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.
1
vote
2answers
94 views
Ricci tensor for a 3-sphere without Math packets
Let's have the metric for a 3-sphere:
$$
dl^{2} = R^{2}\left(d\psi ^{2} + sin^{2}(\psi )(d \theta ^{2} + sin^{2}(\theta ) d \varphi^{2})\right).
$$
I tried to calculate Riemann or Ricci tensor's ...
0
votes
3answers
37 views
Interaction photons-matter and dimensional analysis
I know that when photons pass through matter, the law that describes the intensity in function of the thickness is:
$$I(x)=I_0 e^{-\mu x}$$
where $\mu = \rho \frac{N_a}{A} \sigma$ and ...
-1
votes
1answer
29 views
Terminal velocity and force pull [closed]
I can't figure out this problem .
Buoyancy force and gravity remain constant and viscous force by is $-kv$. And these forces all balance, but data isn't given according to that, or I am not able to ...
-1
votes
1answer
24 views
Charge gained due to photoelectric effect [closed]
Here I think, one beam will knock out just one electron. So, I am not able to even understand what the question says. Please someone give a hint as to what the question asks...
As source of the ...
0
votes
0answers
22 views
Electrostats and Photoelectric effect [closed]
Here I think ,
one beam will knock out just one electron. So I am not able to even understand what the question says , please someone give a hint as to what the question asks
-1
votes
0answers
23 views
Finding rms velocity in isothermal process [closed]
I think none of these options are correct, I just need someone to confirm. Please, as no option is matching with $V_{rms}$.
0
votes
0answers
37 views
Finding the magnetic dipole moment [closed]
I have been stuck on this question for awhile and it should be really easy.
The relative permeability of cobalt is $u_r = 600$ and the density of atoms is $N_e = 9.02*10^{18} [m^{-3}]$, calculate the ...
0
votes
0answers
39 views
Magnetic field of a Ferromagnetic cylinder [closed]
I have been stuck on this problem for hours and have no idea how to go about this problem.
You have an infinitely long cylinder made of linear ferromagnetic material of relative permeability Km, ...
3
votes
1answer
62 views
Tension of rope in the gravitational field of earth
Two balls of mass $m$ each one are connected with mass-less rope with
the same length as the radius of earth. The system is in free fall.
Prove that the tension of the rope when the nearest (to ...
0
votes
1answer
60 views
Two qubits problem [closed]
Given the 2 qubit state:
(a/b) |00> + (c/b) |01> + (c/b) |10> + (d/b) |11>
What is the probability that 2 qubits are equal?
Thanks much!
-1
votes
1answer
87 views
Acceleration, velocity and speed
It is given that acceleration is constant, so can we infer that average speed and velocity are the same?? Moreover, circular motion is out of the question, as the function of x(t) where ...
1
vote
1answer
68 views
Volume of gas at which relative fluctuation of gas density occurs
I have the following question:
In what volume of gas occurs 10 % relative fluctuation of gas density
under pressure of $10^5\text{ Pa}$ and temperature of $293.15\text{ K}$?
I don't understand ...
0
votes
1answer
78 views
Damped oscilator - logarithmic decrement of damping
Could you please tell me, where is the mistake?
What is the logarithmic decrement of damping $Λ$ of damped harmonic oscillator, if its mechanical energy decreases to the 50% of its initial value ...
1
vote
0answers
106 views
How to calculate Riemann and Ricci tensors for a sphere? [closed]
Let's have the metric for a sphere:
$$
dl^{2} = R^{2}\left(d\psi ^{2} + sin^{2}(\psi )(d \theta ^{2} + sin^{2}(\theta ) d \varphi^{2})\right).
$$
I tried to calculate Riemann or Ricci tensor's ...
0
votes
0answers
32 views
2 Masses attached to the roof by strings [closed]
There are two masses $M>>m$. $M$ is attached from a fixed surface by a string and ;m'mass is attached to $M$ by a string. Lengths L1, L2, c/s area A1, A2 of cords are given.I have to find ...
0
votes
1answer
82 views
A Type of Pendulum
Is there any chance that $$rtl(\ddot\omega+\ddot\phi)\cdot\sin{(\phi+\omega t)}- gl\dot\phi \cdot \sin{\phi} + ltr\dot\omega(\dot\phi^2-\dot\omega)\cdot \cos{(\phi+\omega t)}-gtr\cos{(\omega ...
-2
votes
0answers
40 views
Magnetic field due to a rectangular wire [closed]
I have a infinitely long rectangular wire with width = 6cm, height = 0.1cm. A point P is 27 cm away from the center of the wire. I wanna know the magnitude of the magnetic field at this point. The ...
0
votes
0answers
62 views
What does this notation mean in terms of quantic numbers, and how to imagine the electrons in this quantic system? (Helium $2^1$ $P$ and $2^3$ $P$)
Helium atom in the $2^1$ $P$ and $2^3$ $P$ excited states
Now I'm guessing that 1 electron should be considered in the 1s state, but what about the other?
Should I consider the other as simply ...
0
votes
1answer
42 views
Contraction of the metric tensor
This is perhaps a simple tensor calculus problem -- but I just can't see why...
I have notes (in GR) that contains a proof of the statement
In space of constant sectional curvature, $K$ is ...
9
votes
4answers
370 views
Trace and adjoint representation of $SU(N)$
In the adjoint representation of $SU(N)$, the generators $t^a_G$ are chosen as
$$ (t^a_G)_{bc}=-if^{abc} $$
The following identity can be found in Taizo Muta's book "Foundations of Quantum ...
1
vote
1answer
147 views
Projectile along incline [closed]
A particle is projected with velocity $u$ from the bottom of an inclined plane whose inclination with the horizontal is $\beta$. If afterwards the projectile strikes the inclined plane perpendicular ...
1
vote
2answers
79 views
The potential and the intensity of the gravitational field in the axis of a circular plate
Calculate the potential and the intensity of the gravitational field at a distance $x> 0$ in the axis of thin homogeneous circular plate of radius $a$ and mass $M$.
Could anybody describe how to ...
1
vote
1answer
37 views
Finding the coffecient of restitution
A ball moving with velocity $1 \hat i \ ms^{-1}$ and collides with a friction less wall, afetr collision the velocity of ball becomes $1/2 \hat j \ ms^{-1}$. Find the coefficient of restitution ...
3
votes
1answer
139 views
Schrödinger equation for a harmonic oscillator
I have came across this equation for quantum harmonic oscillator
$$
W \psi = - \frac{\hbar^2}{2m}\frac{d^2\psi}{dx^2} + \frac{1}{2} m \omega^2 x^2 \psi
$$
which is often remodelled by defining a new ...
0
votes
2answers
50 views
Impedance of a circuit
I am trying the following problem. A resistor with $295\Omega$ and an inductor are connected in series across an AC source that has voltage amplitude of $550V$. The rate at which electrical energy is ...
0
votes
0answers
36 views
What is the total work done in this problem? [closed]
A 1800 kg trick airplane is 450 m in the air. At this point the plane takes a dive
with an initial speed of
42 m/s and accelerates to 64 m/s, dropping
a total distance of 120 m.
(a) Using the ground ...
1
vote
0answers
54 views
Stress-energy tensor of point particle when the trajectory is a transcendental equation?
I'm working through Carroll's GR book, and Problem 7.8 is not coming together. I'm missing something idiotically simple, but I'm not sure if I can cleanly write a stress-energy tensor for a point ...
0
votes
0answers
39 views
Maximum Height of Potato [closed]
I have a bit of a problem. We made potato launchers and launched the potato's into the air. I need to find out what the maximum height my potato reached was. The givens I have are the following:
Mass ...
0
votes
1answer
27 views
Forces and angles
"The little ball with the mass of 100g has gotten stuck in a chute as depicted in the picture. What forces, and how large are they, that are acting on the ball?"
This is how I solve it:
I find ...
0
votes
0answers
30 views
Understanding Kepler's $2^{nd}$ law in terms of angular momentum conservation
A) Explain how Kepler's $2^{nd}$ law - "The radius vector from the Sun to a planet sweeps out equal areas in equal time intervals" - can be understood in terms of angular momentum conservation.
I ...
0
votes
0answers
30 views
Energy content after moving point charge [closed]
I have the following exercise:
In point A there is a charge with $Q_1 = 3.3\times10^{-5} C$
In point B charge $Q_2 = 8.5\times10^{-6} C$
In point C charge $Q_3 = -6\times10^{-6} C$
This happens ...
0
votes
1answer
58 views
Finding the maximum kinetic energy of any photoelectrons?
An incident photon, $f=5.5\times 10^{14}\ Hz$, hits a metal with a work function of $2.8\ eV$.
Find the maximum kinetic energy of any photo-electrons (in Joules).
I'm confused exactly how to do ...
1
vote
1answer
35 views
Color of a Metal's Threshold Wavelength?
How do I find the color of the threshold wavelength if the metal has a threshold wavelength of $\mathrm{6.5\times 10^{-7}m}$? I know that converts down to $\mathrm{650\ nm}$, but can I still use the ...
1
vote
1answer
27 views
Color of an incident photon?
If the incident light at 360nm causes photoemission of electrons, wouldn't the color be ultraviolet? I know that really isn't a color, but that's what my chart of the light spectrum says.
Unless I ...
0
votes
1answer
36 views
Confused about the Photoelectric Effect?
If an incident photon with frequency f hits a metal with a work force of w:
How do I find the color of the incident photon and the threshold frequency of the metal?
I missed out on the days in my ...
0
votes
2answers
77 views
How to calculate time for ball to reach plane while considering mass, gravity and wind resistance?
The scenario that I'm having is such that a ball of radius $15mm$ is thrown from a location point $\vec{p}=(2, 5, 2)$ in a direction of $\vec{d}=(3, 0, 4)$. The initial velocity is $30m/s$. There were ...
0
votes
1answer
32 views
Violation of the Normalization Constraint?
Say we have two qubits $|a\rangle$ and $|b\rangle$ both initialized to $|0\rangle$. We then apply the rotation gate $R_{x}(\frac{\pi}{2})$ of matrix representation
$\left( \begin{array}{}
...
1
vote
1answer
43 views
Induced voltage by a rotating ring
Two concentric rings dielectrics and uniformly charged are suspended on the same floor.
The outer ring has a radius R and mass M,
while the other has radius r << R and mass M.
The outer ring ...
0
votes
3answers
62 views
Voltage of open circuit
A battery with emf $\varepsilon$ and internal resistance $r$ is connected
with a resistor $R$ in the following open circuit. What is the voltage $V_{ab}=V_a-V_b$?
The answer is $- ...
0
votes
2answers
69 views
Inserting metal into parallel plate capacitor
The plates of an isolated parallel plate capacitor with a
capacitance C carry a charge Q. The plate separation is d.
Initially, the space between the plates contains only air.
Then, an ...
0
votes
2answers
145 views
Perpendicular Elastic Collision (different masses, different velocities)
I'm stuck on a mechanics problem and I can't make any headway past momentum and kinetic energy being conserved. Here is the problem:
Two hover cars are approaching an intersection from ...
0
votes
0answers
28 views
Can a magnet spin a small machine?
If you were to build an upright machine that had a rod fitted into a ball bearing at the base and was secured to another rod at the top of the initial rod extending perpendicularly. To which you ...
3
votes
2answers
117 views
Quantum commutator
I'm given this commutator:
$$\left[PXP,P\right]$$
Being $P\psi=-i\hbar\partial_x\psi$, and $X\psi=x\psi$
I've solved it in two ways, the first one is just aplying the commutator to some function ...
0
votes
1answer
36 views
Zero-zero (lower indicies) term for affine connection ($\Gamma_{00}^\lambda$), why do some terms dissapear?
More simply a tensor algebra question, but in General relativity I have the following when I calculate $\Gamma_{00}^\lambda$:-
$$
\Gamma_{00}^\lambda = \frac{1}{2}g^{\nu\lambda}\left(
\frac{\partial ...
2
votes
0answers
60 views
Wald problem 11.4
Consider a stationary solution with stress-energy $T_{ab}$ in the context of linearized gravity. Choose a global inertial coordinate system for the flat metric $\eta_{ab}$ so that the "time direction" ...
0
votes
0answers
23 views
What are some applications of crystal fabrication? [closed]
I have heard of some applications here or there in certain papers, but I am looking for a broader scope of examples.
0
votes
1answer
37 views
Pendulum system: how is derived the output as Energy?
Good day to everyone,
I want to understand in which way the "Energy equation" is been implemented to this pendulum system.
$x_1(t)$: The angular position of the mass
$x_2(t)$: The angular velocity
...
0
votes
0answers
27 views
Find Resonance Frequencies [closed]
How can I find the resonance frequencies for the harmonic dumped oscillator when it is written in this form?
$$y''\left(t\right)+2\zeta y'\left(t\right)+y\left(t\right)=\sin{(\omega t+\phi)}$$
where ...
1
vote
1answer
51 views
Maximum separation when 2 unlike charges shot apart
I want to compute the maximum separation when 2 unlike charges are shot apart from each other
...
0
votes
2answers
52 views
Relativity Problem Given Rest Length and Proper Time
A relativistic train with a rest length of 500.0 m takes 780 ns to pass a stationary observer
standing on the train platform, as measured by the stationary observer.
(a) What is the speed ...
