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
51 views

Problems with Maxwell density Lagrangian [on hold]

I want to expand the right-hand side of $$\dfrac{1}{4} F_{\mu\nu}F^{\mu\nu} = \dfrac{1}{4}\left(\partial_{\mu}A_{\nu} - \partial_{\nu}A_{\mu}\right)\left(\partial^{\mu}A^{\nu} - ...
0
votes
1answer
46 views

Wave equation on Schwarzschild background

I am trying to follow the solution of the wave equation for a scalar field on Schwarzschild background from http://batteringram.org/science/gr/scalar_wave.pdf. I have a problem on page 2 where they ...
0
votes
0answers
10 views

Kinematic of linear motion [on hold]

Water flows out horizontalmy at the end of a pipe at a height of 52 cm from the floor.If the horizontal distance before it hits the floor is 100 cm,calculate the velocity of the water at the instant ...
0
votes
1answer
48 views

Drag in low earth orbit

So I'm looking at the following problem; Arrow type configuration in orbit. Each end of the arrow has a mass and a cross sectional area (we can assume the head and tail are connected by a ...
-1
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0answers
13 views

Projectile - air drag problem! [on hold]

I know it's not a homework help site, but still, I apace proceeded in this problem, but cannot go further. A body of mass m us thrown at an angle $\alpha $ to the horizontal plane with an ...
1
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0answers
28 views

Scalar field in Reissner-Nordstorm metric

I am trying to solve this kind of problem: For a scalar field $ \psi $ in Reissner-Nordstorm metric \begin{align} g_{ab} dx^{a} dx^{b} = - f dt^{2} + f^{-1} dr^{2} + r^{2} d \Omega_{S^{2}}^{2} ...
3
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1answer
71 views

Simple Harmonic Motion in Special Relativity

I was trying to see what results I would get if I were to incorporate relativistic corrections into the case of a harmonic oscillator in one dimension. I thought that if the maximum velocity of the ...
0
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0answers
27 views

How do we deal with air resistance?

Suppose a body of mass $m$ is projected and given that the air resistance is $kv$, where $v$ is the instantaneous velocity. I need to find the maximum height. I could do it given that the air ...
-1
votes
0answers
44 views

Proving the speed of light to be an invariant and the “ultimate” speed limit using Minkowski diagrams

Can one show that a certain speed, denoted $c$, is invariant and the ultimate speed limit in all reference frames using Minkowski diagrams? As we define $c = 1$, one can find the angle $\tan^{-1}(1) ...
-2
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0answers
23 views

Traveling wave on a (transmission line) rope encountering a mass [on hold]

I would like to know the solution to this problem (or at least some very helpful tips): On a rope with a constant line density $\rho$ we have a traveling wave with wavelength lambda. On a certain ...
3
votes
1answer
66 views

On the Lorentz Group representation [on hold]

I am going through the notes on QFT by Srednicki (which is certainly a worth reading on the subject, and can be found online, see http://web.physics.ucsb.edu/~mark/qft.html). When describing ...
1
vote
0answers
19 views

2D aerofoil fluid analysis - find velocity ratio

I just wondered if you could help me out on this fluid mechanics question? I have worked through it myself but am very unsure of my answer (not yet found the pressure difference) the question is ...
1
vote
2answers
70 views

Calculating euler number of disk

I'm trying to do exercise 3.1 from Polchinski, which should be a rather easy differential geometry problem. I have to calculate the euler number defined by $$\chi = \frac{1}{4\pi}\int_{M}d^{2}\sigma ...
-1
votes
0answers
25 views

Rotating cone magnetic field [on hold]

how to calculate the magnetic field of a rotating cone with a superficial charge distribution that varies with the altitude? It knows : height of the cone , truncated cone rays bases , there are ...
1
vote
0answers
26 views

Fourier transformation and mode expansions [duplicate]

Sorry as this is a rather trivial question, but I'm stuck with a certain implication. I'm working on exercise 1.7 from Polchinski where we are given an open string with boundary conditions ...
-3
votes
0answers
17 views

How does one solve this problem(IOAA 2014, data anlysis)? [on hold]

http://prntscr.com/7nwjrt http://prntscr.com/7nwjvf This problem is from IOAA 2014, second problem from data analysis. I can't connect height with time without velocity or maximum height, which I ...
0
votes
1answer
48 views

Stress-energy tensor on spacetime satisfying Klein-Gordon equation

Consider the stress-energy-momentum tensor $$T_{\alpha \beta}=(\nabla_\alpha \phi )\nabla_\beta \phi -\frac{1}{2}g_{\alpha \beta}((\nabla^\nu \phi ) \nabla_{\nu} \phi +m^2 \phi^2$$ where the ...
0
votes
1answer
44 views

Question about Majorana spinor's property

I am reading the BBS, Exercise 5.1 This exercise is nothing but showing that two Majorana spinors $\Theta_1$ and $\Theta_2$ \begin{align} \bar{\Theta}_1 \Gamma_{\mu} \Theta_2 = -\bar{\Theta}_2 ...
-1
votes
0answers
25 views

Inhomogeneous Maxwell's equations in CGS units [on hold]

What is the basis or the way of writing the given inhomogeneous Maxwell's Equations in CGS units: $$\nabla.E=\frac{\rho}{\epsilon_0}$$ $$\nabla\times B=\mu_0J+\mu_0\epsilon_0\frac{\partial E}{\partial ...
-3
votes
0answers
24 views

SHM by using cos equation [on hold]

A particle start motion according to equation $x=a\cos \left(\omega t+ \frac{\pi}{6}\right)$. Find time required for it to reach mid point. The displacement according to it must be 0 but how?
0
votes
0answers
79 views

Two apparent contradictions in SR involving time dilation and length contraction

So I've encountered the site "Alternative Physics" – a website proposing "alternative" theories to theories in modern physics. Of course, the site claims that both SR and GR are wrong. To show "why" ...
0
votes
1answer
34 views

Intersection of two moving objects [on hold]

There are two objects. The first object moves with speed $U_1$ from known point A to known point B. The second object has speed $U_2$ and starts from known point C. What is the direction the second ...
-5
votes
0answers
23 views

newtonian-mechanics [on hold]

A 1000Kg helicopter hovers over a forest fire and starts to squirt water downward with a speed of 10m/s at the rate of 40Kg/s. What is the initial upward acceleration on the helicopter when it first ...
0
votes
0answers
27 views

Moment of inertia of a connecting rod in a crank-slider mechanism [on hold]

PROBLEM I am trying to determine the moment of inertia of a connecting rod (conrod) in a crank-slider mechanism. $r$ is the crank radius. $l$ is the length of the conrod. $I_{rod}$ is the moment of ...
1
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1answer
50 views

Reduction of Earth's Rotation due to increase in humidity

I came to know that, moon is moving away from earth, resulting reduction of Earth's rotation. Similarly, if Earth's humidity increases, water vapor will be move up to the air from water sources, will ...
-1
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0answers
9 views

Kinematic Motors in series to flywheel calculation problem [on hold]

Motor 1 drives a platform at 15 RPM clockwise steady state always 15 RPM. Call the axis of rotation Y. Motor 2 is mounted on this 15 RPM platform and axis aligned thus also Y axis (parallel Axis). A ...
2
votes
1answer
68 views

Symmetry of extrinsic curvature tensor

I am trying to solve following problem: In a spacetime of signature (+, −, −, −), let $$ u^au_a = 1, \quad A_{ab} = \nabla_cu_dh^c_{\; a}h^d_{\; b}, \quad h_{ab} = g_{ab} - u_au_b $$ Show that ...
0
votes
0answers
43 views

Transforming matrices using tensor transformations? [migrated]

Let us say I started with the matrix $$ A= \begin{pmatrix} x & -y\\ y & x \\ \end{pmatrix} $$ And I wanted to use the tensor transformation: $$ \bar ...
2
votes
1answer
51 views

Raychauduri equation for Milne universe

I am trying to solve this kind of problem: Consider the Milne model, i.e., the empty $ \kappa = −1 $ Friedmann model. Verify by a direct calculation that the expansion $ \Theta $ of the unit ...
0
votes
0answers
39 views

Kinetic Energy and reference frame

Two space ships A and B moving through space in the same direction. Space ship A moves 2m/s faster than B, its weight is 1kg and it has a 2 Watt engine. It is accelerating with full power for one ...
-1
votes
0answers
15 views

Mixture of Gases in kg-mol; Find Temperature [on hold]

A one cubic container contains a mixture of gases composed of 0.02 kg-mol of oxygen and 0.04 kg-mol of helium at a pressure of 220 kPa. What is the temperature of this ideal gas mixture in degrees ...
-1
votes
0answers
30 views

Are there any good places to learn physics without going to university or college? [on hold]

I'm potentially interested in learning physics for my own knowledge, and not to get a degree. I want to start some engineering / technology companies in the future, and having a knowledge in physics ...
0
votes
1answer
21 views

What is the work done in isobaric expansion?

I'm really confused now... I thought the equation was $ W = \int PdV = P\Delta V $ since P is a constant and can be taken outside of the integral. But on the other hand, $ P = \frac{NkT}{V} $, so if I ...
0
votes
3answers
110 views

Gravitational time dilatation calculation [on hold]

I'm proving an example case from Kip Thorne's book, Science of Interstellar for time dilatation at a Kerr black hole with the following data: \begin{align} a & = 0.998, \\ r & = ...
9
votes
0answers
74 views

Degenaracy in mass of $8$ and $27$ reps of $SU(3)$ in Coleman's Aspects of Symmetry

In Coleman's Aspect of symmetry he proposes an amusing problem in the first chapter. It asks us to consider a set of eight pseudo-scalar fields transforming in the adjoint representation of $SU(3)$. ...
1
vote
0answers
79 views

Fourier transformation and commutators

Sorry as this is a rather trivial question, but I'm stuck with a certain implication. I'm working on exercise 1.7 from Polchinski where we are given an open string with boundary conditions ...
0
votes
0answers
36 views

Astronomy - Hubble Law and Universe age [on hold]

We know that the approximated age of the Universe is calculated as 1/H0 where H0 is expressed like (km/s/Mpc). If I try to calculate the dimensional equation I obtain: (10^3 m/(s* 10^6*10^16 m))^-1, ...
2
votes
2answers
92 views

Integration by parts to derive $d\langle x \rangle / dt$

I am reading "Introduction to Quantum Mechanics" by David Griffiths and I am having trouble understanding part of a derivation of $\frac{d\langle x\rangle }{dt}$ in section 1.5 - Momentum - of the ...
0
votes
0answers
19 views

Capacitor problem [on hold]

A charge of 200 micro coulomb is imparted to each of two identical capacitors connected in parallel. At time t=0 the plates of both the capacitors are 0.1 metre apart. The plates of first capacitor ...
-1
votes
0answers
66 views

What is the work being done a girl carrying a bag walking down the street? [on hold]

What is the work being done a girl carrying a bag walking down the street?
0
votes
0answers
37 views

Doubts taking the second functional derivative of the Klein Gordon action

I have very little background with functional derivatives and I would like to clarify some issues. I am trying to compute the second functional derivative of the Klein Gordon action expressed in real ...
-1
votes
0answers
29 views

I tried to solve circuit analysis problem but it failed [on hold]

I tried to solve circuit analysis problem in Hayt's Circuit Analysis 7th. But my answer is wrong and I don't know why I failed to solve this. Here's the question. I have to find what the value of v3 ...
-7
votes
0answers
22 views

Need help please [on hold]

An electron's speed is doubled from $0.2~c$ to $0.4~c$. By what ratio does its momentum increase? What happens to the momentum ratio when the electron's speed is doubled again from $0.4~c$ to $0.8~c$? ...
1
vote
1answer
25 views

Symmetry axis and products of inertia

So if we have an object that has a symmetry axis let us say $z$-axis is a symmetry axis does this mean that the product of inertia $I_{zx} = I_{zy} = 0$? And if that is true why is it true ...
0
votes
0answers
23 views

Flow Rate Calculation from a drain tank through a pipe [duplicate]

I want to know how to calculate the flowrate (Q) of a drain tank flowing through a pipe (see the figure below) All I know is that Q=Velocity(V) x Area (A) V= sqrt(2*g*H)=sqrt(2*9.81*10)=14m/s. I ...
-2
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0answers
26 views

its tricky circuit to given output voltage find without source [on hold]

solve it two quation in that pic,1) find Vout voltage and 2) no source given and find v1,v2,v3.
0
votes
1answer
52 views

Deriving Gravitational Potential Energy using Vectors

Here is my attempt at derivation: First you must find a vector function for the gravitational force. By the inverse square law, the magnitude of gravitational force between two bodies of mass $m$ ...
-4
votes
0answers
23 views

Free fall motion of a body [on hold]

A ball is projected vertically upwards with an initial velocity of 100m/s. Find the speed of the ball at its maximum height Take g=10metre per second square.
-1
votes
0answers
16 views

Average velocity of a clock [on hold]

The minute hand opf a clock is 4 cm long Find the average velocity of the tip of the minute hand between 11:00 a.m to 11:30a.m and 11:00 a.m to 11:30 p.m
-1
votes
0answers
19 views

Motion of particle in $T$ seconds [on hold]

A particle starts moving from rest position ($v = 0$) under a constant acceleration $a$. If it travels a distance $\Delta{x}$ in time $T$, what distance will it travel in the next time $T$?