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
1answer
35 views
Diagonal matrix in k-space
I'm having some trouble with an integration I hope you guys can help me with.
I have that:
${{\mathbf{v}}_{i}}\left( \mathbf{k} \right)=\frac{\hbar {{\mathbf{k}}_{i}}}{m}$
and ...
0
votes
2answers
30 views
Voltmeter forming a closed circuit
A battery is connected to a 10Ω resistor as shown in Figure 2. The emf (electromotive force) of the battery is 6.0 V.
When the switch is open the voltmeter reads 6.0 V and when it is closed it ...
0
votes
0answers
44 views
Air pressure in balloon
I have to calculate the air pressure inside of an hot air balloon. After some searching I found out that I can use the ideal gas law: PV = nRT (from Wikipedia)
So to get the pressure in the balloon I ...
-1
votes
1answer
65 views
Proof $\left[ {\hat H,{{\hat p}_i}} \right] = - \frac{\hbar }{i}\frac{{\partial \hat H}}{{\partial {{\hat q}_i}}}$ [closed]
I have a problem with the Hamiltonian, I don't think anything to solve it!!
So could you give me some hints!
Knowing that:
$$\left[ {{{\hat p}_i},{{\hat q}_k}} \right] = \frac{\hbar }{i}{\delta ...
-4
votes
1answer
41 views
Center of mass of three particles of masses 1kg, 2kg, 3kg lies at the point (1,2,3) [closed]
Center of mass of three particles of masses 1kg, 2kg, 3kg lies at the point (1,2,3) and center of mass of another system of particles 3kg and 2kg lies at the point (-1,3,-2).
Where should we put a ...
0
votes
0answers
56 views
Relativistic canonical transformation
What is relativistic canonical transformation? I need every piece of information about it.
Does anyone know a reference or an article about relativistic canonical transformation?
For example, in ...
1
vote
1answer
41 views
How large of a solar sail would be needed to travel to mars in under a year?
I'm attempting to approach this using the identity
$$F/A = I/c$$
I can solve for Area easily enough
$$A = F(c/I)$$
and I know the distance $d$ is
$$d=1/2(at^2)$$
But I'm having difficulty trying to ...
-1
votes
1answer
202 views
Potential due to a spherical surface charge
The potential at the surface of an insulating sphere (radius R) is given by
$$V(R,\theta) = k \cos(3\theta)$$
where $k$ is a constant. Use separation of variables to find the potential inside the ...
0
votes
1answer
74 views
Simple harmonic oscillator system and changes in its total energy
Suppose I have a body of mass $M$ connected to a spring (which is connected to a vertical wall) with a stiffness coefficient of $k$ on some frictionless surface. The body oscillates from point $C$ to ...
1
vote
1answer
77 views
Killing vector argument gone awry?
What has gone wrong with this argument?!
The original question
A space-time such that $$ds^2=-dt^2+t^2dx^2$$
has Killing vectors $(0,1),(-\exp(x),\frac{\exp(x)}{t}), ...
0
votes
1answer
198 views
Electric field due to nonconducting sphere
For calculating electric field outside a nonconducting sphere with a hollow spherical cavity. When I use the rule (Charge density= $dQ/dV$), I don't know exactly what is $dV$, is the volume here ...
1
vote
1answer
209 views
What is the general relativistic calculation of travel time to Proxima Centauri?
It has already been asked here how fast a probe would have to travel to reach Alpha Centauri within 60 years. NASA has done some research into a probe that would take 100 years to make the trip. But ...
3
votes
2answers
108 views
Geodesic equations
I am having trouble understanding how the following statement (taken from some old notes) is true:
For a 2 dimensional space such that $$ds^2=\frac{1}{u^2}(-du^2+dv^2)$$
the timelike geodesics ...
1
vote
0answers
49 views
A slender rod with a ball at the end
A slender rod is attached to a block accelerating horizontally. The rod is free to rotate without friction. At the end of the rod is a ball.
As the block accelerates, the slender rod will be deviated ...
4
votes
1answer
96 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 ...
3
votes
2answers
572 views
Electric field and electric potential of a point charge in 2D and 1D
in 3D, electric field of a piont charge is inversely proportional to the square of distance while the potential is inversely proportional to distance. We can derive it from Coulomb's law.
however, I ...
0
votes
0answers
35 views
Magnetic Field Lines predict? [closed]
Question: Magnetic field lines determine:
(A) only the direction of the field
(B) the relative strength of the field
(C) both the relative strength and the detection of the field
(D) only the ...
0
votes
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 ...
0
votes
1answer
53 views
The second resonance of string?
What is the relationship between "the second resonance " and string and the wavelength. Like in this question: if the length of the string is 2cm with second resonance, then what is wavelength?
1
vote
2answers
102 views
Potential Inside Conducting Cube
A cubical box with sides of length L consists of six metal plates. Five sides of the box { the plates at $x=0, x=L, y=0, y=L, z=0$ - are grounded. The top of the box (at z = L) is made
of a separate ...
0
votes
1answer
65 views
How is torque equal to moment of inertia times angular acceleration divided by g?
How is the following relation true
$$\tau = \large\frac{I}{g} \times \alpha$$
where $\tau$ is torque,
$I$ is moment of inertia,
$g= 9.8ms^{-2}$,
and $\alpha=$ angular acceleration.
0
votes
0answers
46 views
What force does the seat exert on the rider at the top and bottom of the ride? [closed]
A 75 kg person rides a Ferris wheel which is rotating uniformly. The centripetal force acting on the person is 45 N. What force does the seat exert on the rider at the top and the bottom of the ride?
2
votes
3answers
71 views
How to determine the direction of medium's displacement vectors of a standing wave?
Consider the following problem taken from a problem booklet. My questions are:
What is displacement vector?
And how to determine the direction of displacement vector at a certain point?
Where is the ...
2
votes
1answer
82 views
Liquid benzene magnetic susceptibility
In a solid state physics problem, I'm asked to make a rough estimate of the contribution to the diamagnetic susceptibility of the outer electron of each carbon atom. The wavefunction of these ...
6
votes
2answers
144 views
Why does the quantum Heisenberg model become the classical one when $S\to\infty$?
The Hamiltonian of the spin $S$ quantum Heisenberg model is
$$H = J\sum_{<i,j>}\mathbf{S}_{i}\cdot\mathbf{S}_{j}$$
I have read that when the spin quantum number $S\to\infty$, quantum fluctuation ...
1
vote
1answer
86 views
How can the derivative of this trace be constrained?
I am studying for my exam on relativity and I am going through some problems sets including ones where I was not very successful in so I want to know how to do this problem.
(Convergence of ...
4
votes
3answers
948 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 ...
-3
votes
1answer
26 views
How to find time taken for a spinning top to stop? [closed]
The angular position of a spinning top is given by $\theta = t^3 - 72t$, where $t$ is in seconds
and $\theta$ in "radian".
-1
votes
1answer
44 views
A theoretical problem on Mechanics [closed]
Two particles with masses $m_1$ and $m_2$ are moving in 3D space with some Cartesian coordinate system. There are known the laws of motion of these particles, i.e. the position vectors $\vec{r_1}(t)$ ...
1
vote
3answers
111 views
Projectiles and escape velocity
Q: The escape velocity for a body projected vertically upwards from the surface of earth is 11 km/s. If the body is projected at an angle of $45^\circ$ with vertical, the escape velocity will be?
...
0
votes
0answers
26 views
Assume the following vectors [closed]
Assume the following vectors:
$a = \frac{1}{\sqrt{2}} [0,1,1]$ and
$b = [1,2,3]$.
(a) Calculate the scalar product c = a c b
(b) Calculate the value of vector d where d = b - ca
(c) What is the ...
1
vote
1answer
46 views
Forces on a particle moving in a vertical circle
In the diagrma, a particle (A, mass 0.6kg) is moving in a vertical circle. The question is: When it gets to the lowest point, what is the tension in the light rod that is between the center of the ...
-2
votes
0answers
35 views
Physical Optics [closed]
Monochromatic light is used to illuminate a pair of narrow slits 0.3 mm apart and the interference pattern is observed on a source 0.91 mm away. The second dark band appears 3.0 mm from the center. ...
0
votes
0answers
23 views
Absolute Viscosity of Water at certain temperatures
I just started my class on fluid mechanics. There's a problem that requires absolute viscosity $u$ of water at 25C. I looked up my table in the book and I only have it for water at 20C and 30C. I ...
3
votes
4answers
2k views
Calculating impact force for a falling object?
Good evening, I'm trying to calculate what kind of impact force a falling object would have once it hit something. This is my attempt so far:
Because $x= \frac{1}{2} at^2$, $t=\sqrt{2x/a}$
$v=at$, ...
-3
votes
2answers
105 views
Why is $r'/r^2 = -1/r$? [closed]
If $r=r(t)$, why is $\frac{r'(t)}{(r(t))^2}$ = $\frac{1}{r(t)}$ where $'$ denotes the derivative? I saw it in a lecture.
Can you please explain?
2
votes
1answer
287 views
Simple work and energy problem
I have the following problem:
A man that weights 50kg goes up running the stairs of a tower in Chicago that is 443m tall. What is the power measured in watts if he arrives at the top of the tower ...
0
votes
0answers
45 views
Work And Energy Question [closed]
$H = 3\text{ m}$,$m=2\text{ kg}$
The right side is rough.
I want to figure:
what is the coefficient of friction $\mu$?
How high and exceed the maximum return on the plane right body?
I know ...
1
vote
1answer
81 views
When should angles be expressed in degrees vs. radians?
I am trying to calculate the albedo of a given latitude by following the methods of Brutsaert (1982), I have copied the formula below:
3.6 Shortwave and long-wave radiative fluxes
Albedo ...
0
votes
2answers
48 views
Constant of gravity in earth fixed coordinate system
I have this problem:
If the constant of gravity is measured to be $g_0$ in an earth fixed
coordinate system, what is the difference $g-g_0$ where $g$ is the
real constant of gravity as ...
1
vote
0answers
49 views
Calculating pressure in accelerated fluids in closed and open vessels?
The question asked was "what should be the acceleration such that the pressure at both the points marked by thick dots be equal? the vessel is open and cubic with side 5m?"
Initially i considered ...
1
vote
1answer
90 views
Joule-Thomson effect of Van der Waals gas
I'm supposed to calculate the inversion pressure $p_i$ of a Van der Waals gas. The state equation of the Van der Waals gas is: $$(p + \frac{a}{V^2})(V-b) = RT.$$ To get a hold of the inversion ...
0
votes
2answers
51 views
Electron in an infinite potential well
Does this problem have any sense?
Suppose an electron in an infinite well of length $0.5nm$. The state of the system is the superposition of the ground state and the first excited state. Find the ...
0
votes
0answers
30 views
Lagrangian of electromagnetic tensor in light cone coordinates? [closed]
I have Lagrangian Density of Electromagnetic field Tensor in light cone coordinates using D'Alembertian operator and Lagrangian density in Cartesian coordinates. I couldn't figure out the way to ...
1
vote
0answers
28 views
Mercury's Orbital Precession in Special Relativity
I am researching Mercury's orbital precession. I have considered most perturbations and general relativity. I am still not satisfied. I need your help.
I need a solution to Exercise 13, Chapter 6, in ...
-2
votes
4answers
133 views
Resistors in Parallel
From my book:
"A length of wire is cut into five equal pieces. The five pieces are then connected in parallel, with the resulting resistance being 2.00 Ω. What was the resistance of the ...
0
votes
2answers
54 views
Angle between acceleration and velocity
Problem:
A particle is constrained to move in a circle with a 10-meter radius. At one instant, the particle's speed is 10 meters per second and is increasing at a rate of 10 meters per second ...
0
votes
2answers
66 views
Is it possible to “add cold” or to “add heat” to systems?
Amanda just poured herself a cup of hot coffee to get her day started.
She took her first sip and nearly burned her tongue. Since she didn't
have much time to sit and wait for it to cool down, ...
9
votes
4answers
291 views
Trace and adjoint representation of $SU(N)$
In the adjoint representation of $SU(N)$, the generators $t^a_G$ are chosen as
$$ (t^a_G)_{bc}=-if^{abc} $$
The following identity can be found in Taizo Muta's book "Foundations of Quantum ...
0
votes
2answers
43 views
Simple conservation of momentum and frame of reference problem
I'm making a very simple physics engine based on momentum, and I'm solving what response to use for a collision from each involved object's frame of reference. However, something about how I'm ...
