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Questions tagged [homework-and-exercises]

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.

3
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1answer
1k views

How to calculate the Gibbs free energy out of a given Entropy?

I'm stuck in a small calculation. I try to solve a small exercise. You should calculate for a given entropy the Gibbs free energy, where the entropy is given as $S\left(U,V\right) = \frac{4}{3}\left(...
1
vote
1answer
12k views

How to find maximum velocity

I stack a question about projectile question. The question was A projectile is being launched from ground level with no air resistance. You want to avoid having it enter a temperature inversion ...
0
votes
1answer
11k views

Force and Laws of Motion [closed]

While catching a fast moving cricket ball, why do fielders gradually pull back their hands backwards with the moving ball? Please advice which among the following is the correct answer. If both are ...
6
votes
1answer
643 views

$SU(2)$ Yang-Mills EOM

I'm having trouble with some indices on my yang-mills lagrangian. I have a gauge group $SU(2)$ and a field strength tensor $$ F_{ab}^{i}=\partial_{a}A^{i}_{b}-\partial_{b}A^{i}_{a}+\epsilon^{i}_{\,\,...
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votes
2answers
2k views

How to compute drag coefficient given initial position, initial velocity and final resting position?

The equations I'm using are: x = x + (DT * vx) vx = vx * C My DT is always 0.01 and the coefficient C (related to a linear ...
7
votes
4answers
18k views

Divergence of $\frac{\hat{r}}{r^2}$

In David J. Griffiths's Introduction to Electrodynamics, the author gave the following problem in an exercise. Sketch the vector function $$ \vec{v} ~=~ \frac{\hat{r}}{r^2}, $$ and compute ...
1
vote
1answer
1k views

As ISA Practical - Resistors in Parallel [duplicate]

Possible Duplicate: Current against the inverse of resistance graph, I = V/R +c How would you set up a circuit with a fixed resistor in parallel with a variable one. We are told to measure the ...
2
votes
1answer
412 views

Pushing, inertia of a cart system

I have a cart with another cart on top which gets pulled down by another cart on the side. There is no friction. The question is: How strongly do I have to push with $F$ to keep the cart $m_1$ ...
-2
votes
1answer
839 views

Angular velocity $\omega$ by $v$ [closed]

We have two girls, with mass ($M$). They become close to each other in speed of $V$. The distance between them is $3L$. I was asked to calculate the Angular velocity ($\omega$) of the two girls. So I ...
0
votes
1answer
1k views

Vibrational motion of linear diatomic molecule

This question concerns the following exercise from an old exam: The vibrational motion of a linear diatomic molecule can be approximated as simple harmonic motion. A CO molecule has a bond ...
1
vote
2answers
2k views

Finding coefficients of volumetric expandtion from a know coefficient of linear expandtion

Starting from a homework problem: An aluminum cup of $100 cm^3$ capacity is completely filled with glycerin at $22°C$. How much glycerin, if any, will spill out of the cup if the temperature ...
-1
votes
1answer
281 views

Force on pneumatic cylinder [closed]

A pneumatic cylinder is kept vertically straight on a weighing scale, which is set at 0 for the setup. now a force of 1000 N is applied on the cylinder. what is the reading of scale. assume the ...
0
votes
1answer
364 views

When to use Heat Diffusivity eqn and when to use Fourier's law to find temperature distribution?

Let's say that there is a circular conical section that has diameter $D=.25x$ without any heat generation and I need to find the temperature distribution. Originially I thought I could use the heat ...
1
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1answer
2k views

change of resistance in semiconductors due to temperature change

I have a question that popped up in an old exam I just can't find a way to solve. If you drop the temperature of this material from room temperature (I guess around 300K) to 0C (273K) the ...
0
votes
2answers
385 views

Problem based on Rotational Motion #2 [closed]

One fourth length of a Uniform Rod of mass $m$ and length $x$ is placed on rough horizontal surface and it is held stationary by means of a light thread as shown in the figure. The thread is brunt and ...
0
votes
1answer
889 views

Finding The Resultant Vector [closed]

A plane is flying southeast at 215 km/h. Suddenly there is a wind from the north at 75 km/h. What is the plane's new velocity with respect to the ground in standard position?
0
votes
1answer
488 views

Expansion of Helmholtz energy

To get an expansion of Helmholtz energy of a) an ideal gas b) a Van der waals gas we must integrate $\left ( \frac{\delta A }{\delta V} \right )_{T}=-P$ I saw the solution is : Can you ...
1
vote
3answers
1k views

One dimensional Schrödinger equation equation with initial condition, finding the probability of the particle's future position

A particle of mass $m$ moves freely in the interval $[0,a]$ on the $x$ axis. Initially the wave function is: $$f(x)=\frac{1}{\sqrt{3}}\operatorname{sin}\Big( \frac{\pi x}{a} \Big)\Big[1+2\operatorname{...
0
votes
2answers
5k views

Find the Unit Vector of a Three Dimensional Vector

How can I find the unit vector of a three dimensional vector? For example, I have a problem that I am working on that tells me that I have a vector $\hat{r}$ that is a unit vector, and I am told to ...
-1
votes
1answer
13k views

Electric field due to nonconducting plastic sheets [closed]

Two very large, nonconducting plastic sheets, each 10.0 cm thick, carry uniform charge densities $\sigma_1, \sigma_2, \sigma_3$ and $\sigma_4$ on their surfaces (the four surfaces are in the following ...
7
votes
2answers
718 views

Conformal transformation equation

I am currently reading Kiritsis's string theory book, and something bugs in the CFT (fourth) chapter. He derives the equation that should satisfy an infinitesimal conformal transformation $$x^{\mu} \...
2
votes
0answers
4k views

When do I know if energy stored in an object is 0 or nonzero? (Heat transfer)

There is uniform internal heat generation at $\dot{q}=5\times10^{7} \frac{W}{m^{3}}$ is occurring in a cylindrical nuclear reactor fuel rod of 50 mm diameter and under steady state conditions the ...
0
votes
1answer
1k views

Temperature change effected by electric heater [closed]

A 40-gallon electric water heater has a 10kW heating element. What will the water temperature be after 15 min of heating if the start temp is 50F degrees. There must be an equation. I can't find it ...
2
votes
2answers
28k views

Work done by spring over distance

I'm working through a problem involving energy conservation. Unfortunately, I cannot calculate the work done by a spring. ...
1
vote
1answer
1k views

Problem based on Rotational Motion [closed]

A spool of mass $\mathsf m$ and inner radius $\mathsf r$ and outer radius $\mathsf{2r}$, having moment of inertia $\Large\mathsf{\frac{mr^2}{2}}$ is made to roll without sliding on a rough horizontal ...
3
votes
1answer
3k views

How to calculate time evolution of a wave function in an 1D infinite square well potential?

A particle in an infinite square well has an initial wavefunction $$\psi (x,0) ~=~ Ax(a-x) \qquad \mathrm{for}\qquad 0\leq x\leq a.$$ Now the question is to calculate $\psi (x,t)$. I have ...
1
vote
0answers
572 views

Electron Fermi gas

My question is about 2-dimensional Fermi gas of electrons. What is magnetic susceptibility when $T<<T_F$ (where $T_F$ is Fermi Temperature) and, What is the ratio between Pauli and Curie ...
1
vote
1answer
784 views

Given two boiling temperatures and pressures, how can I find the latent heat?

I am given the fact that at a certain pressure a liquid boils at a corresponding temperature, at a different pressure it boils at a different temperature, and then I am asked to find the latent heat ...
10
votes
3answers
2k views

Gauge invariant Chern-Simons Lagrangian

I have to prove the (non abelian) gauge invariance of the following lagrangian (for a certain value of $\lambda$): $$\mathcal L= -\frac14 F^{\mu\nu}_aF_{\mu\nu}^a + \frac{k}{4\pi}\epsilon^{\mu\nu\...
0
votes
1answer
28k views

To calculate pressure at point in a vertical water pipe of given diameter

Is it possible to calculate pressure at a point in a pipe, where a solenoid valve shall be placed, given that the pipe carries water (utility supplied, regular water) from an overhead tank (at a ...
3
votes
3answers
4k views

Conservation of Momentum/Energy collision Problem

I'm working on a physics problem in preparation for the MCAT and there's this particular problem that's troubling me. I don't know if it's a bad question or if I'm not understanding some sort of ...
2
votes
3answers
134 views

Certain material heating water in a recipient

I don't know how to resolve a problem, but I don't want the answer since I'm almost going to have it resolved. What the problem says is we have 85 liters of water at 7ºC in an iron pot of 29kg. We ...
1
vote
3answers
8k views

How do I find the tension in additional strings in this problem? [closed]

A mass of 5.00 kg hangs attached to three strings as shown in the figure (see image below). Find the tension in each string. Hint: Consider the equilibrium of the point where the strings join. So ...
0
votes
2answers
37k views

What do I need to do to find the stopping time of a decelerating car? [closed]

The question is: A car can be stopped from initial velocity 84 km/h to rest in 55 meters. Assuming constant acceleration, find the stopping time. Sorry for my ignorance, but I need to review ...
0
votes
1answer
225 views

Simple Harmonic Motion. Why am I wrong? Why is my equation wrong more importantly?

Problem/Solution http://img221.imageshack.us/img221/4285/20841428g.jpg! I am deeply confused. B) We know that $x = 2\sin(3\pi t)$. $x' = 6\pi\cos(3\pi t)$ So max speed is $6\pi$ $6\pi = 6\pi \...
0
votes
2answers
855 views

Why wouldn't this system of equations determine where two balls meet?

A ball is thrown vertically upwards at $5\text{ m/s}$ from a roof top of $100\text{ m}$. The ball B is thrown down from the same point $2\text{ s}$ later at $20\text{ m/s}$. Where and when will they ...
2
votes
1answer
494 views

Confusion concerning the use of wavenumber in exercise about Fabry-Perot etalon

In the exercise we are given that the spectrum of a light source consists of two spectral lines, which both have wavelengths around $500 \text{ nm}$ and the separation between them - given in ...
3
votes
1answer
440 views

What would be pressure of 1 kg of photon gas at room temperature put in a volume of 1 liter?

Suppose a number of photons with spectrum corresponding to black body spectrum at 293 K with total energy corresponding to 1 kg put in a box with ideal mirror walls with volume of 1/1000 of a cubic ...
0
votes
1answer
211 views

I think I disprove this with kinematics, but energy says it is right

http://img710.imageshack.us/img710/914/engkin.jpg! Here is my kinematics argument. For now I am only going to look at ball 2 and ball 3. Make note of the following data. $|v_0| = 10m/s$, $y_0 = 10m$,...
0
votes
1answer
224 views

Finding time in ballistic problem

The problem is like this: A bullet with mass 15g moving at a speed of 425 m/s is brought to a stop in 3.5 cm in a block of wood. Find the force on the bullet, assuming that it is constant. So ...
0
votes
2answers
8k views

Internal resistance V=IR circuit question [closed]

I don't know how to resolve this problem. A complete circuit consists of a 18,0 V battery (internal resistance r) and a resistor R. The terminal voltage of the circuit is 15,8 V and the current is ...
-1
votes
1answer
591 views

What happened to the work done by friction here?

Problem/Solution http://img832.imageshack.us/img832/1313/88791065.jpg! Question What happened to the work done by friction BEFORE it touched the spring? Why was that neglected? Also they say there ...
1
vote
1answer
3k views

Two problems with free body diagrams. Contradictions (solutions are provided)

This problem is similar, but also different question from my previous question. They are both unfortunately long. Problem/Solution #1 http://img215.imageshack.us/img215/6696/problem2f.jpg! http://...
1
vote
2answers
3k views

How do I properly use Newton's Third Law and Free Body Diagrams?

A force of 14.0N is applied to a block as shown in the diagram below http://img14.imageshack.us/img14/9337/diagramzw.jpg! If the coefficient of friction is between the surface and the blocks is 0.35 ...
0
votes
3answers
2k views

Forces exerted on a body inside an accelerating elevator

Three masses $m_1 = 3\text{ kg}$, $m_2 = 9\text{ kg}$ and $m_3 = 6\text{ kg}$ hang from three identical springs in a motionless elevator. The elevator is moving downward with a velocity of $v = -2\...
2
votes
1answer
1k views

Counting the number of modes

The exercise is the following Show that the number $N(\lambda) \, \mathrm d \lambda$ of standing electromagnetic waves (modes) in a large cube of volume $V$ with wavelengts within the interval $\...
1
vote
1answer
5k views

Elastic Collision with objects of the same mass

I understand this is a homework problem. I don't expect anyone to do my homework for me, but help me understand what I am doing wrong. The problem goes: Two shuffleboard disks of equal mass, one ...
2
votes
0answers
638 views

Solar wind and the Earth's magnetic field

I have again an old question from a comprehensive exam I took a couple of months ago. Lucky for me one could pick 5 out of 8 questions, because on some of the problems I didn't even know how to start. ...
2
votes
7answers
423 views

My math says this is wrong, but what part of energy conservation am I violating?

Basically energy of conservation says it doesn't matter what path I take, energy will be the same from before as it is after. So I took the block at the top's energy equal to the energy when it lands ...
-1
votes
2answers
179 views

Why is the work positive here?

Problem http://apcentral.collegeboard.com/apc/public/repository/ap11_frq_physics_b_formb.pdf Please refer to question 1f Solutions http://apcentral.collegeboard.com/apc/public/repository/...