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 questions.

learn more… | top users | synonyms (1)

2
votes
1answer
155 views

Refraction of light in water

Here's a question in a paper based on refraction of light. I can't seem to solve it for some reason. A man looks down at a fish length of 20 cm. His eye is 2m above the surface of the water ...
2
votes
2answers
181 views

Question on the uncertainty principle

The problem statement: Measurement detects a position of a proton with accuracy of $\pm10pm$. How much is the position uncertainty $1s$ later? Assume the speed of a proton $v\ll c$. What i ...
0
votes
0answers
75 views

Crystalography - hardest homework problem i have seen so far

The problem statement: We are observing an electron diffraction on a "Ni" crystal. We point the narrow beam of electrons on a crystal which can be rotated so that we are changing the angle ...
2
votes
1answer
122 views

About the conserved charge for the ghost number current in $bc$ conformal field theory

(skip disclaimer) I have a question about the conserved charge for the ghost number current in $bc$ conformal field theory in Polchinski's string theory p62. It is said For the ghost number ...
1
vote
1answer
61 views

Problem similar to Davison & Germer eksperiment but with more angles and the mirror reflection

The problem statement: Narrow beam of electrons hits an aluminium plate under an angle $\alpha=30^\circ$ according to the flat surface. We know the distance between two crystal planes in ...
-3
votes
1answer
4k views

Neutral axis of T shaped beam? [closed]

I am not a mechanics or physics student but a computer science student. I came across a question related to find neutral axis of figure but I do not have slightest idea of what it is and how to find ...
2
votes
1answer
790 views

Calculating specific orbital energy, semi-major axis, and orbital period of an orbiting body

Is it possible to calculate the specific orbital energy $ϵ$, the semi-major axis $a$, and the orbital period $T$ (or $P$) without any of them being available to you? The values I do have available to ...
0
votes
1answer
264 views

Gas mixture flow rate equation [closed]

2 gas flows connect and mix in a pipe. The following is known: $\dot{m_1}$; $\dot{m_2}$ - mass flow rate of the inlet gases $(\frac{kg}{s})$ $A_1$; $A_2$; $A_{mix}$ - the cross section of the pipes ...
3
votes
2answers
141 views

Quantum Mechanics Basics: product space

Consider a coupled harmonic oscillator with their position given by $x_1$ and $x_2$. Say the normal coordinates $x_{\pm}={1\over\sqrt{2}} (x_1\pm x_2)$, in which the harmonic oscillators decouple, ...
4
votes
2answers
235 views

Calculating an expression for the trace of generators of two Lie algebra

Suppose we have $$[Q^a,Q^b]=if^c_{ab}Q^c$$ where Q's are generators of a Lie algebra associated a SU(N) group. So Q's are traceless. Also we have $$[P^a,P^b]=0$$ where P's are generators of a Lie ...
1
vote
1answer
50 views

Escape from gravitational field

Why total energy of a spacecraft must be non-negative if we want spacecraft to escape from the Earth's gravitational field?
1
vote
1answer
169 views

Motion of block on wedge

there is some confusion to me in the case of "motion of block on a frictionless wedge" Below is a simple diagram! Let us consider a situation as above in which there is a block of mass $m$ moving ...
0
votes
0answers
77 views

nuclear physics- Energetics and Mechanics of Nuclear Reaction Homework

A sample of $^{24}_{12}\mathrm{Mg}$ is bombarded by a monoenergetic proton. If the resulting nucleus in a $^{24}_{12}\mathrm{Mg}(p,\gamma)$ reaction; $^{24}_{12}\mathrm{Mg}(p,\gamma)$ has its ...
0
votes
1answer
89 views

Velocity of a spacecraft in a circular orbit

What is a formula for velocity of a spacecraft in a circular orbit? Actually, is there a formula or I can find it from equation of motion?
0
votes
1answer
733 views

Potential energy of the dipole-dipole interaction for two parallel dipole moments

I am looking for an equation that gives me the potential energy of the interaction between two parallel dipoles.
1
vote
1answer
236 views

What is the friction between cylinder and wall (ground)?

A hollow cylinder (radius $R$) is rolling against the wall at angular speed $\omega$. The coefficient of friction between the cylinder and the wall(ground) is $\mu$. After how many rotations the ...
1
vote
2answers
269 views

At what starting angle will a billiard ball hit a pocket? [closed]

Let's say you have a square pool table with one pocket. At what angles can you hit the ball to make it hit the pocket? What relation do these angles hare? Constraints: Table is square: $\ell=b$. ...
0
votes
3answers
177 views

Which parameter determines how much the 'recoil' or force applied by an object hurts?

Another question from my textbook, directly following after this question: 2 identical bullets are fired, on from a lighter rifle and one from a heavier one, with the same amount of force. Which ...
6
votes
4answers
561 views

Symmetry in resistor circuits

Given 6 points that are connected with each other with a resistor of resistance $R$, find the resistance between any two points. (Answer: $R/3$) (All the conducting wires have the same ...
1
vote
2answers
101 views

Possibility of determining the mass of the water without a scale?

Possibility of determining the mass of the water by knowing water volume, water temperature and atmospheric pressure. I want to know if I can determine the mass of $V=0.01\,m^3$ of water in ...
2
votes
2answers
378 views

Falling chain with friction

I really need some help with a physics problem, and I guess my doubt is more conceptual, than question based. But still, let me pose it to you: A uniform chain of mass $M$ and length $L$ lies on ...
1
vote
4answers
1k views

Velocity of 2 balls with different masses on a moving train after the train brakes

Question This is from my textbook: There are 2 balls of the same size made of rubber and iron respectively kept on the smooth surface of a moving train. Which ball will move faster when the train ...
1
vote
1answer
151 views

Compute the central charge of $bc$ conformal field theory

I have a s****d question, how to calculate the central charge of $bc$ conformal-field theory in Polchinski's string theory, Eq. (2.5.12)? For a $bc$ CFT given by $$S=\frac{1}{2\pi } \int d^2 z \,\,b ...
0
votes
1answer
103 views

Atom emitts a photon - what is the frequency of the energy of the photon?

Lets say I have an atom of known mass who goes from excited state to the ground state whose energy is $14.4 keV$ lower. I know that the emitted energy $14.4keV$ got converted into an energy of a ...
0
votes
0answers
71 views

Question on Binet's equation

Is there a difference in Binet's equation when the force acting on a body is attractive and repulsive? I mean, if the force has a magnitude $F(r)$, is it Binet's equation always ...
0
votes
1answer
85 views

Calculating angular momentum

A particle of mass $m$ is at a very large distance $p$ from origin $O$ and is moving with velocity $\vec{V}$ which is perpendicular to $\vec{OP}$. I have to calculate angular momentum $L$ of the ...
0
votes
1answer
44 views

Conservation of energy constant

A particle $P$ of mass $m$ moves under the repulsive inverse cube field $\vec{F}=\frac{m\gamma}{r^3}\vec{e_r}$ ($\vec{e_r}$ is a unit vector along a position vector $\vec{r}$). Initially $P$ is at a ...
-2
votes
1answer
114 views

Temperature of rod if end points are different temperature [closed]

How does the temperature vary along the length of the rod if its both ends are at different temperature. As an example, consider the problem: 20 cm long rod has rod at one end 100 ºC and another ...
1
vote
1answer
105 views

Distance of image from concave mirror

A concave mirror of focal length $8cm$ forms an inverted image of an object placed at a certain distance. If the image is twice as large as the object, what is the distance of the object and the image ...
4
votes
2answers
304 views

FFT distortion, harmonics (singing wine glass)

I'm doing a school assignment on Singing Wine glasses (you rub the rim of the wine glass with a wet finger and it produces a pure tone). I have recorded $30\,\text{ms}$ of the "singing" at a sampling ...
1
vote
1answer
128 views

How to derive Eq. (2.4.23) in Polchinski's string theory book

Given the Operator Product Expansion (OPE) of a product of the energy momentum tensors $$T(z)T(0) = \frac{ \eta^{\mu}_{\mu} }{2z^4} - \frac{2}{\alpha' z^2} :\partial X^{\mu} \partial X_{\mu}(0): + ...
2
votes
2answers
270 views

2 protons collision (both with different kinetic energies) - I don't know what to put in for $p^2c^2$

The problem statement: Two protons with kinetic energies $W_{k1}=4GeV$ and $W_{k2}=2GeV$ colide and form new particles. What is the mass of newly born particles? There are as many as possible ...
1
vote
1answer
66 views

Identify the weight of operator under conformal transformation

I have a stupid (homework tag may be suitable =_=) question about the problem 2.7 in Polchinski's string theory volume 1. Why the weight of operator $:e^{ik\cdot X}:$ is $(\frac{\alpha'k^2}{4}, ...
2
votes
2answers
174 views

$\left(H^\dagger H\right)^2$ is invariant under $U(1)\times SU(2)$?

Is it true that $\left(H^\dagger H\right)^2$ is invariant under $U\left(1\right) \times SU\left(2\right)$ where $H$ is the Higgs field $(1,2,1/2)$? Does this invariance imply that its hypercharge ...
-3
votes
1answer
135 views

2 particles collision - deriving the equation for an energy which can become mass of new particles!

Q1: Lets say a particle with mass $m_1$ and kinetic energy $W_k$ colides with a particle of mass $m_2$ at rest. Can anyone explain why do both particless ($m_1$ and $m_2$) plus an aditionall mass $m$ ...
2
votes
2answers
103 views

Identity of Operator Product Expansion (OPE)

I have one more s****d question in Polchinski's string theory book, Eqs. (2.3.14a) $$ j^{\mu}(z) :e^{ik \cdot X(0,0)}:~ \sim~ \frac{k^{\mu}}{2 z} :e^{ik \cdot X(0,0)}:,$$ where $j^{\mu}_a ...
0
votes
0answers
141 views

Pair annihilation - how to generaly solve these types of problems?

Simple question regarding pair production: Can a collision of an electron with a positron result in two photons? I have this problem to solve and I doubt this is even possible. Please provide some ...
5
votes
1answer
121 views

A question about conformal transformation in Polchinski's string theory

I have one more stupid question in Polchinski's string theory book. P. 46, it is said It is convenient to take a basis of local operators that are eigenstates under rigid transformation (2.4.9) ...
1
vote
1answer
107 views

Compton scattering - Does this example have a solution if i get negative number under the square of quadratic eq.?

1. The problem statement Incomming photon gives half of its energy to an electron during scatering. After scattering, photon is headed $\phi=120^\circ$ according to the original direction. ...
1
vote
0answers
121 views

How to prove Eq. (2.4.5) in Polchinski's string theory book?

I got one more stupid question in Polchinski's string theory book. In p. 44, it is said The currents $$j(z)=i v(z) T(z), \tilde{j}(\bar{z}) = i v(z)^* \tilde{T}(\bar{z}) \tag{2.4.5}$$ are ...
2
votes
1answer
86 views

Compton scatering problem - How to solve this large problem faster using the momentum system?

I have encountered a hard exercise which i cannot quite solve. Could anyone help me with it? This is the exercise: Lets say we have a photon whose energy $W_f$ is equal to the relativistic ...
2
votes
1answer
186 views

How to derive Eq. (2.1.24) in Polchinski's string theory book

Excuse me, I got one more stupid question in Polchinski's string theory book :( $$\partial \bar{\partial} \ln |z|^2 = 2 \pi \delta^2 (z,\bar{z}) (1) $$ I shall check this equation by integrating both ...
5
votes
1answer
237 views

Divergence theorem in complex coordinates

This question is related to Stokes' theorem in complex coordinates (CFT) but, I still don't understand :( Namely how to prove the divergence theorem in complex coordinate in Eq (2.1.9) in ...
0
votes
1answer
215 views

Coordinate system

Quoting from 'Nuclear Physics - Theory and Experiment' by RR Roy, BP Nigam 2005 edition Link to text How did the author arrive at equations (23a, 23b,23c)? Chapter 8 Nuclear model II, 8.7 ...
-2
votes
2answers
230 views

RC Parallel Circuits [closed]

I am trying to solve the following exercise: A capacitor 27 and $\mu$F is connected to a battery of 21 V through a resistance of 14 M$\Omega$. 1) What 's the charge on his parallel-plate? 2) ...
0
votes
1answer
44 views

Are these data enough to solve this simulataneity problem

observers: S and S'. S stands in midway between two events (say, 2 flashes) located 20 metres apart. He sees two flashes occurring simultaneously. S' notes a 20ns difference in them. S' is moving ...
0
votes
0answers
59 views

How can this formula be analyzed intuitively?

Given $$ \bar{U}_{vib}(T) = R \sum_{\text{n.m.}} \left( \frac{\theta_{vib, n.m.}}{2} \right) + \frac{\theta_{vib, n.m.} e^{-\theta_{vib, n.m.}/T}}{1-e^{-\theta_{vib, n.m.}/T}} $$ where ...
2
votes
1answer
114 views

Where does $p^i/p^+$ come from in the EOM of an open string?

I have a stupid question about Eq. (1.3.22) in Polchinski's string theory volume 1. In the equation of motion for an open string, Eq. (1.3.22), $$X^i (\tau, \sigma) = x^i + \frac{ p^i}{p^+} \tau + ...
3
votes
2answers
272 views

Restriction of a Lagrangian

I'm wondering if anyone could help me with the following questions. Let $M$ be the Minkowski spacetime, given $f\in C^{\infty}(M) ; f(m)=x^{0}(m)$, with $\{x^{\mu}\}$ being a global Cartesian ...