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6
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1answer
272 views

few fermions in a harmonic trap — position density matrix from diagrammatics

I'm trying to calculate the momentum distribution of a 1D system of non-interacting identical fermions in a harmonic trap. Given Feynman's answer (from his Statistical Mechanics book) for the ...
1
vote
1answer
526 views

Finding the electric field for a shell of charge

Suppose we have charge density defined $$ \rho(x,y,z) = \begin{cases} 0 & 0 \leq r < a \\ K & a \leq r\leq b\\ 0 & b< r \end{cases} $$ For some constants $K,a,b$ How would ...
0
votes
0answers
84 views

Using thermodynamics and Kinematics together to solve a parachuter problem?

I need to find a parachutist's displacement after a given height (nearly 37000m) and at a given latitude. I have his mass, area, parachute area, drop height, parachute deployment height, data about ...
0
votes
2answers
839 views

Showing that the probability density of a linear harmonic oscillator is periodic

The complete question I am trying to answer is the following: Show that the probability density of a linear harmonic oscillator in an arbitrary superposition state is periodic with period equal to ...
5
votes
1answer
190 views

Potential Energy tends to infinity on the N-Body Problem

I need help to solve this problem related with the N-Body problem, i dont understand quite well what I need to define or to express in order to solve it. We assume a particular solution to the N-Body ...
1
vote
1answer
94 views

Problem evaluating moles in a an isochor transformation

I have a problem with an isochor transformation. Me and my group of study made an experiment that want to check Gay-Lussac’s law. We registered the equilibrium states and fitted the $P = nRT / V$, ...
1
vote
1answer
107 views

Thermodynamic process when nebula is heated

The basic thermodynamics problem is stated as follows. The nebula contains a very tenuous gas of a given number density (atoms per volume) that is being heated to a given temperature. What is the ...
4
votes
3answers
149 views

Effect variable cylinder radius has on airflow

Can someone shed some light on the following problem? I'm unable to get any traction on it as I know nothing about fluid dynamics (I'm a computer engineer). It's a problem I made up over a lunch ...
0
votes
1answer
455 views

Point where resulting magnetic field is equal to 0

I am trying to solve the following task Given 3 infinitely long current carrying wires, as shown on image ($I_1=I_2$ , $I_3=2I_1$) and $r=5 cm$ calculate the point between them (in the same plane; ...
5
votes
1answer
194 views

Is it possible to determine particles' velocities from their crossings on a ring?

2 particles are constrained to move in a ring. Both particles begin moving at $t=0$ from $p=0$, each particle moving in the opposite direction of the other. It is known that they are moving at ...
1
vote
0answers
43 views

Calculate Rotational Intertia

If a can of soup, and a can of beans (tightly packed), are set in a race down a rough hill (has friction), the soup wins, because the inside of the can (soup) is not drawing energy from the system. ...
0
votes
1answer
344 views

CGS Units to SI Units

This may be a stupid question but I am having trouble getting the same result in CGS units as I would if I used SI units for a unitless calculation. I have to calculate $E_c=\frac{m_e c\, ...
1
vote
2answers
137 views

Bell State, if Bob applies a Pauli Gate?

After Alice and Bob share a Bell state, Bob applies a Pauli gate to his qubit. What will be the situation of the Bell state? What happens? Then Alice applies the same gate to her qubit – again, what ...
2
votes
1answer
275 views

A point in multiple magnetic fields

Given two parallel wires carrying a current (e.g. 2 and 3 Amperes) and the distance between them, 5 cm, how do I determine the magnitude of the magnetic field at a point M mid-distance between the ...
0
votes
2answers
240 views

Colors in the secondary rainbow reverse of that in the primary rainbow

Why the colors of Secondary rainbow is reverse of that in the color in the Primary rainbow? What can be the possible reason among the following options Because it is formed by one internal ...
1
vote
0answers
141 views

Braess's Paradox in Electrical Circuits

According to this and other similar papers, http://www.nature.com/nature/journal/v352/n6337/abs/352699a0.html, adding a current-carrying path can increase the voltage drop across a circuit. What is ...
2
votes
3answers
1k views

Delta Dirac Charge Density question

I have to write an expression for the charge density $\rho(\vec{r})$ of a point charge $q$ at $\vec{r}^{\prime}$, ensuring that the volume integral equals $q$. The only place any charge exists is at ...
3
votes
1answer
350 views

Spin-orbit coupling constant for rubidium

I have come across the following question in my course notes: The $5s\to 5p$ transition in rubidium is split into two components with wavelengths of 780nm and 795nm respectively. For the $5p$ state, ...
4
votes
2answers
395 views

Is the proper interpretation of temperature missing in this book?

In Randall T. Knight’s textbook “Physics for Scientists and Engineers” in the first chapter on thermodynamics (Ch. 16: A Macroscopic Description of Matter) one of the first conceptual questions is ...
1
vote
1answer
1k views

Partition function of bosons vs fermions

I have two atoms, both of which are either bosons or fermions, with four allowed energy states: $E_1 = 0$, $E_2 = E$, $E_3 = 2E$, with degeneracies 1, 1, 2 respectively. What's the difference between ...
7
votes
1answer
1k views

One strategy in a snowball fight

Here's a common college physics problem: One strategy in a snowball fight is to throw a first snowball at a high angle over level ground. While your opponent is watching the first one, you ...
2
votes
1answer
802 views

Elastic collision in two dimensions

Suppose a particle with mass $m_1$ and speed $v_{1i}$ undergoes an elastic collision with stationary particle of mass $m_2$. After the collision, particle of mass $m_1$ moves with speed $v_{1f}$ in a ...
0
votes
1answer
78 views

Not working brakes: just another energy conservation problem

A car is driving down a mountain ($v=90 km/h=25 m/s$, when the driver realizes that brakes aren't working. He try to lose velocity going up an inclined ($20°$) plane, with a friction coefficient of ...
0
votes
2answers
105 views

Energy problem: What's wrong here?

A car ($m=540\,\text{kg}$) engine, has a power of $60\,\text{kW}$. The static friction coefficient between wheels and road is $k=0.6$. How long does it take to reach the speed of ...
0
votes
2answers
549 views

What is the force of friction between two bodies given their masses and a force pulling them as a unit accross a surface?

Where a force of 200N pulls two blocks together(as one system) across a horizontal table top(µ=0.800) $m_A$ = 5.00kg, $m_B$ = 10.0kg Find the acceleration of the system. Find f$_k$ between B and A ...
1
vote
0answers
306 views

Malus' Law (Polarization) [closed]

Consider the image below: Let $\theta_2$ be the angle between the axes of transmission of Polarizer 2 and Polarizer 3 and $\theta_3$ be the angle between the axes of transmission of Polarizer 3 ...
2
votes
1answer
380 views

What is the energy of interaction between a point charge and an infinite cylinder?

I don't remember enough from my electromagnetism course and I can't find any simple, full example on this subject. I know, that I can consider the cylinder as a wire with the same charge density ...
1
vote
1answer
73 views

How to express answers where inequalities are involved?

Part of the floor of a workshop is inclined at 10 degrees to the horizontal. This is to allow the safe storage of steel cylinders. A cylinder of mass 7000 kg is stored as shown in the diagram (c the ...
0
votes
1answer
604 views

A problem from Pathria, canonical ensemble, how to calculate $\left\langle \left(\Delta E\right)^{3}\right\rangle $

This is problem 3.18, from the book Statistical Mechanics by Pathria. Show that for a system in the canonical ensemble $$\left\langle \left(\Delta E\right)^{3}\right\rangle =k^{2}\left\{ ...
1
vote
1answer
639 views

Kinetic Energy And Rotational Motion

The problem is, "A metal can containing condensed mushroom soup has mass 220 g, height 11.0 cm and diameter 6.38 cm. It is placed at rest on its side at the top of a 3.00-m-long incline that is at ...
2
votes
1answer
258 views

Euler-Lagrange Equation

A particle moving towards the origin has initial conditions $x(t=0) = 1$ and $\dot{x}(t=0)=0$ If the Lagrangian is L:=$\frac{m}{2}\dot{x}^2 -\frac{m}{2}ln|x|$ This should satisfy Euler Lagrange ...
0
votes
0answers
311 views

What is the pressure in a vertical pipe that has moving fluid through it?

I have a system like this When the valve is closed, pressure along any point along the thin tube may be found thru pgh. 1. What about when the valve is opened? 2. Does the top of the jar need to be ...
0
votes
1answer
698 views

Mass-Spring Damper system - moving surface

I need help with a physics problem, I don't know much about dampers, how can this be solved? we have $y_0(x)=\mu\sin(\Omega x)$ We arrive at this equation for motion (where we define $b$ and ...
0
votes
1answer
85 views

Lorentz Transformations and Aberration

Why does the azimuthal angle, $\phi$, remain unchanged between reference frames in special relativity? Edit: Here is a link from "Radiative Processes in Astrophysics": ...
0
votes
1answer
678 views

Finding Rotational Kinetic Energy Of A Clock

The problem I am working on is: Big Ben, the Parliament tower clock in London, has an hour hand 2.70 m long with a mass of 300 kg, and a minute hand 4.20 m long with a mass of 100 kg (see figure ...
0
votes
3answers
222 views

Doubts with levers

I'm studying for a test of static rigid body and I'm having doubts on how to solve problems involving levers with weight. If I have, for example, a lever 10 kilograms and 3 meters long and one support ...
0
votes
1answer
518 views

Diffraction through a single slit?

I'm having some trouble finding the answer to this homework question: A wide beam of light from a laser with a wavelength of 450.0 nm is incident on a long narrow opening that is 0.300 mm wide. ...
3
votes
1answer
883 views

Electric Potential of a Charged Sphere

Problem Consider a sphere with radius $R$, and with a charge distribution $\rho(r)=\rho_0r$. Using Poisson's equation, calculate the electric potential inside and outside the sphere. Solution I don't ...
0
votes
0answers
513 views

Question about the refraction of light in a swimming pool

Here is a homework question that I'm having a hard time understanding: Out of pure intellectual curiosity you have donned a snorkeling face mask and allowed yourself to sink to the bottom of the ...
1
vote
1answer
705 views

Solving a Statically Indeterminate Truss with Energy Equations (Tricky)

Okay, first off, I know that there is a very similar problem about statically indeterminate trusses here, but my problem was a bit different and I didn't know how to adapt the answer for that problem ...
0
votes
2answers
4k views

Tensions And Pulleys With Masses

The problem I am working on is: "A block of mass m1 = 1.80 kg and a block of mass m2 = 6.30 kg are connected by a massless string over a pulley in the shape of a solid disk having radius R = 0.250 m ...
0
votes
2answers
74 views

How to show that if two masses($m_1$ and $m_2$) are connected($r_1<r_2$) with a string moving around a point A, they move with same angular velocity?

How to show that if two masses($m_1$ and $m_2$) are connected($r_1<r_2$) with a string moving around a point A( circular motion), they move with same angular velocity? In this special case if ...
0
votes
1answer
146 views

What's the proper way to approximate the position uncertainty of a particle?

In this problem: shouldn't $\Delta x\sim\lambda/\sin\theta$ be $$\Delta x\sim \frac{\lambda}{\sin\theta} - \left(\frac{-\lambda}{\sin\theta}\right) = 2\frac{\lambda}{\sin\theta}$$ instead such ...
1
vote
1answer
144 views

Spring coupled platforms & conservation of momentum - can it be solved with freshman physics?

This question came up as an exercise in a first year undergraduate course I was a TA for. It turned out to be a lot more difficult (impossible?) than anticipated... Two platforms of mass $M_1$ and ...
-1
votes
1answer
52 views

Is this $W_{r} = \frac{1}{2}kx^2 $ procedure correct [closed]

I have a $0.40 Kg$ of mass that has $v= 0.5 m/s$, and then crashes with a spring with constant $k = 750N/m$ Whats the distance that the spring was compressed? I was using the equation $W_{r} = ...
0
votes
1answer
1k views

How to calculate the supporting force for a cantilever beam? [closed]

How do I find the supporting force $R_A$ and the moment $M_A$? Should I use the sum of moments or?
4
votes
2answers
285 views

What was the muzzle velocity of a home made gun launch strait in the air if air time was 8.2sec?

I built a potato gun and wanted to calculate the muzzle velocity. I remember from physics that I could run the numbers by calculating time from launch until landing. After pointing strait into the air ...
0
votes
1answer
88 views

How to find the distance of the source of sound in this problem? [closed]

On a cloudy day, the sound of thunder was heard 4.5 s after the flash of light was seen. How far was the cloud? Given that speed of sound = 340 m/s This is what I've tried: speed of light(c) = 10^6 ...
0
votes
1answer
221 views

How do you find the speed of sound in the problem? [closed]

A stone is dropped into a 40m deep well. The sound of the splash is heard 2.95 seconds after the stone is dropped. Find the speed of sound.
1
vote
2answers
63 views

thermodynamics in air and water

In perspective of thermodynamics,a person can survive a temperature of twenty degrees Celsius in air, but a person cannot survive in water with same temp.(it's not because he can't breathe) why is ...