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.
4
votes
1answer
513 views
Hooke's Law question
I have the following question to answer:
a force of160 N stretches a spring 0.050m from its equilibrium position.
A. what is the spring constant of the spring?
The equation for Hooke's Law in my ...
4
votes
1answer
355 views
Plotting a SHO in matlab
I have no prior experience of using matlab. My teacher want me to solve this question. I have been trying for a couple of hours now with no luck, please help!
The mass of 100 g hanging in a spring ...
4
votes
1answer
336 views
Electric Potential
A nonuniform linear charge distribution given by λ = bx, where b is a constant, is located along an x axis from x = 0 to x = L. What is the electric potential at a point on the y axis? Set potential ...
4
votes
2answers
109 views
Vector potential
I have difficulty understanding the following vector calculus example. Text can be found here. It is the 5th Q&A -- starting with equation (31.1035).It concerns finding the vector potential of a ...
4
votes
1answer
118 views
Frictional force on a rope wrapped around a drum
So there's a rope wrapped around a circular drum, subtending an angle $\theta$. A large force, $T_A$, pulls in in one direction, and a much smaller force $T_B$ pulls in in another direction.
The ...
4
votes
2answers
176 views
Technical detail in the solution of the hydrogen atom
I'm trying to do an exercise in which you solve the Schrödinger equation for the hydrogen atom. Through the exercise, I've already shown that the wavefunction is:
$$ \psi_{n\ell m}(r,\theta,\varphi) ...
4
votes
3answers
178 views
How can a vertical force cause motion at an angle?
I just started learning physics 3 days ago and am having trouble understanding what I am doing wrong. Can someone please explain my error(s)? Thanks!
We have a 1kg object on a plane at a 30 degree ...
4
votes
2answers
108 views
What was the muzzle velocity of a home made gun launch strait in the air if air time was 8.2sec?
I built a potato gun and wanted to calculate the muzzle velocity. I remember from physics that I could run the numbers by calculating time from launch until landing. After pointing strait into the air ...
4
votes
2answers
143 views
Calculating the period of a quasi-circular orbit
In solving an exercise I had to find the equation of the quasi-circular orbits of an object with the potential $V(r)=-\alpha r^{-1-\eta}$ and I expressed it as:
$$r(\phi)=\frac{r_c}{1+\epsilon ...
4
votes
4answers
531 views
What identifies an action-reaction pair of forces?
This is a question on a worksheet that I'm giving to a student:
A man of weight $W$ steps into the lift. The lift then moves downwards with a constant speed. If $R$ represents the force acting on ...
4
votes
2answers
334 views
Deriving Birkhoff's Theorem
I am trying to derive Birkhoff's theorem in GR as an exercise: a spherically symmetric gravitational field is static in the vacuum area. I managed to prove that $g_{00}$ is independent of t in the ...
4
votes
3answers
182 views
What does it mean if a = g in this situation?
I basically solved this problem, but I am unsure what the final equation actually means.
Write an expression for solving the mass of block C if mass B moves to the right with an acceleration ...
4
votes
1answer
163 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 ...
4
votes
2answers
46 views
Equivalent RC circuit to a RRC circuit?
I'm in doubt about a situation that I've seen sometimes: imagine we have a resistor in parallel with a resistor and a capacitor in series. Since I don't know how to generate figures of circuits to ...
4
votes
1answer
109 views
Quantum Mechanics - Hidden Variables
In Steven Weinberg's Lecture on Quantum Mechanics (p. 342), he writes:
The correlation between the spins of the two particles can be
expressed as the average value of the product of the ...
4
votes
1answer
106 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 ...
4
votes
1answer
135 views
Maximum probability of success for distinguishing between two pure states with one measurement
Suppose you have the states such that $\langle \phi | \theta \rangle = cos(x)$ and you have one measurement to distinguish between the two. It is claimed that the probability of success at guessing ...
4
votes
3answers
960 views
“Find the net force the southern hemisphere of a uniformly charged sphere exerts on the northern hemisphere”
This is Griffiths, Introduction to Electrodynamics, 2.43, if you have the book.
The problem states Find the net force that the southern hemisphere of a uniformly charged sphere exerts on the ...
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
81 views
Finding the acceleration of a cart rolling on a table
The cart is rolls frictionless on the table. It has a mass of $1 kg$. Attached to it are 2 strings, that go through two frictionless sheaves. The weights have masses as in the picture.
...
4
votes
3answers
65 views
Length of day of a gas giant
How can the rotational speed, or the length of a day be determined or estimated in a planet which is composed entirely of non homogeneous fluids? There must be internal forces (pressure gradients, ...
4
votes
3answers
129 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 ...
4
votes
1answer
749 views
Solving a statically indeterminate truss
I'm trying to solve the statically indeterminate truss shown below and I'm having a little trouble.
$H$, $P$ and $\beta$ are given. The material is aluminum (density is 2700 ${kg/m^3}$) and has a ...
4
votes
1answer
468 views
Force on Earth due to Sun's radiation pressure
I have been asked by my Classical Electrodynamics professor to calculate the force that the Sun exerts in the Earth's surface due to its radiation pressure supposing that all radiation is absorbed and ...
4
votes
1answer
936 views
Shouldn't the electric field in a solid insulating sphere be linear with radius?
I am a senior in High School who is taking the course AP Physics Electricity and Magnetism.
I was studying Gauss's laws and I found this problem:
A solid insulating sphere of radius R contains a ...
4
votes
1answer
508 views
Tight Binding Model in Graphene
I'm following a calculation done by a guy who's done it a bit different than what I've done before (used nearest neighbour vectors and a DFT instead of what I will show below), I'm not quite sure how ...
4
votes
1answer
141 views
Transpose Map Positive, But Not Completely Positive?
I am reading Introduction to Quantum Computing by Kaye, Laflamme, and Mosca. Here is a question I am struggling with:
Exercise 3.5.6: Prove that the transpose map, which maps $\rho \mapsto ...
4
votes
1answer
494 views
Finding the position of a planet between two other planets of known mass and distance
Here is the question:
A planet with mass $m$ and a second with mass $M$ are separated by a
distance $d$. A third planet with mass $m_3$ happens to be midway
between $M$ and $m$. Where could ...
4
votes
2answers
60 views
How to directly calculate the infinitesimal generator of SU(2)
We commonly investigate the properties of SU(2) on the basis of SO(3). However, I want to directly calculte the infinitesimal generator of SU(2) according to the definition $$X_{i}=\frac{\partial ...
4
votes
1answer
102 views
Chain of balls on an inclined plane
Suppose we have some inclined plane, and there is some chain of balls of length $l$ and mass $m$ lying on it. No friction at all in the system.
1) What is $x_0$ (the vertical hanging part of the ...
4
votes
2answers
340 views
Quantum Mechanics: Show that the expectation value of angular momentum does not change with time
The potential is given by $V\left(\left\|(x,y,z)\right\|\right)$, so $[\hat{L}, \hat{H}] = 0$.
Using the definition of $\langle \hat{L} \rangle$ and the time-dependent Schrödinger equation, show that ...
4
votes
1answer
169 views
Voltage drop over a cell membrane
Again, a problem from exam preparation:
[A] cell's membrane allows sodium ions to pass through it, but not chlorine ions. The cell is placed in a salty solution with a ten times higher ...
4
votes
0answers
58 views
I am trying to calculate the branching ration of higgs goes to 2 photons using the standard model [closed]
I need to use the three lowest order feynman diagrams to first calculate the squared matrix element to put into fermis golden rule formula and then from there get the branching ratio of higgs decays ...
4
votes
1answer
88 views
Application of Archimedes Principle
A candle is floating in a liquid placed in a container. The container is a cylinder of diameter $D$, and the candle is of width $d$. ($D>d$) The height of the liquid from the bottom of the ...
4
votes
1answer
89 views
“Redshifting” of forces in stationary space - times
Here's the problem statement:
Let $(M,g_{ab})$ be a stationary spacetime with timelike killing field $\xi ^{a}$. Let $V^{2} = -\xi _{a}\xi ^{a}$ ($V$ is called the redshift factor).
(a) Show that the ...
4
votes
0answers
139 views
Friction in circular motion [closed]
A circular horizontal table spins around its axis of rotation with a constant angular speed. It completes one revolution in 2 seconds. An object whose mass is $M = 0.8 \ \text{kg}$ is placed on the ...
4
votes
0answers
212 views
Derivation of the oil drop experiment [closed]
The oil drop experiment performed by Robert Millikan in 1909 enables us to calculate the elementary charge.
The following forces have effect on the droplet:
$F_\mathrm{G} = m g = \frac{4}{3} \, \pi ...
4
votes
1answer
734 views
Infinite-range 1D Ising model + Hubbard-Stratonovich-Transformation
I have a probably quite simple question RE the HST.
After some work, I obtain as the partition function for the infinite range 1D Ising model
$$Z = \int_{-\infty}^\infty \frac{dy}{\sqrt{2\pi / ...
3
votes
4answers
575 views
Why does an object thrown parallel to the ground eventually fall down?
Suppose an object is thrown parallel to the ground. The gravity acts downward (ie. perpendicular to the direction of motion of the object). The work done by gravity on that object will be given by :
...
3
votes
3answers
139 views
Non-SHM oscillatory motion
How to solve these kind of questions , where $|F| \propto x^2$?
How to find time period and velocity type related things to the oscillatory motion?
...
3
votes
2answers
218 views
Quantum Mechanics Notation for BRA KET
I've been given this homework problem, but I do not understand its notation.
Please perform the following where the wavefunctions are the normalized eigenfunctions of the harmonic oscillator ...
3
votes
2answers
197 views
Simple QFT exercise
Consider a particle on the real line with:
$L=\frac{1}{2}(\partial_0q)^2 + f(q)\partial_0q$
the equation of motion is that of a free particle $\partial_0^2q=0$. In fact $\delta[f(q)\partial_0q]=0$. ...
3
votes
3answers
193 views
Irreversible process
I have this problem. I have an ideal gas that goes through an irreversible adiabatic decompression. I have the initial state (P,T,V), and the final pressure, and I have to calculate the entropy ...
3
votes
3answers
1k views
3
votes
3answers
1k views
Maximum range of a projectile launched from elevation “dumbed down”
I am trying to conceptually understand why the angle which produces the greatest range for a projectile launched with an elevation is not 45 degrees. I have exausted all other options, and I hope that ...
3
votes
1answer
196 views
Do the trigonometric functions preserve units?
I saw an exercise where you had to calculate the units of $C_i, i=1,2$ from an equation like this:
$v^2=2\cdot C_1x$ and
$x=C_1\cdot \cos(C_2\cdot t)$
where
$x$ means meters,
$t$ means seconds ...
3
votes
2answers
306 views
The quantized energy level E depends on which power of n?
A particle in one dimension moves under the influence of a potential $V(x)= ax^6$, where $a$ is a real constant. For large $n$, what is the form of the dependence of the energy $E$ on $n$?
3
votes
2answers
67 views
Sign of acceleration
I'm developing an application using accelerometer sensor. I'm not good at physics so forgive me if the question is trivial. If I have 3 values of acceleration: $x$, $y$, $z$, I find acceleration ...
3
votes
4answers
377 views
why does a larger thermal conductivity provide a smaller temperature gradient?
I was thinking about Fourier's Law in heat transfer today and for some reason I am just not understanding the relationships it gives us. Fourier's tells us that if the heat transfer rate is kept ...
3
votes
2answers
326 views
Delta-function and integration
How to "take" following integral?
$$
\int \limits_{-\infty}^{\infty}\frac{3\gamma a^{2}d^{3}\mathbf r}{4 \pi \left( r^{2} + \frac{\gamma^{2}}{c^{2}}(\mathbf r \cdot \mathbf u)^{2} + ...
