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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0answers
28 views

Calculating OPE of Graviton Vertex Operator

Consider Exercise 2.8 in Polchinski's String Theory book. We are asked to compute the weight of $$f_{\mu \nu}:\partial X^{\mu} \bar{\partial}X^{\nu}e^{ik\cdot X}:$$ I have carried out the usual ...
-6
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0answers
24 views

Speed at time of impact [on hold]

A car runs off the road and hits a stationery rock. Some glass pieces from the windscreen are projected forward and were found at an average distance of 15 meters from the car. The average height ...
-3
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0answers
16 views

At time t=o a body is thrown vertically upward with a velocity u at time t=t [on hold]

At time t=o a body is thrown vertically upward with a velocity u at time t=t another body is thrown with same velocity u . when that two body will meet together ?
-2
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1answer
21 views

Thermodynamic process Question [on hold]

A 3.1-mole sample of an ideal gas is gently heated at constant temperature 290 K. The gas expands to 1.5 times its initial volume as its pressure decreases. What is the change in the internal energy ...
-2
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0answers
15 views

problems on power, force and velocity [on hold]

A car of 1200kg is moving along a horizontal road, the engine of the car is working at a constant power of 24kilowatts. The frictional resistant to motion has a magnitude of 400N. When the speed of ...
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2answers
27 views

Resistors and dependent voltage sources

why not $i(2\;kΩ)=15\;mA$ and $i(5\;kΩ)=4\;mA$?
0
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1answer
28 views

Gauss's Law for slabs [on hold]

When you have a slab with thickness $t$ and infinite in other two directions , why $E \cdot A$ (flux) is zero in z direction ( shown in figure ) Problem 57 in the picture
-3
votes
0answers
19 views

Reference wave and wave travelling through a medium - refractive index [on hold]

To determine the refractive index of a medium two identical em-waves are generated. One wave goes through unknown medium whereas the other one stays in vacuum. Determine the refractive index ...
-4
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0answers
35 views

Problem based on equations of motion [on hold]

Two balls A and B are thrown simultaneously, A vertically upwards with a speed of 20 m/s from the ground and B vertically downwards from a height of 40m from the same speed along the same line of ...
-6
votes
0answers
27 views

Vector addition [on hold]

A pilot sets a course of 040$^{\circ}$ at an airspeed of $750$$km$$h^{-1}$. A wind blows SE at $140$$km$$h^{-1}$. Find the plane's ground speed and track. Can anyone illustrate this in a diagram?I ...
0
votes
1answer
87 views

Work done in a capacitor with a conductor between its plates [on hold]

I have been agonizing over this problem, and have tried for more than two hours two find it with Kirchhoff equations, but I end up with 0=0 or some algebraic monstrosity. I have the following ...
0
votes
3answers
63 views

How much force does it take to move the Earth? [on hold]

If you could fly and survive in space, how much force would you need to move Earth at 1 meter per second?
0
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0answers
72 views

Spin 1/2 particle in rotating magnetic field [on hold]

So, I'm attempting a solution to the following problem: I am given a particle with spin 1/2 in a constant vertical magnetic field and a rotating field in the (x,y) plane. $$B(t) = [B_1\cos(\omega ...
0
votes
0answers
29 views

What is the kinetic energy of a proton as it is falling towards the core of the star?

Given: Star's mass = $1.99e10^{30}kg$ Type II supernova occurs (in a simple form: the outer layers of the star fall into the now inactive core at speeds of 126000km/s) $10^{44}$ joules of energy is ...
0
votes
1answer
28 views

How to know if a body is travelling with uniform acceleration or not?

One of the Example in my Textbook is A body travels a distance of 2 m in 2 seconds and 2.2 m in next 4 seconds. What will be the velocity of the body at the end of 7th second from the start. In ...
-2
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1answer
21 views

Torque-direction of rotation

Taking moment about $A$, the mass of 2.0g and 0.50g is rotated in the clockwise direction is because they are both directed in downwards position? while $R2$ is acted in upward direction, so it is ...
-2
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0answers
49 views

Maxwell said the dimension of charge is $L^3 T^{-2}$ and of mass is $L^3 T^{-2}$, is he right? [on hold]

If his right then, from Lorentz magnetic force, $$F= BQV$$ where $F$ is force, $B$ is magnetic flux density, $Q$ is charge and $V$ is velocity. Therefore, $$M L T^{-2} = B Q V \, .$$ Substituting for ...
0
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0answers
73 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
53 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
1answer
55 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
votes
0answers
19 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 ...
2
votes
0answers
39 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} ...
4
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2answers
123 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
votes
0answers
32 views

How do we deal with air resistance? [on hold]

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
53 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
24 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
79 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
23 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
75 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
26 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
27 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
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0answers
19 views

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

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
51 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
48 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 [closed]

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
92 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
36 views

Intersection of two moving objects [closed]

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 ...
-6
votes
0answers
25 views

newtonian-mechanics [closed]

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
38 views

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

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
vote
1answer
52 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
votes
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
71 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
52 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
42 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
16 views

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

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
31 views

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

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
22 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
2answers
121 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 & = ...