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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0
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1answer
16 views

Determine normal forces on tilted table

A table with the mass m, the height a and the width b is resting on a tilted floor with the angle of rake $\alpha$. A block at A prevents the table from sliding. Determine the normal forces from the ...
1
vote
0answers
8 views

Gamma matrices in Decay Calculations

I am trying to calculate the transition amplitude, $\lvert M^2\rvert$, for the process where a top quark (momentum p) decays to a bottom quark (momentum q) and a $W^+$ boson (momentum k). I am ...
0
votes
0answers
14 views

The angle of incidence of prism if emergent angle is 90°. Answer in terms of angle of prism and refractive index of prism [on hold]

A ray of light is incident at an angle of incidence i on one face of a prism of angle A (assumed to be small) and emerges normally from the opposite face. If the refractive index of the prism is n, ...
0
votes
3answers
21 views

Path followed by a free test charge in an electric field region represent the electric lines of forces?

As we know that if we put any free test charge in an electric field which is moving horizontally then it will experience an repulsive force and move in that direction so by that we can say that for ...
2
votes
2answers
99 views

Why Electric field lines starts and ends at 90 Degree at the surface of Conductor?

There is one property of Electric lines of forces which states that: Electric field lines start and end at 90 degree at the surface of the conductor. But why is that so? Is there any prove for ...
2
votes
2answers
33 views

Work done by frictional force on a sliding block

A block slides across a table horizontally with an initial velocity $V$. The frictional force $F$ brings it to rest after its Centre of Mass covers distance $S$. What is the work done by the ...
0
votes
0answers
19 views

RC Circuit Analysis

I want to correct three misunderstandings about the following RC circuit. 1) How does current move directly after switch S gets closed? Current runs through all three circuit components to begin ...
-2
votes
0answers
13 views

Find magnitude of the net magnetic field at points A, B, and C between two parallel wires [on hold]

I've been having a lot of problems with this problem. Been working on it for over to an hour and nothing. I've been using amphere's law but I think I'm getting r wrong, here's one of my most recent ...
0
votes
0answers
22 views

Help with inclined slope motion? [on hold]

I have this question (5ii) and I was wondering why this wouldn't work, why can't I solve for acceleration using $v^{2}-u^{2}/2s$=$a$ then solve for $t$? I obviously have to resolve and use $p=fv$ but ...
1
vote
2answers
34 views

Electric field due to a finite line charge

I was wondering what would happen if we were to calculate electric field due to a finite line charge. Most books have this for an infinite line charge. In the given figure if I remove the portion of ...
1
vote
1answer
69 views

When will the pringles can fall?

When I'm pushing a tall, cylindrical object stood on its end (like a Pringles can) I can push it near its bottom, and it will slide forwards, or push it near the top, and it will topple over. So, ...
2
votes
0answers
21 views

Current carrying conductor with a loop at a distance [on hold]

In Fig, there are an infinitely long conductive wire and a circle conductive loop C with the radius r and the number of turns n. The distance L between the wire and the loop is much larger than r, ...
0
votes
0answers
14 views

water pressure relationship to volume [on hold]

I have a one inch pipe 120 feet long and caped. I have a 60 pound input of pressure. I put a half inch pipe at the end and open it. What happens to the pressure in the one inch pipe? If I add another ...
1
vote
1answer
32 views

About shift operators

The question is this: Does $$L_+ L_- Y_{lm} $$ ,where $Y_{lm}$ is a spherical harmonic function, equals to zero. If so, why? The two operators above are defined as $$L_+ ={L_x + iL_y } $$ $$L_-={L_x ...
3
votes
0answers
21 views

Average force for repeated elastic collisions

I was asked to compute the average force exerted by a rigid spherical ball (mass m) on the floor over infinite time, in a situation where the ball is dropped vertically from a height (h) and makes ...
2
votes
0answers
21 views

Distribution forces

I've got two trees and between them is a rope. When someone about 100kg sits at the center of the rope, what are the forces on both parts of rope (to the left-hand side from the person and right-hand ...
1
vote
1answer
48 views

Where should five charges be placed inside a hollow sphere so as the system has the lowest potential energy? [duplicate]

I want to know the configuration where the system would be most stable.I only know basic Physics & Maths and cannot start solving the problem.I can understand it for 4 charges and 6 charges as a ...
0
votes
0answers
20 views

Charged particle inside a hole in a charged layer

The layer $-a\le x \le a$ is homogeneously charged with a electric charge density $\rho \gt 0$ and on the x-axis a thin hole is made. A particle with mass $m$ and charge $q \gt 0$ move on the ...
0
votes
0answers
47 views

Where should five charges be placed inside a hollow sphere so as the system has the lowest potential energy? [on hold]

I want to know the configuration where the system would be most stable. But I cannot think of a way to start solving the problem. I've solved it for 4 charges and 6 charges and they have a symmetric ...
1
vote
0answers
37 views

In Solution of the equation $\Box A_\sigma=0$. I find some mathematical difficulties

The solution of the equation $$\Box A^\mu(x)=0$$ has the form $$A^\mu(x)=\int\frac{d^4k}{(2\pi)^4} e^{ik^\mu x_\mu}A^\mu(k)\delta(k^2).$$ Where we take $A^\mu(k)$ of the form $$A^\mu(k)=\sum ...
0
votes
1answer
32 views

Working out length of taut rope orbiting the earth remaining straight and radially directed [on hold]

A taut rope is orbiting the earth with angular velocity $a$. It remains straight and radially directed. If the midpoint is located at radius $r$, find the length of the rope. I can't even imagine ...
0
votes
0answers
27 views

How to calculate the potential energy of a magnetic dipole (with zero torque)

In the following problem, I would like to know how they arrived at their solution for finding the potential energy of a magnetic dipole (boxed in red). I understand I probably have to use the ...
-3
votes
0answers
21 views

Electric field calculation and its graph vs x [on hold]

Electric field along the line passing through two point charges, A point charge $Q_1= +3 \mu C$ is placed at the origin and a point charge $Q_2= -7\mu C $is placed at $x=0.4 m$ on the $x$-axis of a ...
3
votes
3answers
54 views

Does my (too low) tire pressure, affect the speedometer of my car?

Does my (too low) tire pressure, affect the speedometer of my car? How big is this effect?
0
votes
0answers
16 views

Nuclear medical physics [on hold]

How do you calculate the half life of daughter isotope (in secular equilibrium) Given a table of countrate(cps) and time (min). Like: Time (minutes) Count Rate (cps) time (min) Count Rate ...
-5
votes
0answers
30 views

Electric circuit question [on hold]

A 100W bulb is connected in series with a room heater.If now 100W bulb is replaced by a 40W bulb,the heater output will (Decrease) (Increase) (remain same)? And explain why?
0
votes
1answer
87 views

Commutator and Hamiltonian [on hold]

Assume that $[\hat{A},\hat{H}]_-=0$ and $[\hat{B},\hat{H}]_-=0$ but we know that $[\hat{A},\hat{B}]_-\neq 0$. Then there exists degenerate stationary states of $H$. How to prove it?
0
votes
1answer
28 views

Continuity equation for charge and current densities of an accelerated point charge

For a point charge that moves with the trajectory $ \vec r(t)$, we know that it has the singular charge and current densities $$ \rho (\vec x, t) = q \delta^3(\vec x - \vec r(t)) $$ $$ \vec J(\vec x, ...
0
votes
1answer
41 views

Determining the path of a light ray

Let's consider a ray in the plane $xy$. Let the refractive index be defined in any point of the plane with the function $n(x,y)$. In time $t=0$ the ray is located in coordinates $(x_0, y_0)$ and its ...
0
votes
0answers
23 views

Find the acceleration of a stirrer [on hold]

Question: There is a fixed 55 cm long rod (vertical) that is charged uniformly up to +137 nC, Place a 0.5 g plastic 6 cm long stirrer (horizontal) that is charge to +93 nC, 2.3 cm away from the rod ...
0
votes
1answer
29 views

Is this the correct entropy? [on hold]

In order to take a nice warm bath, you mix $50$ liters of hot water at $55^{\circ} C$ with $25$ liters of cold water at $10^{\circ} C$. How much new entropy have you created by mixing the water? What ...
0
votes
0answers
35 views

Find resistance of inductor in series RLC circuit [on hold]

We have a resistance of 350 Ohm, an inductor of 0.15H and a capacitor of 10 microfarad, connected in series to an AC source of voltage 240 V and frequency 50 Hz. The measured effective voltage over ...
0
votes
1answer
33 views

Derive the Sackur-Tetrode equation

How do you derive the Sackur-Tetrode equation? I know that you must start off with the multiplicity of a mono-atomic ideal gas: ...
-1
votes
1answer
28 views

Parabolic slide [on hold]

Can you give a big hint for this problem please? A material point is released at the end of a track in the form of a vertical arc of parabola $y = x^2$ in $[-1,1]$, the meter is selected as a unit ...
0
votes
0answers
29 views

Determine the speed of balls after collision [on hold]

I am sorry for my English it is not perfect at this moment but I hope you will understand my question. I have a problem with solving this exercise. The two balls of the same density and radius r and ...
1
vote
1answer
43 views

Circular motion from rest [on hold]

A particle $P$ is suspended from a fixed point $O$ by a light string of length $a$. When hanging at rest under gravity at $A$ it is given a horizontal velocity $u$. The particle moves freely in a ...
0
votes
0answers
40 views

Simple question. Where to find coefficients of friction? [on hold]

I am trying to calculate the force the Terracotta Warriors apply to the Lykan Hypersport in the Fast and Furious 7 movie. I am trying to find the frictional force of the statues applied against the ...
0
votes
0answers
21 views

Calculate small small oscillations of a pendulum

The system is setup as follows: A point $O_1$ moves along the $x$ axis with it's $x$ coordinate being $a\sin(\omega t)$ and $\omega\ne\sqrt{\frac{g}{l}}$. There's a pendulum attached to $O_1$ of ...
1
vote
3answers
60 views

Work-Energy conservation with friction

I didn't go to the lesson of work-energy theorem, so I miss something about this subject. I know the formulas, but I can't figure it out. This question has many quantities. Here is the problem, ...
1
vote
1answer
35 views

Cosmological Boltzmann equation

Consider the Boltzmann equation: $$\frac{d \ln{n^c(T)}}{d \ln{T}} = \frac{\Gamma}{H}(1 - \frac{n^c_{eq}(T)}{n^c(T)})$$ We know that the ratio $\Gamma/H$ can be considered constant, let us put it ...
2
votes
3answers
222 views

Normalization problem with hydrogen wavefunction

Suppose you have a mix of states made up of the Hydrogen $\lvert nlm \rangle$ states where one of the coefficients is unknown. For example: $$ \lvert \psi\rangle=A\lvert 100\rangle + ...
0
votes
1answer
27 views

How is angle measured by telescope in spectrometer is twice the prism angle [on hold]

While measuring the angle of the prism with the help of telescope in a spectrometer (see figure below), the observed angle is twice the angle of the prism. Can someone prove it geometrically? Edit : ...
1
vote
1answer
23 views

Superstring vacuum amplitude on the torus

My question is how to obtain the superstring (Type II A and B) vacuum amplitudes on a torus. They are given in Polchinski's String Theory Vol. 2 equation (10.7.9): ...
-1
votes
0answers
48 views

Center of Mass of Snowman [on hold]

Suppose you build a snowman 6 feet tall by stacking 3 spheres. The bottom sphere has a diameter of 3, the middle one a diameter of 2, and the head a diameter of 1. What is the center of mass?
1
vote
2answers
24 views

How do I choose the right value of $r$ to find where the electric field is zero?

Sorry for the long question. I'm having a difficult time trying to explain my confusion. I have a positive point charge$\ Q_1 =+q$ at the origin and a negative point charge $\ Q_2 = -2q$ at $\ x=2$ ...
0
votes
1answer
44 views

What's the geometry of a gravitational field at the flat end of a cylinder?

Gauss's law is fairly straightforward in explaining the gravitational field strength around the curved sides of a cylinder - but what is the geometry of the field at the flat end? For example, does ...
0
votes
0answers
17 views

From Eulerian to Lagrangian coordinates (Rigid Body) [on hold]

so I got this problem: Given $u=((x_1-x_3)^2, (x_2+x_3)^2, (-x_1x_2))$ Find $x(X) ; X(x) ; u(X)$ I know you have to use the fact that u=x-X, and that x=x(X) ; X=X(x) Any ideas?
1
vote
1answer
30 views

What is the position as a function of time for a mass falling down a cycloid curve?

In the brachistochrone problem and in the tautochrone problem it is easy to see that a cycloid is the curve that satisfies both problems. If we consider $x$ the horizontal axis and $y$ the vertical ...
3
votes
0answers
50 views

Escape velocity for Schwarzschild metric

I can't fill in the gaps in my solution to this and assistance or a reference would be appreciated. The question begins with the straightforward derivation of the EoM for a massive particle orbiting ...
0
votes
1answer
59 views

How to find the number of distinct contraction cases in Wick's Theorem?

Let $\mathcal{G}^8_{un}:=(t_1,t_2,t_1'^3,t_2'^3)=\langle 0 \mid T[Q_{un}(t_1)Q_{un}(t_2)Q(t_1')^3Q(t_2')^3] \mid 0 \rangle_{un}$ We want to use Wicks theorem to write this function as the sum of ...