

The source of a magnetic field (it could be a magnetic dipole) is moving at relativistic speed. This magnetic field encounters a hydrogen atom at rest with respect to the source. Will this encounter ...
0
votes
1answer
49 views

Normalisation of a wavefunction [on hold]

If the system if found in the state: $$\psi=\sqrt{\frac{1}{2\pi}}(\frac1{\sqrt3}e^{-i3\phi}+ce^{-i4\phi})$$ what value of $c$ normalizes the wavefunction? Clearly: $$\int_0^{2\pi}\psi^*\psi ...
1
vote
1answer
62 views

Radiation pressure (Jackson exercise)

Here's an exercise from Jackson: A plane wave is incident normally on a perfectly absorbing flat screen. From the law of conservation of linear momentum show that the pressure exerted from the ...
-1
votes
0answers
25 views

Balancing an equation [on hold]

I had an exam today and I havn't be able to find an answer to the question asking to balance this reaction equation, about oxalic and permanganate. Could you balance it or is it just impossible ? ...
-2
votes
0answers
43 views

Establish a relation between height of the water column and time in a spherical container with a hole in the bottom [on hold]

Suppose we have half of a sphere of radius $R$ filled with water. In the bottom of the container there is a hole of area $S_0$. Velocity of the water flow out of the hole is $$V=\lambda(2gh)^{1/2}.$$ ...
0
votes
1answer
30 views

Solving Lagrangian equations of motion for two point-bodies with gravitational interaction

I would like to solve the equations of motion with the Lagrangian function for two point-bodies that interact gravitationally via the potential $$V= {-Gm_1m_2 \over r_{12}} $$ where $$r_{12} = **r_1 ...
4
votes
2answers
52 views

Electric field due to a uniformly charged FINITE rectangular plate

I was teaching kids about how to find electric field using the superposition principle for continuous charge distributions, I thought may be I should derive the formula for electric field due to the ...
0
votes
1answer
53 views

Probability of finding particle in infinite square well, displaced walls

Initially a quantum particle moves in a one-dimensional well ($x$-axis) from $-a$ to $ a$, $ V = \infty $ outside and $ V = 0 $ inside the well. So initially, the wave-function $$ \psi_0 = ...