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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19 views

Vibrating water container problem

I am struggling with visualising and understanding the phrasing of this question - cross posted from Math stack exchange since this forum is more appropriate: "A water-filled container is ...
1
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1answer
10 views

can any one please explain in simple terms phase change of reflected light

Phase change is used to explain interference in thin films . The concept is not explained there . Does the change in direction by 180 mean phase change ?
0
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0answers
9 views

Electric field in a non-uniformly charged sheet

So if we have a large sheet that is not uniformly charged and is NOT a conductor, how can I find an expression for the electric field everywhere? Things we know about the sheet: the width is 2b it ...
0
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0answers
48 views

How long will it take before the orbit of the earth is 365 days exactly? [on hold]

I understand that the number of days per year has changed throughout the history of the Earth. Apparently there were once over 400 days per trip around the sun. How long will it take approximately ...
-1
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0answers
36 views

How do I calculate momentum for a particle in a box, using the momentum quantum operator?

For a particle in a one dimensional box with $U(x) = 0$ between $x = 0$ and $x = L$ (infinite Potential well) the momentum for $n = 1,2,3,...$ is given by: $$p_n = \frac{nh}{2 L}$$ The wave ...
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0answers
22 views

Variation of quadratic term in modified Einstein-Hilbert actions

In the context of mimetic gravity at some point one try to add to an already modified Einstein-Hilbert action also a term like $$ S_\chi=\int\,d^4x\,\sqrt{-g}\frac{1}{2}\gamma\chi^2,\qquad(\star) $$ ...
0
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1answer
25 views

Slingshot Energy into traveled distance [on hold]

If I have a slingshot and I am told that when pulled it has about $E$ joules of energy, how do I compute the height at which an object of mass $m$ would travel if the slingshot is released in the ...
-3
votes
1answer
33 views

Question to find distance at which 2 trains will pass each other [on hold]

I was solving some questions related to motion, and I came through a question which I cant understand. The question goes like this: Two trains A and B, 100 km apart are approaching towards each ...
-6
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0answers
29 views

Particle Physics [on hold]

Which of these statements are true? A. temperature becomes very high during this process. B. pressure becomes very high in this process C. fission fragments are radioactive D. alpha-rays are ...
-1
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0answers
37 views

Proof of Poisson equation in a gravitational field [on hold]

Can anybody show me how to prove Poisson's equation in Newtonian Gravitational field?
-1
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3answers
39 views

What is extension produced in spring? [on hold]

Consider a spring -block system given in the figure : Note: K is spring Constant. Initially, the spring was unloaded. After that, a block of mass M is loaded to spring and as a result, the spring ...
-2
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0answers
20 views

Are there any benefits from doing Physics MCQs without a given answer key? [on hold]

If I have a answer key, I can know where I'm wrong and solve again. But many books give MCQs without answer key, how should I use them?
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0answers
43 views

Angle sum of triangle in Schwarzschild solution

Curvature of space is often intuitively explained as angles of a triangle not adding up to 180 degrees. My questions concerns that. Suppose you have a perfectly spherical star of uniform density - so ...
17
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6answers
2k views

Conical train wheels

I've been reading about how the conical shape of train wheels helps trains round turns without a differential. For those who are unfamiliar with the idea, the conical shape allows the wheels to shift ...
-1
votes
0answers
42 views

Using Lagrangian to get the equations of motion [on hold]

The thin hollow cylinder rolls on the plane without slipping. My attempt is summarised in the sketch below. X and Theta are my coordinates. I got stuck while writing the T
0
votes
1answer
26 views

Calculate constant acceleration for a length of track with start at rest and final velocity known [on hold]

My maths and physics are rusty... I'm a bit ashamed, but I have this problem: Let's have a 1000m linear accelerator (mass driver) propel a payload at constant acceleration, with a final velocity of ...
-1
votes
0answers
44 views

Commutation relation between linear momentum and vector Potential [on hold]

Does the linear momentum and vector potential commute? How can we show their commutation relation ? I am actually trying to find the commutation relation between both linear momentum of an electron ...
-4
votes
2answers
70 views

Would a piece of paper look as big as a small bedroom to an atom?, or bigger? [on hold]

This evening my six year old asked me "Would a piece of paper look as big as this room to an atom?, or bigger?" ('this room' being a small sized bedroom) A friend suggested it would probably ...
1
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2answers
36 views

Magnetic field of three infinite co-planar wires

I'm reviewing test questions from an old exam, but getting a little confused. a) I thought one of the locations of 0 magnetic field would be right where the middle wire is, since the fields from ...
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0answers
42 views

Showing that a mass moves a half cycle

Consider a mass $m$ at position $x(t)$ on a rough horizontal table attached to the origin by a spring with constant $k$ (restoring force $-kx$) and with a dry friction force $f$ $$\begin{cases} ...
-1
votes
1answer
15 views

Help with this cyclic thermodynamic process question? [on hold]

So we had this question on a midterm: A heat engine runs by taking in 2000 J of heat energy, doing 1450 J of work, and expelling the remaining energy to the cold reservoir. If the cold reservoir is ...
1
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0answers
60 views

How to integrate this physics model? [on hold]

OK, this might seem easy... but I'm having trouble solving it. Let's say I have a force acting on a body whose mass is known, by this law: $$F(t) = bt^2$$ where $b$ is a known constant, and $t$ is ...
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votes
1answer
16 views

magnetic intensity of cylinder sheet of charge having surface charge density K

Here is the problem I have by using $$a_z\times R = (0,0,1) \times (\rho-1.2,0,-z) = (\rho -1.2)a_\phi $$ $$H=\int_{-\infty}^{\infty}\int_{0}^{2\pi}\frac{2.4(\rho-1.2)d\phi dz}{4\pi ...
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2answers
47 views

Finding out the direction of $E$ field

In the Figure we can see a conducting loop of radius $r$ situated in a uniform magnetic field that is perpendicular to the plane of the loop. If we change the magnetic field, then according to the ...
2
votes
1answer
36 views

Lorentz-invariant phase space of a three-body decay process

I am not following the use of delta function in the 3-body decay process. In $\gamma^* \to gq\bar{q}$ process, with $\gamma^*$ being a virtual photon, we have a phase space factor $$d^9R_3 = ...
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0answers
22 views

Vertical throwing exercise [on hold]

I'm struggling with this question Assume an object is resting on the ground, in $t=0$ there is a force that starting to work on it, which grants him vertical acceleration towards up, $a=3m/s^2$, ...
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0answers
22 views

A problem from the Rudolf Ortvay Competition in Physics [on hold]

PROBLEM STATEMENT: A uniform circular disc is kept face down on a table which has a uniform friction coefficient.It is pushed in such a way that there is translational as well as rotational motion of ...
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0answers
20 views

Correlator $bc$ system

I have da doubt with bc system. Polchinski says (2.5.10) $$ b(z)b(0)~=~O(z). \tag{2.5.10} $$ I tried to compute the correlation function With eom, using eq (2.5.6b) by Polchinki, removing the ...
1
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1answer
29 views

Complex scalar theory: annihilation and creation operators give wrong commutators with Hamiltonian

The theory of a real (hermitian) scalar field can be found in many books and everywhere online. On the other hand, if we take the field non-hermitian, then I can only find notes on path integrals. I ...
0
votes
1answer
56 views

Can forces in N be thought of as a length? [on hold]

A solid uniform 45.0 kg ball of diameter 32.0 cm is supported against a vertical friction-less wall using a thin 30 cm wire of negligible mass. (a)make a free body diagram for the ball and use it to ...
1
vote
1answer
56 views

Polchinski Equation (10.4.7) [on hold]

I'm having a problem to interpret eq. (10.4.7) in Polchinski: $$ \gamma(z)\delta(\gamma(0)) = O(z), \qquad \beta(z)\delta(\gamma(0)) = O(z^{-1}). $$ What does he mean by $\delta$? He tries to ...
-2
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0answers
26 views

Finding root mean square speed of an ideal gas [on hold]

Again a question ive never seen in my life... The molecules of an ideal gas have a root mean square speed of 520 meters per second at a temperature of 27 degrees celcius. Calculate the r.m.s speed ...
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0answers
12 views

To change the path of an electron beam travelling horizontally using a uniform electric field [on hold]

Problem: A beam of 20kV electrons is travelling horizontally. If one was to bend the path to make them travel vertically at the same speed, using a region with a uniform electric field, what are the ...
-1
votes
2answers
98 views

Find an angle in 3 dimensions [on hold]

I've had a good attempt at this question trying everything I could think of and the answer I got was 60 degrees yet still incorrect. It would be great if anyone could help me out. The boom ...
-3
votes
2answers
30 views

Finding velocity using given angle [on hold]

Question: A body is projected with $50 \text{m/sec}$ at an angle of $53$ degree with the horizontal. After what time velocity makes an angle $33.7$ degree with horizontal? ($\tan 33.7 = 2/3$) My ...
-1
votes
0answers
23 views

2-Dimensional motion query [on hold]

Question: A body is projected with speed u at an angle theta with the horizontal. (Following are the three parts in the question). Find the change in velocity in time t. Find the speed of body at a ...
1
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1answer
51 views

Can mass-less spring system be solved?

Suppose we have typical chain of strings with masses, attached to the walls (W) at each side W-----m-----m--------W x=0 x=6 x=12 x=21 So if we let ...
6
votes
2answers
84 views

Twin paradox modified for a periodic universe [on hold]

I had received a homework problem by my professor. Please consider part (b) of the problem. Can part (b) be solved only using special relativity? And how can you solve this? I think that Alice will ...
0
votes
2answers
84 views

$[A_1, H] =[A_2, H] = 0$ but $[A_1, A_2] \neq 0$?

I am having a difficult time understanding this problem. Suppose $[A_1, A_2] \ne 0,$ $[A_1, H] = 0,$ $[A_2, H] = 0.$ Show that the energy eigenstates of $H$ are in general ...
-3
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0answers
19 views

On Fig 4.1, draw the normal to the mirror at end P [on hold]

I got given some homework to do over summer. It's a completely different exam board with a different specification. I understand it's hard to draw lines on this site but any help you could give me, ...
-1
votes
1answer
21 views

Problem related to Motion in 2-Dimension [on hold]

Question: The height y and the distance x along the horizontal place of a projectile on a certain planer (with no surrounding ...
-4
votes
0answers
30 views

Relative motion problem [on hold]

A fighter plane flies at a velocity of 300 m/sec. On the fighter plane there is a gun which shoots at a rate of 40 rounds per second with a muzzle velocity of 1200m/sec. The shots are aimed at another ...
-6
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0answers
53 views

Speed of the train is faster than a car, why? [on hold]

What are the physical quantities required to prove this?
-9
votes
1answer
50 views

Can you help me with physics homework please? [on hold]

An object with a mass M is suspended from an elastic spring with a spring constant k. The object oscillates with period T on the surface of Earth. If the oscillating system is moved to the ...
1
vote
0answers
26 views

Simple mean-field “lattice gas” model

I'm having some conceptual problems with a simple mean-field "lattice gas" model and I'd be glad if someone could help me go further. Basically the model consists of the following hamiltonian: $$ H ...
0
votes
3answers
51 views

Reaction Force on String Wrapped Around Circular Peg

A massless inextensible string is wrapped around a frictionless circular peg. The string is taut, with tension $T_2$ and $T_1$ at the points where it leaves of the pg as shown. The segment wrapped ...
2
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0answers
51 views

Problem with OPE (from Polchinski) [on hold]

I was reading Polchinski, Vol. 2 pag 12, while I found (10.3.12a): $$ e^{iH(z)}e^{-iH(z)}=\frac{1}{2z} + i\partial H(0) + 2zT^H_B(0) + O(z^2).\tag{10.3.12a} $$ Now I tried to do the OPE, what I ...
-2
votes
0answers
24 views

Calculating deceleration time - help! [on hold]

I'm struggling with a question due to not having the correct formula, so I hope someone can help! A 10000kg object is travelling at 15m/s, and a force of 600N is applied in the opposite direction to ...
1
vote
1answer
40 views

Electric field between capacitors [on hold]

A parallel-plate capacitor consists of two parallel, conducting plates of area $A$, separated by a distance $d$. Each carries a charge of magnitude $Q$; positive on one, negative on the other. ...
-5
votes
0answers
20 views

Find tf and end distance [on hold]

Mass = 30kg, Velocity = 30, Acceleration = unknown, end distance = unknown, force = -20N. Please breakdown formula in elementary physics terms thanks! a)Find time the object came to a stop b)the end ...