Tagged Questions

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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4 views

Problem with the notation

We have this stationary metric, $$ds^2 = e^{2U}(dt+\omega_idx^i)^2 -e^{-2U}dx^2$$ The book wrote down the spin connections of this: $$ \omega^{0i}=\partial_ie^{U}e^0 +e^{3U}\partial_{[_i\omega _k]}e^k ...
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0answers
47 views

Horrifying electron gas model

I am given the Hamiltonian (in an exercise called: plasmons) (and $\langle, \rangle $ denotes the expectation value) $$ H = \sum_{k} \varepsilon_k a_k^{\dagger} a_k + \sum_{k_1,k_2,q} V_q ...
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1answer
24 views

How to determine velocity of two objects after they collide? [on hold]

Let's say an object of mass 1 kg is moving 10 m/s. A second object of mass $m_2$ is at rest. When the two objects collide, they stick together and move 8 m/s. what is $m_2$? I understand how to solve ...
-1
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0answers
14 views

How to relate the height from which a object can fall with the young modulus of that object? [on hold]

How to relate the height from which a object can fall without breaking with the Young modulus of that object?
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0answers
19 views
0
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0answers
19 views

Determine coefficients so that $f(x,y)$ is a conserved quantity [migrated]

Given the function $f(x,y) = ax^2y + by + ct^2 $ with $x(t)=2t$ and $y(t)=3t^2$ I shall determine the coefficients a,b,c so that $f(x,y)$ is a conserved quantity. My approach is $$\frac{df}{dt} = ...
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0answers
36 views

Computation of spin connection 1-form

If stationary metric is given by $$ds^2 = e^{2U}(dt+\omega_idx^i)^2 - e^{-2U}dx^2$$ and I want to try to solve for this. After doing the effort to find the metric and its inverse, I used the tetrad ...
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0answers
27 views

Deduce the 3 dimensional Anti de Sitter space from the 4 dimensional case

From $$AdS^4=\big\{(u,w,x,y,z) \in \mathbb{R} | -u²-w² +x² + y² + z²=-1 \big\}$$ parametrized by $$f(t,\rho, \theta, \phi) = (\sin t \cosh \rho, \cos t \cosh \rho, \sinh \rho \cos \theta, \sinh \rho ...
0
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1answer
42 views

Uncertainty Principle and single-slit diffraction [on hold]

A beam of 50eV electrons travel in the x direction towards a slit of width 6 micro metres which is parallel to the y direction. The diffraction pattern is observed on a screen 2 metres away. Use ...
0
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1answer
52 views

Does varying centripetal force produce circular motion?

Centripetal force $\vec{F}$ is responsible for circular motion and is given by $$ \left|\vec{F}\right| = \dfrac{m\left|\vec{v}\right|^2}{r}$$ If the linear velocity $\vec{v}$ varies, the centripetal ...
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0answers
13 views

capacitors - Square circuit of two capacitors charging in a changing magnetic field

I've been studying capacitors and DC circuits, and I received this question as part of my homework: The above circuit is a square circuit with side a. E and F are the midpoints of AB and CD ...
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0answers
40 views

Please help with quantum mechanics [on hold]

Let the Hamiltonian of two nonidentical spin 1/2 particles be where and are constants having the dimensions of energy. Find the energy levels and their degeneracies.
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0answers
22 views

Translational acceleration of cylinders [on hold]

Two cylinders with total mass $M$ and radius $R$ are connected by a massless rod along their axis of rotation and rest on a horizontal surface. A frictionsless ring at the center of the rod is ...
1
vote
1answer
43 views

Calculate the mass of a Schwarzchild black hole with Komar integral

In Wald's GR, Komar integral is Eq. (11.2.9): $$M=-\frac{1}{8\pi}\int_S\epsilon_{abcd}\nabla^c\xi^d$$ $S$ can be chosen as a 2-sphere, the boundary of a spacelike hypersurface $\Sigma$ such that the ...
0
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1answer
50 views

Work-Energy theorem vs conservation of mechanical energy? [on hold]

Bodies A and B are moving in the same direction in a straight line with constant velocities on a frictionless surface. the mass and the velocity of A are: 2kg and 10m/s. The mass and the velocity ...
0
votes
1answer
47 views

Spring on a cart problem [on hold]

The problem is two dimensional. At t = 0, a stationary cart at initial position (0,0) starts to accelerate with acceleration a in the negative "x-axis" direction. On the cart, there is an isosceles ...
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0answers
63 views

Green's Function

The potential due to a point charge at $\vec{r}_1$ or the Green's function is $G(\vec{r},\vec{r}_1)=-\frac{1}{4\pi|\vec{r}-\vec{r}_1|}.$ To satisfy a boundary condition for zero potential on a ...
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0answers
64 views

Why the sum of energy don't increase in this device? [on hold]

The black arm is turning clockwise at $w1$. The blue axis is fixed to the ground. The grey cruz (grey arms) is turning at $w2$, with $w2 < w1$, so the grey cruz is turning around itself ...
19
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6answers
3k views

“Falling upward” - how far you have to be from Earth to start falling to the Moon?

Talking about gravity with my 9 y/o she asked when do we start "falling upward" to the Moon. What is the distance at which the Moon's gravitational attraction is higher than that of the Earth and thus ...
1
vote
1answer
44 views

Integration of $e^{-it\sqrt{\mathbf{p}^2 + m^2}}$ for QM amplitude

My question might be more about maths than physics, but it originated in a Physics context. Take $\hbar$ = $c$ = 1. I was looking at the amplitude for a free particle to propagate from an initial ...
1
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1answer
25 views

Calculating speed of spheres in an asymmetric Newton's cradle?

If we assume there is an asymmetric Newton's cradle with 3 spheres of different masses and the first one is initialized with some certain speed can we calculate the outcome speed of the all the three ...
0
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0answers
23 views

How are Newton's Laws of motion related to the amount of work exhibited by the balloon vehicle? [on hold]

A long string is running through a straw with a balloon taped onto the straw, and a penny taped onto the balloon.
1
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1answer
53 views

What is the point of this type of graph? (Projectile) [on hold]

What is the point of graphing vs range, during kinematic motion?
0
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1answer
25 views

How to find amplitude?

What are the general methods of finding amplitude in various cases of simple harmonic motion containing spring block system? I always try with the basic sine formula of SHM to find amplitude.
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1answer
33 views

Energy of a charge inside spherical conducting shell

A point charge $q$ is at the center of an uncharged conducting spherical shell of finite thickness, with inner and outer radii $a$ and $b$ respectively. Find the work done on the system when $q$ is ...
0
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1answer
31 views

What is the movement of the material point? [on hold]

The parametric formulas for movement of a material point are: $x(t) = A \sin (kt)$ $y(t) = B(1 - \sin(kt))$ Describe the movement of the MP. That is the entire question and I can't find a ...
3
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3answers
72 views

Variable mass dynamics: Particle and Rigid Body

I'm encountering some issues in the understanding of some basic concepts about the dynamics of variable-mass particles and rigid bodies. For what I found, for example reading On the use and abuse of ...
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votes
2answers
32 views

Emf induced on the wings of an airplane

An airplane goes over antartica. According to a passenger who always faces towards the front of the airplane, which side of the wing has a higher electric potential (right or left)? I realise this ...
-1
votes
0answers
23 views

Rotation of a rigid body [on hold]

There is a pen of length l = 12cm, which initially is in vertical position and falls by rotating through the center Q which coincides with the extreme of the pen on which it was initially standing. ...
0
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0answers
26 views

Why we don't integrate intital velocity in body cast equation?

On this site I've found a formula for calculating the $x, y$ coordinates for a body throwed by an angle to a horizon. It looks like this: $$x(t) = V_0 t \cos(\alpha); $$ $$y(t) = V_0 t ...
-2
votes
0answers
20 views

Calculating errors?

Hollow cylinder of length l, inner and outer radii r1= 2.5+-0.3mm, r2= 5.5+-0.3mm, density= 7.88g/cm^(3), mass density= p, mass= (pi)xpxlx(r2^(2)-r1^(2)). What is the mass?
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1answer
28 views

Spin operator eigenstate in Fock space

I am creating an operator group from representation of spin 1 operators $$J_{x} = \frac{1}{\sqrt{2}}\left(\begin{array}{ccc} 0 & 1 & 0\\ 1 & 0 & 1\\ 0 & 1 & 0 \end{array} ...
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votes
2answers
22 views

Energy Continuity Equation [on hold]

I have the following equation (Energy Continuity Equation) derived from Einstein/Friedmann equations: $$ \dot \rho + 3\left(\rho + P\right)\frac{\dot a}{a} = 0 $$ Now, I then have to show that $\rho ...
0
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0answers
14 views

What is the minimum velocity an object needs to hit a vertical hanging rod with, so it it can make a revolution? [on hold]

The Rod is handing vertically, Attached to a firm end. An object of mass m hits it with velocity v, so what minimum value does v should have so this rod can rotate about its pivot?
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0answers
23 views

How to calculate orbital eccentricity from the ratio of satellites' velocities?

I have a problem with working out how to calculate the orbital eccentricity from the ratio of two satellites' linear velocities. We know that two satellites encircles the Earth in two elliptical ...
-1
votes
1answer
28 views

What is the radius of the circular orbit? [on hold]

Find the radius of the circular orbit of a satellite moving with an angular speed equal to the angular speed of Earth's rotation. I started with the relation of time period ad the radius.
0
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1answer
34 views

Conservation of momentum 1-D problem: track is not level

This problem is a standard kind of 1-D problem. You have 2 equal carts approaching each other and collide on linear air track. BUT the person setting up the track did NOT do a good job at making the ...
0
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0answers
28 views

What is the true weight in the geostationary satellite? [on hold]

What is the true weight of an object in a geostationary satellite that weighed exactly 10.0N at the north pole? I did: At north pole the weight of the body is 10 N. Therefore the mass of the body is ...
0
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0answers
17 views

Max thickness that can be cooled in a certain time

A frozen-food company freezes its spinach by first compressing it into large slabs and then exposing the slab of spinach to a low-temperature cooling medium. The large slab of compressed spinach is ...
0
votes
1answer
26 views

Will a heavier or lighter tank shell / bullet/ etc. travel further given the same starting energy?

Let's say I have for example two tank shells, both being the same size and having the same shape. Let's say the starting energy is 100kg*m/s^2, and the one shell is 1kg, the other 25kg. 1kg * ...
0
votes
2answers
22 views

Damping Constant for a Ball Rolling in a Bowl

How would you find the damping constant of a ball rolling without slipping in half of a sphere? I know how to find the period of the oscillation, but how would I find the damping constant ...
-2
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0answers
9 views

Parabolic motion exercise [on hold]

From the top of a building, when t=0, a ball is thrown with a speed of 50m/s, forming a 30° angle over the horizontal. The ball hits the street, wuth a 40° angle below the horizontal. Find: a) Time in ...
0
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0answers
13 views

What is the total energy? [on hold]

So, there was this example in my book with a rock being lifted upwards. The person lifting the rock does 20J of work and gravity does 20J of work. I know Wnet = 0, since there isn't a change in ...
0
votes
2answers
111 views

How can Rosetta spacecraft orbit an object with such a low mass?

Comet 67P Churyumov–Gerasimenko's mass is 11 orders of magnitude lower than Earth's. That means that the comet's gravity force is also weaker than Earth's in the same proportion. Provided that also ...
0
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0answers
19 views

Magnetic,Uniform Magnetic field [on hold]

If a circular current carrying loop ,carrying current $I$, is placed in a uniform magnetic field $B$,calculate the force of expansion on the loop. Radius is $R$. Is the answer $IRB~~$ or $~~2 IRB$?
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1answer
32 views

Dimensional analysis of sums [on hold]

Quick question: What is the dimension of the following fraction ($\sum_{i=1}^{n}a_i b_i )/ \sum_{i}^{n}a_i$ where $a_1,a_2,...,a_n$ are in kg and $b_1,b_2,...,b_n$ are in Newton?
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2answers
28 views

Photoelectric effect intensity

I understand the PE effect quite well but I'm failing to understand one thing. Intensity is the amount of energy per second incident to a given area. So can you can increase the intensity by either ...
0
votes
1answer
52 views

What would be observed by two observers traveling away from one another at 0.75c? (And why?) [on hold]

What would be observed by two observers traveling away from one another at 0.75c, and why? Suppose that two observers O1 and O2 are at a point S. Then, they begin traveling away from one another at ...
0
votes
0answers
29 views

Angle of throwing a ball in a train moving in an arc [on hold]

Suppose a man is standing in the middle of a train car, when the train is riding with speed $V_T$ on a track section with circular curvature of radius $R$. The man wants to throw a ball with velocity ...
1
vote
0answers
47 views

Finding the metric [on hold]

If given the metric: $$ds^2=e^{2U}(dt+w_idx^i)^2-e^{-2U} d\overrightarrow{x}^2$$ where $w = w_idx^i$ is one form How to find the metric in order to find the inverse metric? The new thing about this ...