# 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.

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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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: $$F(r, t) = \frac{k}{r^2}e^{-at}$$ ...
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 ...
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 ...
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 ...
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 ...
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 ...
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?
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).
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 ...
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. ...
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, ...
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. ...
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 ...
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} ...
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 ...
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 ...
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 ...
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?
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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.
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