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.
2
votes
1answer
169 views
Inhomogeneous Effective Mass in a 2D Lattice
Consider a tight-binding square lattice in 2D. This lattice has two different nearest neighbor tunneling rates along the x and y directions; call them $J_{x}$ and $J_{y}$. All longer range tunneling ...
1
vote
1answer
140 views
Clarification on a Goldstein formula steps (classical mechanics)
At page 20 of Classical Mechanics' Goldstein (Third edition), there are these two steps given between eqs. (1.51) and (1.52):
$$\sum_i m_i \ddot {\bf r}_i \cdot \frac{\partial {\bf r_i}}{ \partial ...
1
vote
0answers
168 views
Simple heat transfer question [closed]
You add an unknown volume of milk of $5.2 ^\circ C$ to a cup of coffee ($40 mL$ of water, temperature: $80.3 ^\circ C$). After a while of stirring the temperature reaches $73.2 ^\circ C$. The ...
0
votes
3answers
2k views
Finding Angular Acceleration of rod given radius and angle
A uniform rod is 2.0 m long. The rod is pivoted about a horizontal, frictionless pin through one end. The rod is released from rest at an angle of 30° above the horizontal. What is the angular ...
2
votes
1answer
154 views
Finding Max Radius of Propeller (angular velocity)
I am looking at a question from University Physics
The given answer
Whats the intuition behind using the below diagram
of finding $v_{tip}$? I was looking the the $v_{tan}$ not knowing how ...
3
votes
1answer
300 views
How do I find the initial velocity in this problem?
An X-ray tube gives electrons constant acceleration over a distance of $20\text{ cm}$. If their final speed is $2.0\times 10^7\text{ m/s}$, what are the electrons' acceleration?
I know this ...
0
votes
2answers
108 views
Solar Thermal/Solar Photo-voltaic calculations [closed]
This is my final high school assignment. I'm asked to prepare a research document on Solar Thermal and Solar Photo-voltaic, to prove that these two are feasible alternative power sources to power the ...
0
votes
1answer
419 views
Maximum Kinetic energy of a spring
The block in the figure below lies on a horizontal frictionless
surface and is attached to the free end of the spring, with a spring
constant of 35 N/m. Initially, the spring is at its relaxed ...
2
votes
0answers
86 views
Bandgap Spacing in Photonic Crystals
I am doing some self-study on photonics and have encountered the following question:
We know that amorphous electronic crystals such as amorphous silicon have a bandgap. Can amorphous photonic ...
0
votes
2answers
258 views
Linear motion with variable acceleration
Consider the following problem
I pull a mass m resting at x = 0 on a frictionless table connected to a spring with some k by an amount A and let it go. What will be its speed at x=0?
I know how to ...
1
vote
1answer
335 views
Find magnetic scalar potential for superconducting sphere
In regions where $J = 0$, the curl of the magnetic field $B$ is necessarily zero (since $\nabla \times B = \mu_0 J$). Therefore $B$ can be written as $B = -\nabla V_m$, where $V_m$ is a scalar ...
2
votes
1answer
4k views
Abiotic oil vs the traditional theory of oil deposit formation [closed]
I am curious to see what people think of the abiotic theory of oil deposit formation versus the traditional theory.
I have long wondered how enough organic material became trapped underground to ...
0
votes
0answers
489 views
Finding the period and frequency for simple harmonic motion [closed]
A 1 lb weight is suspended from a spring. Let y give the deflection (in inches) of the weight from its static deflection position, where “up” is the positive direction for y. If the static ...
2
votes
2answers
587 views
Why might the normal force on a box not be equal to its weight?
Very simple homework question which I managed to get wrong:
"The weight of a box sitting on the floor points directly down. The normal force of the floor on the box points directly up. Need these ...
0
votes
1answer
95 views
Is Force equal to components in different dimensions of Force or distance of those components
I'm trying to understand to find my homework problem. Its a simple concept of finding force. I wonder if you have two formulas then is the total Force = force from x + force from y. Or ...
0
votes
4answers
175 views
Find total energy and momentum of an moving electron in a rest frame
I have an electron moving with speed $u'$ in a frame $S'$ moving with speed $v'$ relative to a rest frame $S$.
How do I find the total energy and momentum of the electron in the rest frame $S$?
I ...
0
votes
0answers
80 views
How to figure out an elastic constant? [closed]
Im doing this study and i have this question which I'm not 100% sure on, got me pretty stumped. anyone think they can help me?!
When a bowstring is pulled back in preparation in preparation for ...
3
votes
1answer
173 views
Can I find a potential function in the usual way if the central field contains $t$ in its magnitude?
I'm working on a classical mechanics problem in which the problem states that a particle of mass $m$ moves in a central field of attractive force of magnitude:
$$F(r, t) = \frac{k}{r^2}e^{-at}$$
...
0
votes
0answers
182 views
Finding transcendental equation for the energy of a particle in delta potential well near infinite potential barrier [closed]
I'm having trouble finding the transcendental equation for a particle in a delta potential settled near an infinite potential wall. The potential is given by
$$
V(x) =
\begin{cases}
\infty & x ...
0
votes
0answers
84 views
Wireless signal strength
My question is possibly somewhat misplaced, but I'll try to explain as best as I can. Suppose I have a transmitter with a frequency of 2500MHz and a power of 1W. It radiates uniformly in all ...
-2
votes
1answer
404 views
Calculate acceleration and time given initial speed, final speed, and travelling distance? [closed]
A motorcycle is known to accelerate from rest to 190km/h in 402m.
Considering the rate of acceleration is constant, how should I go about calculating the acceleration rate and the time it took the ...
2
votes
2answers
783 views
Calculating work done on an ideal gas
I am trying to calculate the work done on an ideal gas in a piston set up where temperature is kept constant. I am given the volume, pressure and temperature.
I know from Boyle's law that volume is ...
1
vote
3answers
259 views
Same momentum, different mass
The question is: if
A bowling ball and ping pong ball
are moving at same momentum
and you exert same force to stop each one
which will take a longer time? or some?
which will have a longer ...
2
votes
1answer
112 views
helium balloon tied to a car [closed]
A helium balloon is tied to the seat of a car. The doors and windows of the car is closed. If the car now starts moving, in which direction will the balloon move- front or back?
0
votes
2answers
236 views
How does one calculate the volume of a nucleus and the volume of an atom (in this case hydrogen)?
The hydrogen atom contains 1 proton and 1 electron. The radius of the proton is approximately 1.0 fm (femtometers), and the radius of the hydrogen atom is approximately 53 pm (picometers).
0
votes
1answer
320 views
Calculate the UPS Capacity in amp-hours [closed]
I am trying to find out the UPS capacity in amp-hours for my HP UPS system.
I've already done some calculations based on the UPS information from the HP Power Manager software.
Bellow are my ...
-1
votes
1answer
259 views
Internal forces in a truss and its geometry
I'm to work out the internal forces in a truss, but I can't get my head around the geometry of the truss itself. I'm starting to think there may have been information on the diagram which I missed. ...
0
votes
1answer
58 views
1D Acoustical Relations beyond nearest neighbor couplings
Consider some 1D Lattice of atoms with nth neighbor coupling of strength k_{n}. I'm looking for the dispersion relation for acoustical phonons under these conditions.
I start with the Lagrangian,
...
6
votes
0answers
221 views
Capacitance of this unusual capacitor
This capacitor is composed of two half spherical shelled conductors both with radius $r$. There is a very small space between the two parts seeing to that no charge will exchange between them.
...
1
vote
1answer
44 views
Rays in Symmetric Resonator
I'm having some trouble figuring out how to get started on this question:
If I have a symmetric resonator with two concave mirrors of radii $R$ separated by a certain distance, after how many round ...
0
votes
2answers
366 views
Deriving the Poynting Theorem
I am trying to derive the Poynting theorem. So far, I've only been able to narrow down which equations I think I'll need to do so. These are the equations:
Maxwell's Equations:
$$
\nabla\times{\bf E} ...
0
votes
0answers
74 views
How do I figure out the normal force on a person on an accelerating platform? [closed]
My question is an elaboration on this question: Force on rope with accelerating mass on pulley
The elaboration is to determine what the Force that the platform exerts on the person. Assume the ...
1
vote
1answer
208 views
What is the general relativistic calculation of travel time to Proxima Centauri?
It has already been asked here how fast a probe would have to travel to reach Alpha Centauri within 60 years. NASA has done some research into a probe that would take 100 years to make the trip. But ...
2
votes
3answers
294 views
Jumping on earth versus jumping on the moon
Given the following problem:
On the moon the acceleration due to gravity is $g_m = 1.62 m/s^2$. On earth, a person of mass $m = 80 kg$ manages to jump $1.4 m$. Find the height this person will ...
0
votes
1answer
305 views
How to get the gradient potential in polar coordinate
In polar coordinate,
$$\nabla U = \frac{\partial U}{\partial r}\hat{\mathbf{r}} + \frac{1}{r}\frac{\partial U}{\partial \theta}\hat{\mathbf{\theta}} .$$
Can anyone show me how to get this result?
1
vote
1answer
163 views
Bouncing Ball Pattern
If a ball is simply dropped, each time a ball bounces, it's height decreases in what appears to be an exponential rate.
Let's suppose that the ball is thrown horizontally instead of being simply ...
1
vote
2answers
164 views
Proof of $Dq-qD=1$ where $D=\frac{\partial }{\partial q}$ is the differential operator
Can anyone provie me the proof of $Dq-qD=1$ where $D=\frac{\partial }{\partial q}$ refers to the differential operator?
Or if it's something special to quantum mechanics, why is it?
Is this ...
0
votes
2answers
88 views
Proof of $T=\sqrt{2y/a}$ in uniformaly accelerating object [closed]
Suppose that there is a object that does a y-axis-only free fall to ground. The initial distance from the ground is defined as $H$.
How does one prove that time the object takes to reach the ground ...
0
votes
0answers
72 views
Describing the movement of the object in a particular situation in Lagrangian way
Suppose there is a object M, (sliding motion) moving by the initial speed $v$ and the initial location $x_0$. Otherwise noted, friction is assumed to be nonexistent. It then meets a circular mold ...
2
votes
3answers
206 views
How do I integrate $\frac{1}{\Psi}\frac{\partial \Psi}{\partial x} = Cx$
How do I integrate the following?
$$\frac{1}{\Psi}\frac{\partial \Psi}{\partial x} = Cx$$
where $C$ is a constant.
I'm supposed to get a Gaussian function out of the above by integrating but don't ...
1
vote
1answer
151 views
Questions regarding solving the Brachistochrone problem using Lagrangian
brachistochrone problem: Suppose that there is a rollercoaster. There is point 1 ($0,0$) and point 2 ($x_2, y_2)$. Point 1 is at the higher place when compared to the point 2, so the rollercoaster ...
0
votes
2answers
125 views
Frequency of a tuning fork in a vacuum
Consider this equation of a damped harmonic oscillator such that:
$$
\ddot{x}+2\gamma\dot{x}+\omega^2_0=0
$$
with: $\gamma=\frac{b}{2m}$ and $\omega_0=\sqrt{\frac{k}{m}}$
Finally, we know that the ...
1
vote
1answer
359 views
Adiabatic expansion [closed]
I'll start off by saying this is homework, but I ask because I don't understand how the math should work (I don't just want an answer, I'd like an explanation if possible). I understand if this is ...
0
votes
0answers
110 views
Antenna Power and gain calculation [closed]
I have a wireless security related question, the second part confused me:
Your wireless network usually has a range of 100 feet. However you
are having a (confidential) meeting in a 10’ x 10’ x ...
0
votes
1answer
1k views
Distribution of charge on a hollow metal sphere
A hollow metal sphere is electrically neutral (no excess
charge). A small amount of negative charge is suddenly
placed at one point P on this metal sphere. If we check on
this excess negative charge a ...
1
vote
1answer
410 views
Friction due to air drag at high speeds
I am trying to set up this problem, but I am not sure how to go about doing so. (From University Physics, Young & Freedman):
You throw a baseball straight up. The drag force is proportional to
...
0
votes
0answers
36 views
What is some analogous experiment about the black holes by using the diary product like eggs, milk? [duplicate]
Possible Duplicate:
Black hole analog experiment?
I will explain my situation a little bit:
My teacher assign a experimental project that must be including the diary product: egg and milk ...
2
votes
1answer
170 views
A differential equation of Buckling Rod
I tried to solve a differential equation, but unfortunately got stuck at some point.
The problem is to solve the diff. eq. of hard clamped on both ends rod.
And the force compresses the rod at both ...
1
vote
2answers
492 views
Barrier in an infinite double well
I am stuck on a QM homework problem. The setup is this:
(To be clear, the potential in the left and rightmost regions is $0$ while the potential in the center region is $V_0$, and the wavefunction ...
2
votes
1answer
357 views
Schrödinger equation with complex potential
In 1 dimension what is the solution of the Schrödinger equation with potential
$$ V(x) = V_r + i V_i $$
Potentials are constant.
