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.
3
votes
2answers
112 views
Derivatives of delta function and equation of continuity for a single charge…
For a single charge $e$ with position vector $\textbf R$, the charge density $\rho$ and and current density $\textbf{j}$ are fiven by:
\begin{equation} \rho(\textbf{r},t)= ...
0
votes
1answer
288 views
How do I get the focal length from a camera matrix?
Given the camera matrix, can I find the focal length of this camera ?
0
votes
2answers
412 views
Basic Free Fall Problem [closed]
So I am trying to remember my childhood doing some Physics problems, but seems that I forgot almost everything. But it's not a big thing... that's why I'm training!
My problem is pretty basic ...
0
votes
1answer
139 views
Figuring out the force using Newton's 2nd Law
Here's a problem on my test review:
A 12 kg crate resta on a horizontal surface and a boy pulls on it with
a force that is 30° below the horizontal. If the coefficient of
static friction is ...
0
votes
3answers
327 views
How can I show that an arbitrary wavefunction in a 1D SHO is periodic in time?
I want to show that an arbitrary wavefunction $f$ in a one dimensional harmonic potential reproduces itself after a period T up to a phase factor: $f(x,t+T)=Af(x,t)$, $|A|=1$
I am not sure if this ...
0
votes
2answers
2k views
Finding initial velocity using conservation of momentum and energy [closed]
A 14 g bullet is fired into a 120 g wooden block initially at rest on a horizontal surface. The acceleration of gravity is 9.8 m/s^2. After impact, the block slides 6.97 m before coming to rest.
If ...
1
vote
1answer
445 views
Dimensional Analysis: Buckingham Pi Theorem Using a Matrix
I have the following problem - which is basically that $\omega = f(g,h,l)$ now the book claims that the two $\pi$ terms as follows $\pi_1 = \omega \sqrt{\frac{l}{g}}$ and $\pi_2 = \frac{h}{l}$. Now we ...
1
vote
0answers
127 views
Wave propagation in anisotropic media
I'm given a wave propagation in an anisotropic medium with the following properties:
$\epsilon=\left[\begin{array}{ccc} \epsilon_{11} & 0 & 0 \\
\epsilon_{21} & \epsilon_{22} & ...
2
votes
2answers
180 views
Modeling galactic colonization
I found a really cool sounding order-of-magnitude modeling question but am a bit at a loss on how to approach this:
The Milky Way contains 100 billion stars and has a radius of 250,000 light ...
1
vote
0answers
314 views
Find equations of motion from given Lagrangian density [closed]
Could someone help me solve this probably not very hard problem?
Given Lagrangian Density:
$\mathcal ...
0
votes
0answers
413 views
Word problem - Potential/Elastic Energy [closed]
A man (weighing $86\space Kg$) is climbing a mountain when he suddenly
falls. His security rope works like a spring with spring constant $1.2
> \times \space 10^3 N/m$ and after a fall of ...
2
votes
3answers
96 views
How can I modify the bullet trajectory based on the ballistic coefficient? [duplicate]
Possible Duplicate:
How to modify the bullet trajectory based on the ballistic coefficient?
I am new to the physics surrounding bullet trajectory and how it is calculated. I am a software ...
1
vote
0answers
184 views
How to model an accelerometer measurements on a car wheel?
I am working on kinematically modelling an accelerometer on a car wheel.
When working on the initial conditions, I am confused whether or not I should use the gravitational acceleration since there ...
1
vote
1answer
848 views
Dimensional Analysis: Buckingham Pi Theorem
I am studying for a fluids quiz and I am having a few problems relating to dimensional analysis but for the time being fundamentally I have a problem selecting the repeating variables. Like does ...
1
vote
2answers
492 views
how can I find the potential created by spherical capacitor with dielectric material
If we have a spherical capacitor with inner radius or r1 and outer radius of r2, with charges (+/-)q on them and there is a dielectric material (with constant e) in between them with.
What kind of a ...
0
votes
1answer
444 views
Solving an indeterminate triangle truss structure (statics)
From the setup of two trusses as shown in this illustration, how can I solve for the axial reaction forces in the trusses, and in the points A and B (Ax, Ay, Bx, By)? It is assumed that the joints ...
-1
votes
2answers
287 views
A proton is trapped in an infinitely deep well of L meters. The proton is in the first excited state. How does the excited state change the question.
The real question was: A proton is trapped in an infinitely deep well of 1*10^-14m. I suppose that is unimportant as that should only help us decided the limits of our integration. What I'm worried ...
0
votes
0answers
106 views
How long until “final totality”? [closed]
It is given that the angular size of the Sun as viewed from Earth is $0.533^\circ$, the distance of the Moon from Earth at perigee is $3.633\times 10^5$ km, and the mean radius of the Moon is $1737.1$ ...
4
votes
3answers
403 views
Ice cream cone and loop-de-loop
Was siting in class thinking about this problem, did some rough sketches of a solution but never really managed to solve it.
Assume a boy starts at the top of a circle with radius R as described ...
4
votes
1answer
161 views
Imaginary angle on simple centrifugal problem
Say somebody is spinning a mass on a string with a period $T$ around. If $T$ is very big, the mass will describe a small circle around the person's waist. If $T$ is very small, the plane of movement ...
0
votes
0answers
78 views
2 streams going into an engine and 2 come out. Find the enthalpy?
Problem: Steam supply to an engine is made of two streams that mix before entering the engine. Stream 1 flows at a $.01\frac{kg}{s}$ with an enthalpy of $2952\frac{kJ}{kg}$ and a velocity of ...
-2
votes
1answer
55 views
what is a difference in the width of the spinning bar?
The bar with length l, density r, diametr d, Young's modulus E, Poisson's ratio mu, is spinning around the cross-section, what is the change in the width of this bar?
0
votes
1answer
719 views
What happens to temperature when pressure is constant in a cylindrical piston of saturated liquid ammonia?
Let's say I have a cylindrical piston containing saturated liquid ammonia that is fitted with an electrical heater and a paddle wheel for stirring at an initial pressure and an initial temperature. ...
0
votes
0answers
77 views
Equalizing Oil and Water Separation in Sample Tube [closed]
I hope I'm posting this in the correct forum.
My son is working on a science project, and I am stuck in trying to figure out how to help him. He's doing conductivity testing in various liquids, and ...
2
votes
2answers
922 views
Book against a wall and forces
If you take a book with mass of 1kg and push it against the wall. With how much force do you have to push the book so it does not fall?
The problem is I know how to calculate this problem, you say ...
0
votes
0answers
385 views
How to determine donor or acceptor impurity
I am working on a practice exam for semiconductor physics. Can anyone help me figure out how to determine whether we have donor or acceptor impurity in Gallium Arsenide?
...
1
vote
1answer
161 views
Proof of the Hydrostatic weighing equation?
I've been trying to derive this equation for about two hours:
But I can't seem to get it. In my class, we are using it to accurately measure the volume of irregulary-shaped objects, but I ...
0
votes
0answers
348 views
Reynolds Number and relation to Moody Diagram for a Small Pipe
I am having the following problem on a lab write up:
Calculate the approximate distance downstream of the inlet that
corresponds to the hydrodynamic entry length for Reynolds numbers of
3,000 ...
1
vote
2answers
529 views
Calculating the gravitational acceleration inside of a planet
Plant X has a radius of 5000 km and is composed of two layers.
The first inner layer ranges from the centre to 2000 km from centre, it's density is 8 kg / dm^3.
The second layer ranges from 2000 ...
-1
votes
1answer
3k views
Finding induced current in a loop at an instant [closed]
I'm working on this problem; however, I cannot seem to get anywhere.
Given information:
The rectangular loop in the figure has 2.1x10^-2 ohm resistance.
What is the induced current in the loop at ...
0
votes
2answers
287 views
Finding work done by rotational force?
A disk with a rotational inertia of 5.0 kg·m2 and a radius of 0.25 m
rotates on a fixed axis perpendicular to the disk and through its
center. A force of 2.0 N is applied tangentially to the ...
1
vote
1answer
161 views
Finding stopping time when only given initial angular velocity and an expression for angular acceleration?
Question:
A wheel starts is spinning at $27\text{ rad/s}$ but is slowing with an angular
acceleration that has a magnitude given by $\alpha(t) = (3.0\;\mathrm{rad/s^4})t^2$. It stops in a time ...
1
vote
2answers
109 views
Is stopping something work?
If somebody pushes against a mass moving with $3 \frac{m}{s}$ to slow it down to $2 \frac{m}{s}$, he will drain the moving system of kinetic energy. Does he do work then or does he consume work?
My ...
3
votes
1answer
533 views
Escape velocity of a rocket standing on Ganymede (Moon of Jupiter)
I want to calculate the escape velocity of a rocket, standing on the surface of Ganymede (moon of Jupiter) and trying to leave Ganymede.
My thinking was, the kinetic energy $E_{\text{KIN}}$ must be ...
-1
votes
2answers
147 views
Fuel Tank Question
Could some one please help me with this question as my physics knowledge does not extend this far.
I have a large fuel tank with a capacity of 21000L. The tank currently is holding 2000L of fuel. On ...
0
votes
1answer
365 views
How do I express the Kepler general orbit $r(\phi)$ in rectangular coordinates?
How do I express the Kepler general orbit $r(\phi)$ in rectangular coordinates?
I use the identities $x=r\cos\phi$, $y=r\sin\phi$, and $r^2 = x^2 + y^2$, but I block at some point.
0
votes
2answers
331 views
Does power from a resistor perform work or heat transfer?
Let's say I have a cylindrical piston filled with air and fitted with an electrical resistor. The resistor has a current going across it and a voltage for a certain amount of time. My system would be ...
2
votes
0answers
374 views
Bungee jump physics
Question:
A bungee jumper jumps from a bridge. The length of the loose rope is 30 m. When the jumper reach the lowest point possible, the rope stretches 10 m. What is the final stretch of the rope, ...
3
votes
1answer
486 views
Conservation of angular momentum for a nonrigid body
Question:
The sun is not a rigid body but a hot ball of gas. The period of rotation varies from 37 days at the pole to 26 days at the equator. The mean radius of the sun is $7\times 10^8\text{ ...
2
votes
2answers
482 views
Generating Pink Noise
I've generated some white noise in Excel by using the formula
$$2*\mathrm{Math.RAND}()-1$$
I would now like to create some pink noise. I believe this is done by applying some sort of filter to the ...
0
votes
1answer
377 views
Plotting Hydrogen's $2P_{x,y,z}$ Probability Densities in MATLAB [closed]
I have spent an unreasonable amount of time trying to plot $F(r,\theta,\phi)$ plane slices in MATLAB. I want to look at $x-y,y-z,x-z$ planes. Here's the function, specifically:
...
0
votes
0answers
331 views
Calculate Electric field distribution of infinite plane in the presence of conductors. (using the method of image charges) [closed]
Two solid conductors are attached together orthogonally and there's an infinite plane with charge density σ at π/4 Rad with the conductors. Calculate the field dist. In the region with the plane
...
2
votes
2answers
274 views
Can you please help with our water tank and spray system?
For our design project we are working with a client in Wisconsin to help them remove snow from their roof-mounted solar panels in the winter. We are designing a system to spray water a vertical ...
2
votes
1answer
421 views
Force on rope with accelerating mass on pulley
I have a pretty basic pulley problem where I lack the right start.
A child sits on a seat which is held by a rope going to a cable roll (attached to a tree) and back into the kid's hands.
When it ...
0
votes
0answers
312 views
Ground state energies with fermions of same spin?
Consider two non-interacting Fermions (half-integer spin) confined in
a 'box'. Construct the anti-symmetric wavefunctions and compare the
corresponding ground-state energies of the two systems; ...
1
vote
1answer
100 views
What lepton neutrino would be in this reaction?
If I have a reaction (shown below), and I'm supposed to work out what the products will be, what would the lepton neutrino $\nu$ be? e.g electron neutrino:
$$ \nu + p \to $$
2
votes
2answers
312 views
Gauss' law giving zero field where field is not zero?
Two plastic sheets with charged densities as shown:
I'm trying to find the field at $B$. I obtained the correct answer by adding up the fields created by each charge density. But I realized that ...
0
votes
0answers
79 views
How long will the bar to travel 1.50 cm threaded rod? [closed]
A threaded rod with 12.0 turns / cm and 1.18 cm in diameter is mounted horizontally. I screwed a threaded rod with a hole to attach to the bar. The bar rotates at 237 revolutions / min. How long will ...
3
votes
2answers
553 views
The Energy-Momentum Tensor and the Ward Identity
I have a question regarding a homework problem for my quantum field theory assignment.
For the purposes of the question, we can just assume the Lagrangian is that of a real scalar field:
...
3
votes
1answer
309 views
Compound lenses and virtual images
Ok so I have this problem where I have a system of two lenses. All I know is that the distance between the object and the first lens is 30cm, the distance between the object and the final image is ...
