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.
0
votes
0answers
8 views
physics (measurement) [closed]
The grains of California beach sand have an average radius of 50μ meter.What mass of sand grains would have a total surface area equal to the surface area of a cube exactly 1m on an edge?Sand is made ...
2
votes
0answers
20 views
What is the distance traveled
I have newly started to study mechanical physics. based on study, I conduct a simple experiment. But unfortunately i am unable apply the laws in reality.
Experiment:
I placed a coin (2 Indian Rupee ...
0
votes
0answers
32 views
How fast could a football drop? [closed]
Suppose we have a perfect spherical football with d=22.5cm and m=420g. The football has a initial vertical speed of v(around 20m/s, not sure if it will cause a turbulence) and zero angular speed. If ...
1
vote
2answers
44 views
Collision between a photon and an electron
Looking through this AP Physics question, I was struck by how the 'collision' between a photon and electron looks so much like a macroscopic collision. Is this even physically possible?
Look at the ...
-2
votes
0answers
28 views
Why is the expectation value of the Hamiltonian/position commutator equal to zero?
I know that the commutator $[H,x]$ is $-i \hbar \,p/m$, but how do I show that the modulus of the expectation value of this is zero?
2
votes
1answer
38 views
Time evolution of a quantum state
I have another point in QM that I would like clarified. Suppose $$\{|n\rangle\}$$ is a set of eigenstates of both the Hamiltonian $H$ and another operator $\hat O$ corresponding to an observable also. ...
0
votes
0answers
22 views
stable equilibrium in the system
I'm trying to understand the solution of the following problem.
A solid cube of uniform density and sides of b is in equilibrium on top of a cylinder of radius R. The planes of four sides of the ...
-3
votes
0answers
41 views
What are the concepts and explanation to these questions?
I'm looking through my old quiz, and I don't understand these concepts. Could someone explain the concepts to me so that I can answer these questions?
1) Consider a 3,000 Ohm resistor cmposed of a ...
3
votes
1answer
37 views
Why doesn't this equation for orbital motion change with position in the orbit?
The question and answer are on pg.8-10 of this PDF:
At first, I went through it, thinking nothing of it. But then, I wondered: "What if we picked a final state in which the space junk was NOT at ...
2
votes
1answer
21 views
What is the difference between material fatigue test with 1 sec cycle and 1 hour cycle?
Assume I have two pieces of metal. I want to test the material fatigue (e.g. how many cycles can the material stand before it will break). Onto one of them I apply a tension force every second. Onto ...
4
votes
2answers
52 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 $$X_{i}=\frac{\partial ...
0
votes
0answers
19 views
Specular intensity [closed]
Im currently studying for an exam and have been going through some past papers on the subject, however i have come across a question that has recursively come up each year and the notes on it are not ...
3
votes
3answers
42 views
Showing Hubble constant is time-independent
I have the following question for homework:
Show that the Hubble constant $H$ is time-independent in a universe in which the only contribution to energy density comes from vacuum energy.
So in ...
0
votes
1answer
28 views
Optical element fitting to Jones matrix
I need help with finding out which optical element is this.
I read about Jones matrix and vectors, but this is not one of the standards.
$$
\begin{pmatrix}
1 & i \\
-i & 1\end{pmatrix}
\cdot ...
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 ...
0
votes
0answers
22 views
electrostatic repulsion [closed]
Consider a homogeneous ring of radius $R$ made of a thin wire of cross section $A$ with $R^2≫A$. A charge $Q_1=10\mu C$, uniformly distributed over the ring, broke it apart due to electrostatic ...
-1
votes
1answer
37 views
A double-sided lever and multiple forces
Assume a double-sided lever where the left arm has a length of a and the right arm has a length of b + c. Please refer to this ...
0
votes
0answers
39 views
How does the volume change affect an adiabatic process?
A gas consisting of rigid diatomic molecules was initially under standard conditions. Then the gas was compressed adiabatically, 5 times the original volume. Find the mean KE of rotating molecules ...
0
votes
2answers
46 views
Dividing values with units
I'm reading about the subject of heat in a basic physics book. If I am not mistaken the formula to work out how much energy is required to increase the temperature of water is
...
0
votes
0answers
44 views
Gradient involved commutator in $\phi^4$ theory
In a phi fourth theory, the Hamiltonian density is:
$$\mathcal{H}=\frac{1}{2}\pi^2+\frac{1}{2}(\nabla \phi)^2+\frac{1}{2}m^2\phi^2+\frac{\lambda}{4!}\phi^4$$
Now I impose the usual equal time ...
-2
votes
0answers
19 views
Circuits using generators and cells and capacitors [closed]
This question was in a science worksheet we had, the description read as follows:
Tim has designed a generator that uses wind power and provides 9V. He decides to design a circuit using diodes ...
-2
votes
0answers
27 views
Finding Clebsh-Gordan coefficient [closed]
Two electrons , their angular quantum number =1 . Find the inner product of < 2 0 | 00 > ?
0
votes
1answer
23 views
Kinetic energy when $F(t)$ graph is given
The very basic problem here is that the point where this graph ends is ambiguous. I mean, what is the corresponding $y$-value for the point whose $x$-valu is $4.5s$? Then I think we should find the ...
0
votes
1answer
36 views
Relative Velocity problem
I was solving many problems on relative velocity and observed that when the question asks,
"what is the minimum time taken by the boat/man to cross the river.?"
so the solution says that he ...
0
votes
1answer
21 views
Doubt in problem regarding resistance a of cylinder
I've been working on some exercises and I'm in doubt if my procedure with this one is correct. We have a hollow cylinder with internal radius $r_a$, external radius $r_b$, resistivity $\rho$ and ...
1
vote
2answers
41 views
Electric potential energy after nuclear fission
We have a uranium-236 nucleus that fissions into two equal fragments, and I'm supposed to find the electrical potential energy just as the two fragments split apart. No other information is provided.
...
0
votes
2answers
45 views
Kinetic energy of two charged balls at infinite distance between them
If I have two balls with masses and charges $m_1, q_1^{+}$, $m_2, q_2^{+}$, initially held at distance $d$, and then released, how can I know the kinetic energies of each of the balls at infinite ...
1
vote
0answers
33 views
Solar sail area going to Proxima Centari [closed]
I have a physics question that I need some help with:
"Proxima Centauri is a star in the Alpha Centauri solar system, it’s the
nearest star to our sun (4.24light−years) ...
2
votes
1answer
40 views
Escape velocity for hydrogen molecules
The question:
At what temperature is the RMS speed of Hydrogen molecules equal to the escape speed from the earth's surface? Values of radius of earth($r$) and gas constant $R$ has been supplied ...
1
vote
0answers
27 views
Induced current using a reference system bound with a moving charge
Let's suppose to have a charge moving at velocity $\textbf{v}$ and a square wire.
If I choose a reference system bound with the square wire, since the charge is moving at velocity $\textbf{v}$, in ...
-3
votes
0answers
22 views
Length Traveled from Complete Stop [closed]
A one-meter long rifle fires a bullet at a velocity of 800 m/s. What is the average acceleration experienced by the bullet when it is fired? (Hint: You need to find the time it takes the bullet to ...
-1
votes
1answer
48 views
Convert units from cal cm^-1 day^-1. to Wm-2
I am trying to calculate the energy budget of a water body and the last term I'm trying to calculate is the energy advected from the water body ($Q_v$). This equation is given by:
$Q_v = V ( (T_1 - ...
0
votes
0answers
24 views
Existence of Boltzmann Distribution With Constraints [closed]
I have a problem with showing the existence of Boltzmann distribution given some constraints.
Consider $p_1,...,p_n$ a Boltzmann distibution, where $p_i=\frac{\epsilon^{-\beta \cdot E_i}}{\sum_{j}^{} ...
0
votes
0answers
17 views
Image formed by a spherical ball [closed]
There is an object on one end of a sphere and an image is formed on the other end. I am given the index of refraction,radius and the distance of the image from the center ball. How do I find the ...
0
votes
1answer
41 views
Pulley system with rope
Two masses $m_1$ and $m_2$ are connected through a weighty, uniform and an unstretchable rope of length $L$. The surface is frictionless. Masses of the objects are $m_1=m$, $m_2=\frac{2}{3}m$, mass ...
0
votes
2answers
41 views
Electric power transmission
If we want to transmit electic current for a long distance, we must minimize a heat that releases because of the resistanse. We cannot make a cable wide because it is expensive and it will be massive. ...
2
votes
2answers
65 views
Intuition contradicting answer for calculating pressure
I have a cylindrical container say of some base area $A$. The atmospheric pressure is $P_0$. The cylinder is moved at an acceleration $a$ horizontally right. I need to find which base would have to ...
-2
votes
0answers
29 views
The Hartree solution of two harmonic oscillator coupled by potential $V \propto ({\bf r}_1-{\bf r}_2)^2$ [closed]
$H={\bf p}_1^2+{\bf p}_2^2+{\bf r}_1^2+{\bf r}_2^2+x({\bf r}_1-{\bf r}_2)^2$.
$x$ is the coupling factor.
-2
votes
1answer
34 views
Resistance of a copper wire [closed]
Resistance of a copper wire is 20ohm. If the diameter of the wire is 5mm and length 1ohm. Find the resistivity of the copper.
0
votes
1answer
70 views
Derivation of Dirac equation using the Lagrangian density for Dirac field
How can I find Dirac equation using the Lagrangian density for Dirac field?
0
votes
1answer
59 views
Periodic boundary condition on a Wave Function of a Particle in a Box
Until now solving the Schrodinger Equation for a particle in a box was relatively easy because the boundaries conditions imposed zero value on the wave function at the boundaries. But now I must find ...
0
votes
0answers
21 views
The stress in copper and steel parallel compound members [closed]
Came across this question and just needed abit of help undertanding how i should go about this question
I have also attempted to solve it to the best of my ability but im stuck.
A concrete column of ...
0
votes
0answers
28 views
Maxwell-Boltzmann distribution for transport equations
I have to calculate the transport coefficients for the Maxwell-Boltzmann distribution. But I'm not sure what distribution I have to use.
As far as I know it should not be the MB distribution for ...
1
vote
1answer
54 views
How do determine the equation of motion of slinky?
I'm trying to analyze the forces exerted to slinky. First, I've divided both ends.
The red vector is the force exerted by slinky itself, and the green vector is gravitational force.
Force exerted ...
1
vote
1answer
37 views
Problem with a rotating frame of reference on the South pole
Consider this problem:
A high-speed train is traveling at a constant 150 m/s (about 300 mph)
on a straight horizontal track across the south pole. Find the angle
between a plumb line suspended ...
1
vote
1answer
38 views
Finding Hamilton's equations given a Hamiltonian
I am trying to find Hamilton's equations for a general Hamiltonian given by $$H[u]=\int_\mathbf{R} \phi(u,u_x)dx$$
Suppose $$\frac{\delta f[u]}{\delta u(x)}\equiv \frac{\partial f}{\partial ...
2
votes
2answers
83 views
Force Between Two Conducting Spheres
Aakash PHYSICS JEE (Main & Advanced) Study Package - 5 & 6 (Class XII)
Chapter - Electric Charges and Field
Assignment (page 12)
SECTION - A; Q.no - 1
...
1
vote
1answer
61 views
Friction on a conveyor belt
I was surfing through an Olympiad paper and I was caught with this question.
A block of mass 1 kg is stationary with respect to a conveyor belt that is accelerating with $1\, \tfrac{m}{s^2}$ ...
1
vote
1answer
29 views
Determine the velocity and acceleration of the vertex $B$
1) The bent rod $ABCD$ rotates about the line $AD$ whit a constant angular velocity of $90 rad / s$. Determine the velocity and acceleration of the vertex $B$ when the rod is in the position shown in ...
3
votes
0answers
59 views
Effective mass in Spring-with-mass/mass system
Suppose you have a particle of mass $m$ fixed to a spring of mass $m_0$ that, in turn, is fixed to some wall. I'm trying to calculate the effective mass $m'$ that appears in the law of motion of the ...
