# 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 questions.

1answer
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### Simple pendulum and planet mass [closed]

A simple pendulum, $20cm$ in length, has the period of $2.7s$ on a certain planet. Find the mass of the planet if its diameter is $18000km$. $G=6.67\times10^{-11}Nm^2/kg^2$ I have no idea how to get ...
2answers
366 views

### How to find $\tau$ in this circuit?

1) 2) I know, that $$τ = \frac{L}{R}$$ but what is $R$ in this formula? It seems to be the total resistance, but how to find it in 1) and 2)?
1answer
277 views

### Infinite Potential Well Energy for Piece-wise Constant Wave Function

I'm trying to compute the expectation value of energy for a certain state in an infinite potential well but I'm getting contradictory answers. The well has potential \begin{align} V(x) = \left\{ ...
3answers
207 views

### Forces involved in a climbing jet aircraft

I have a problem with which I need help. The question is: A jet aircraft is climbing at an angle of 45° above the horizontal and is accelerating at 4.5m/s2. What is the total force that the cockpit ...
1answer
518 views

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456 views

### Newton's law of cooling: changing temperature of environment

A metal ball having temperature of $80^\circ C$ is placed into $m$ grams of water at $0^\circ C$. After ten minutes, it was found that the temperature of ball and water are $60^\circ C$ and ...
1answer
116 views

### Two masses on rope spinning around

Two balls of the same mass $m$ are connected to each other with rope of length $l$. One of the balls is also connected to the ceiling with a rope of the same length $l$. The balls are spinning ...
0answers
79 views

### A quicker way to verify that a function is a constant of motion?

I have three particles that we can indicate with $\alpha$ ($\alpha$=0,1,2), they are identified by the $r^i_\alpha$ coordinates and $p^\beta_j$ conjugata momenta ($\beta=0,1,2$ and $i,j=1,2,3$). I ...
2answers
151 views

### Generalizing a relativistic kinematics formula for spatial-acceleration dependence

I'm starting from this expression $$\alpha dt = \gamma^3 dv$$ where $\alpha$ is proper acceleration of a point particle, $dv$ and $dt$ are coordinate differentials of velocity and time, and ...
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644 views

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### Bose–Einstein statistics exercise

I've a basic Bose–Einstein statistics exercise. I've tried to solve it in two ways, but each way gives a different result. We have $n$ identical bosons without interactions at temperature $T$. There ...
2answers
100 views

### 3D Vector Rotation of Point Mass

Triangle defined by points OA, OB and OC : (-i,3 j,-4 k), (i,2 j,2 k) and (3 i,7 j,- k) where i, j, k are unit vectors along x,y,z axis. Point mass is placed at OA. Triangle rotates with angular ...
2answers
217 views

### Frictional Forces

In the figure, blocks A and B have weights of 45 N and 23 N, respectively. (a) Determine the minimum weight of block C to keep A from sliding if μs between A and the table is 0.21. (b) Block C ...
0answers
132 views

### Proving conservation of angular momentum in an elliptic billiard problem

This is for a course focusing on the connections between Newtonian, Lagrangian and Hamiltonian formalisms. We're given an elliptic billiard table with foci 1 and 2, where $L_1$ and $L_2$ are the ...
1answer
388 views

### How to calculate flow rate from given stream function

I am stuck on a homework question, all because I dozed off in class. I understand what a stream function is but I don't know how to apply it to calculate the flow in pretty much anything. Here's the ...
1answer
1k views

### A Doppler Effect problem with a moving medium

I tried solving the following question and started having multiple doubts: Two cars A and B are moving towards each other with some speed $25$ m/s. Wind is blowing with speed $5$ m/s in the ...
1answer
295 views

### Proton mass and Weizsäcker's formula

I'm so confused in the use of nuclear masses and atomic masses. I have two questions. From the book "Outline of Modern Physics" by Ronald, I understand that the semiempirical mass formula ...
1answer
163 views

### Adding heat to a system doesn't equal the work done by gas?

So, I answered a physics question for a class that goes as follows: A gas in a cylinder is kept at a constant pressure of $250000\: \mathrm{Pa}$ while $300\: \mathrm{kJ}$ of heat are added to it, ...
1answer
504 views

### Superconducting diamagnetic sphere in uniform magnetic field

What is the size of the magnetic dipole moment $\vec m$ of a superconducting diamagnetic sphere $radius=R$ in a uniform magnetic field $\vec B_0$? Since there is no free current, we can solve for ...
1answer
54 views

### Doubt about coordinates and point of equilibrium

I'm solving an exercise about small oscillations and I have a doubt about coordinates that I have to use. This is the text of the exercise: "A bar has mass M and lenght l. Its extremity A is hooked ...
1answer
844 views

### Electric field a distance $z$ above the center of a circular loop. The Hard way [closed]

Problem 2.5: Find the electric field a distance $z$ above the center of a circular loop of radius $r$ which carries a uniform line charge $\lambda$. This problem is in refereced here (with ...
2answers
187 views

### Velocity of an object undergoing homogenous acceleration

So I was considering the following problem within the context of Special Relativity: Given an object O, with initial velocity v, undergoing constant acceleration at a rate of a, I want to express the ...
1answer
191 views

### Harmonic oscillator - wavefunctions

I understand now how I can derive the lowest energy state $W_0 = \tfrac{1}{2}\hbar \omega$ of the quantum harmonic oscillator (HO) using the ladder operators. What is the easiest way to now derive ...
1answer
53 views

### Charge in a layered ball

A ball (radius $R$) has three layers. For $0<r<a$ it is a conductor with free charge $+Q$. For $a<r<b$ it is a linear dielectric $\epsilon$ with free charge embedded in it with density ...
1answer
84 views

### Summing moments, where have I gone wrong?

Because I don't have enough rep to post images, the question is question 3 part iii) from here: http://www.mei.org.uk/files/papers/m209ju_jk32.pdf The surface at the contact point B is rough. The ...
1answer
95 views

### Finding cooling time using an impulse of heat energy

A chip has a thermal resistance of 0.007 K/J s and thermal capacitance of 1.6 J/K. You give it an impulse of heat equal to 100 J. How long does it take for the chip to cool to within 10% of the ...
2answers
1k views

### Slowdown rate of rotating body due to friction force [closed]

This isn't a homework question, but it might as well be. The problem I have been pondering is: If a disc (or children's roundabout if you like), of radius r, mass m, is spun around it's center ...
1answer
101 views

### Photon Escape Angle From Black Hole

Consider a photon source emitting photons near the surface of a Schwarzschild black hole. What angle, as a function of the source's radius from the event horizon, must the photons be emitted at such ...
0answers
76 views

### Calculate heat loss of aquifer with unknown shape

I am trying to calculate the heat loss of an aquifer. An aquifer is basically a hot-cold storage $80\: \mathrm{meters}$ under the ground. It is an open system and utilizes groundwater. By pumping for ...
2answers
337 views

### Why don't the components of these forces add up correctly? [closed]

In the arrangement as shown in the figure below, the ends P and Q of an inextensible string move downwards with uniform speed $u$. Pulleys A and B are fixed. Mass $M$ moves upward with a speed: ...
1answer
341 views

### How do you calculate the Milky Way’s galactic year? [closed]

The Solar system moves at a speed of 220 km / s around the galaxy. It’s about 27,000 light years from the Galactic Centre. How long does it take for the solar system to orbit around the Milky Way? ...
1answer
170 views

### Boundary conditions for Laplace's equation

Given a grounded conducting sphere, $V=0$ and $radius = R$, centered at the origin with a pure electric dipole (dipole moment $\vec p$) situated at the origin and pointing along the positive $z$ axis, ...
2answers
160 views

### $Q$-value in nuclear reaction $^{9}Be\left(\gamma,n\right)^{8}Be$

I have two questions. First, I understand that in a nuclear reaction $$Q:=K_{after}-K_{before}\equiv E_{0,before}-E_{0,after} \qquad (1)$$ where $K$ is the total kinetic energy, and $E_0$ is the ...
1answer
289 views

### Young's Double slit experiment (Finding the value for slit separation) [closed]

The question illustrates mono-chromatic red light passing through 2 slits and forming an interference pattern on the screen. The question then asks to suggest a suitable value for the slit separation. ...
0answers
59 views

### error propagation and collision in ideal gas

When dealing with gas, a statistical approach is needed because For N particles, you have to solve 6N equations which cant be done analytically. To know our time step for numerical solving, you can ...
2answers
195 views

### Find E and B from vector potential

I have a vector potential given by: $\mathbf{A}(x,t) = \mathbf{e}_{y}\frac{1}{2} e^{-(x-ct)^{2}/{4a^{2}}}$ Now, the question is "Determine the E and B under the condition that the scalar potential ...