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
246 views
Finding Speed with the Energy Principle - Nuclear Fission
The question goes along the lines:
Uranium-$236$ fissions when it absorbs a slow-moving neutron. The two fission fragments can be almost any two nuclei whose charges $Q_1$ and $Q_2$ add up to ...
4
votes
1answer
512 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
731 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
$$Z = \int_{-\infty}^\infty \frac{dy}{\sqrt{2\pi / ...
1
vote
0answers
95 views
To determine the age of the Earth by the cooling of the centre of the Earth [closed]
I was given an exercise to make a simple model that suggests that the age of the Earth is 78000 years old.
My current starting point is
$$u_{tt} = u_{xx}$$
so a wave equation. So in polar ...
0
votes
0answers
624 views
Calculating weight using moments [A-Level] [closed]
A uniform plank of weight 150N and of length 4.0m rests horizontally on two bricks. One
of the bricks is at the end of the beam. The other brick is 1.0m from the other end of
the plank.
a) Sketch the ...
1
vote
2answers
288 views
How do you choose the locations of forces when calculating moments?
A uniform bridge of weight 1200kN and of length 17m rests on supports at each end which are 1m wide. A stationary lorry of weight 60kN has it’s centre of mass 3.0m from the centre of the bridge.
...
2
votes
4answers
268 views
How are we able to view an object in a room with bulb..?
This is a very basic question on optics. How are we able to view an object kept in a room with a bulb?
From what I understand, light rays from bulb will hit the object and some colour will be ...
2
votes
1answer
290 views
How to set up a very simple experiment in optics?
This might come across as a very rudimentary question.
My fundamentals of Optics are weak. In the optics chapter of my physics text book I saw diagrams each depicting an object on the left and a lens ...
3
votes
2answers
371 views
How do I calculate the position on the Bloch sphere of a quantum gate with a given diagonal matrix?
In quantum computation there are several principal quantum gates that have corresponding matrix representations. One of these is the Z gate, whose matrix is $\left[\begin{smallmatrix} 1 & 0 \\ 0 ...
5
votes
1answer
5k views
How do you find the uncertainty of an weighted average?
The following is taken from a practice GRE question:
Two experimental techniques determine the
mass of an object to be $11\pm 1\, \mathrm{kg}$ and $10\pm 2\, \mathrm{kg}$.
These two ...
0
votes
1answer
201 views
Fluid Mechanics Problem - Ball in Reservoir
I'm having a problem starting and solving the following problem:
My attempt at the solution was to realize you need to balance the upward forces with those of the downward forces in which case you ...
2
votes
1answer
2k views
Best way to solve P-V-T Question
Typically in an exam or problem set we have a problem that's like this:
Determine the specified property at the indicated state of water. Locate the state on a sketch of the T-v and P-v diagram.
p= 3 ...
0
votes
1answer
199 views
Calculating vector of plane resulting from tailwind
In some basic physics homework I have, it asks what the resulting vector would be for a plane traveling at x m/s in y direction that is affected by a tailwind going in z direction at w m/s. How would ...
1
vote
3answers
124 views
Question about orthonormal decompositions over unitary operators
I'm teaching myself quantum information theory using Nielson and Chuang's "Quantum Computation and Quantum Information" and I'm at a point in the book where the formalism is starting to make my eyes ...
0
votes
0answers
50 views
on interference [closed]
A laser is oscillating at two frequencies v1 and v2 such that v2-v1=c/2l where resonator length. If the laser is used in an interference experiment,what is the minimum path difference between ...
0
votes
4answers
12k views
Heat transfer calculated from the specific heat formula
Say I have 10g of silver, whose specific heat is known to be 0.235. I've heated it up from 50.0C to 60.0C. How much heat has been transferred?
The equation is:
$$ Q = C_pm\Delta t $$
where Cp is ...
3
votes
1answer
1k views
Reversible adiabatic expansion
I am confused with adiabatic expansions. I have a homework problem wherein 2.75 moles of an ideal gas at 375 K expands adiabatically with an initial pressure of 4.75 bar and a final pressure of 1.00 ...
2
votes
2answers
518 views
Calculating uncertainty in the final result (combining uncertainties)
I'm struggling to determine the uncertainty in $F$ so it would match the textbook answer.
The problem statement is: A force F is obtained using the equation: $F = \frac{mv^2}{2\pi(x_2 - x_1)}$.
The ...
0
votes
2answers
3k views
Not sure of a homework solution involving average acceleration
A 50.0 g Super Ball traveling at 25.0 m/s bounces off a brick wall and rebounds at 22.0 m/s. A high-speed camera records this event. If the ball is in contact with the wall for 3.50 ms, what is the ...
3
votes
2answers
315 views
Paramagnet: Negative specific heat?
for a simple paramagnet ($N$ magnetic moments with values
$-\mu m_i$ and $m_i = -s, ..., s$) in an external magnetic field $B$, I have computed the Gibbs partition function and thus the Gibbs free ...
0
votes
0answers
53 views
Finding Necessary Velocity of a Projectile? [closed]
Imagine, you're throwing a ball 19.2 meters and you need to throw it over a 6.2 meter obstruction that is half way (9.6m) between the release point and the catch point (assuming these are both 1.2 ...
3
votes
4answers
924 views
How can I explain why the weak nuclear interaction between individual nucleons is 'weak'?
By considering the energy-time uncertainty principle, estimate the range of the weak nuclear interaction at low energies. Compare this range to the size of a typical nucleon (for example, a proton) ...
3
votes
2answers
78 views
Balmer lines and their positions
I have two stars, both of which have Balmer lines at the same wavelength positions.
Why does this pattern exist?
2
votes
3answers
2k views
why do we test electric fields with positive charges? not negative ones?
Is there any difference between using a positive vs a negative charge to test an electric field? If so, please explain.
5
votes
2answers
176 views
Can the process $u\overline{u} \rightarrow s\overline{s}$ be mediated by the EM interaction?
I’m working on a homework problem which asks for the dominant contribution (e.g. EM, strong, or weak) to the process $p + \overline{p} \rightarrow \Lambda + \overline{\Lambda}$).
I know that the EM ...
0
votes
0answers
112 views
Vectors and finding lines on a point [closed]
Long John Silver, a pirate, has buried his treasure on an island with five trees located at the following points: A (30.0m, -20.0m), B(60.0 m, 80.0m), C (-10.0m, -10.0m), D (40.0m, -30.0m), and ...
0
votes
2answers
267 views
Vectors, Component Addition, and Significant Figures
I have two vectors $\vec{A}$ and $\vec{B}$ and I need to find the x- and y-components of $\vec{C} = \vec{A} + \vec{B}$. Here's what I have so far:
$$|\vec{A}| = 50.0 m, \theta = -20.0^\circ$$
...
0
votes
0answers
325 views
A question related to Gauss's Law [closed]
I'm unable to solve this numerical based on Gauss's Law:
A charge Q is distributed uniformly within the material of a hollow sphere of inner and outer radii: r1 and r2. Find the electric field at a ...
6
votes
2answers
749 views
How do Leptons arise from Lambda decay?
I have a question for an assignment:
Use your understanding of the quark model of hadrons and the boson model of the weak nuclear interaction to explain how leptons can arise from lambda decay, ...
3
votes
1answer
124 views
Equation of Motion in a Non-Inertial (Rotating) Frame
Let me preface my question by informing you this is for an assignment, so I would rather not have explicit answers but rather be given guidance in arriving at the correct solution.
The question is ...
0
votes
1answer
478 views
Changing magnetic flux graph?
In regards to a graph of the changing magnetic flux in a generator such as this one:
a) The equation of the graph should be $\Phi = BA \cos \theta$. As $\theta=\omega t$ (angular velocity*time), ...
6
votes
2answers
234 views
Conservation of energy in a non-linear oscillator
I have a homework question about a "non-linear oscillator". I actually have an answer to this question, but the answer I get is stronger than what is needed according to the question. The question ...
2
votes
1answer
1k views
Degeneracies of the first excited state
I have a box with $x,y,z$ all ranging from 0 to $l$. It has $V(x)$=0 inside and =$\infty$ outside. By extending the 1D Schrodinger equation, I have that the allowed energy eigenvalues are ...
3
votes
1answer
191 views
Projection of states after measurement
Continuing from the my previous 2-state system problem, I am told that the observable corresponding to the linear operator $\hat{L}$ is measured and we get the +1 state. Then it asks for the ...
0
votes
1answer
263 views
Two-state system problem
Given a 2-state system with (complete set) orthonormal eigenstates $u_1, u_2$ with eigenvalues $E_1, E_2$ respectively, where $E_2>E_1$, and there exists a linear operator $\hat{L}$ with ...
-1
votes
1answer
253 views
Functional Derivative of Convolution
How to carry out the following functional derivative?
$$\frac{\delta F}{\delta n(r)}$$ where $$F=\int dr n(r) \int C(|r-r'|) n(r') dr'$$
is it simply:
$$2 \int dr' C(|r-r'|) ...
1
vote
1answer
262 views
Representations of gamma matrices
I have to do this exercise for homework. Find a representation of the gamma matrices unitarily connected to the standard representation for wich the spinors $u(p)$ that satisfy the equation $(p_\mu ...
1
vote
0answers
248 views
Understanding 1D Kinematics and Uniform Acceleration? [closed]
So I was given the following homework problem:
You land on an unknown planet somewhere in the universe that clearly has weaker gravity than Earth. To measure g on this planet you do the following ...
2
votes
3answers
972 views
Why is the central maximum the brightest?
This is a question directly from a homework sheet, however I can't find any decent answers online so hopefully someone can help!
In single slit diffraction, why is the central maximum double the ...
2
votes
1answer
391 views
Lean angle of a turning bicycle
I'm asked to derive a relationship for the leaning angle of a bicycle with the following specs:
Center of gravity for bike and rider is a distance $L$ above the ground when vertical, and the total ...
5
votes
1answer
412 views
Magnetic field from a half-cylinder [closed]
I am preparing for an exam, on this problem I had the opposite direction of the magnetic field.
A conductor-cylinder with radius R has been cut in half ($\phi \in [0,\pi]$)
A DC current $I$ runs ...
3
votes
0answers
115 views
Matter-wave interference from free falling cold atoms
and another exam question, this is about current research:
Interference of matter waves has been studied using ultra-cold atoms. The phase of a matter wave for free-falling cold-atoms at time $t$ ...
3
votes
2answers
303 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
280 views
Four vectors from spinors
In Exercise 2.3 of A modern introduction to Quantum Field Theory by Michele Maggiore I am asked to show that, if $\xi_R$ and $\psi_R$ are right-handed spinors, then
$$ V^\mu = \xi_R^\dagger \sigma^\mu ...
7
votes
2answers
840 views
Transformation of angles in special relativity
My question is about the problem below
Depicted are two space ships (the USS Voyager and the USS Enterprise), each with velocity $v=c/2$ relative to the space station (Babylon 5).
At the exact ...
3
votes
2answers
446 views
Complete vs General Integral of first order PDE
The following is an excerpt from Landau's Course on Theoretical Physics Vol.1 Mechanics:
... we should recall the fact that every first-order partial differential equation has a solution depending ...
3
votes
1answer
4k views
How do I solve for the phase constant given the amplitude and the angular frequency?
A piston (with mass M) in a car engine is in vertical simple harmonic
motion with amplitude A. The engine is running at a period T. Suppose
a small piece of metal with mass m were to break ...
4
votes
1answer
166 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 ...
-1
votes
1answer
124 views
Radiation dosage problem [closed]
A large room is filled with air polluted with Caesium-137 at an average density of 1 Cs-137 particle per $\text{cm}^3$. A human is sitting inside the room for duration of $T$. What is the total energy ...
11
votes
1answer
566 views
Mechanics + Thermodynamics: Bouncing Ball
In preparation for an exam, I'm revisiting old exam questions. This one seems neat, but also quite complicated:
A soccer ball with Radius $R=11cm$ is inflated at a pressure of $P =9 \times 10^4 ...
