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
3answers
569 views
Unit conversion help (school)
Can someone please help me explaining how to convert units in problems like the one below? I've done many things, but I am very unsure of all my methods. Could you tell me what steps should I take?
...
3
votes
1answer
708 views
How do I construct the $SU(2)$ representation of the Lorentz Group using $SU(2)\times SU(2)\sim SO(3,1)$ ?
This question is based on problem II.3.1 in Anthony Zee's book Quantum Field Theory in a Nutshell (I'm reading this for fun- it isn't a homework problem.)
Show, by explicit calculation, that ...
8
votes
9answers
5k views
What's the core difference between the electric and magnetic forces?
I require only a simple answer. One sentence is enough... (It's for high school physics)
1
vote
1answer
315 views
Schrödinger function: Separable wave function with even potential function of x
I have done the Problem 2.1 in Griffiths' quantum mechanics,
and it seems not making sense to me.
What if the wave function isn't symmetric at all?
Then obviously the proof doesn't work. The ...
0
votes
2answers
458 views
What will be the relative speed of the fly? [duplicate]
It has happened many times and i have ignored it everytime.
Yesterday it happened again .
I was travelling in a train and saw a fly (insect) flying near my seat.
Train was running at a speed of ...
4
votes
2answers
820 views
Make a semi transparent mirror with copper
The question:
How would you make a semi transparent mirror (50% reflection, 50% transmission) with glass with a layer of copper. For light $\lambda$ = 500nm Try to be as realistic as possible
What ...
2
votes
1answer
157 views
Find the Hamiltonian given $\dot p$ and $\dot q$
I have these equations:
$$\dot p=ap+bq,$$
$$\dot q=cp+dq,$$
and I have to find the conditions such as the equations are canonical. Then, I have to find the Hamiltonian $H$.
To answer to the first ...
2
votes
1answer
416 views
Force on rope with accelerating mass on pulley
I have a pretty basic pulley problem where I lack the right start.
A child sits on a seat which is held by a rope going to a cable roll (attached to a tree) and back into the kid's hands.
When it ...
1
vote
3answers
74 views
Capacitor Charging and Discharging when connected to the ground
When we charge a capacitor using a battery and then remove the battery, the plates of capacitor becomes charged. One holds positive charge and the other one gets equal negative charge. o. k. ?
Now ...
1
vote
1answer
290 views
Finding the electric field for a shell of charge
Suppose we have charge density defined
$$
\rho(x,y,z) =
\begin{cases}
0 & 0 \leq r < a \\
K & a \leq r\leq b\\
0 & b< r
\end{cases}
$$
For some constants $K,a,b$
How would ...
1
vote
2answers
218 views
Doubt concerning centripetal acceleration
What is the centripetal acceleration and angular velocity of a child located 8.2 m
the center of a carousel? The speed (size of the tangential velocity) of the child is
2.1 m / s
A train moves in a ...
-1
votes
1answer
227 views
What's the electric field with a point charge not in the center of the sphere? [closed]
That's a hollow conducting sphere link
My charge is at P (10uC). R = 0.15m. PS = 0.05m. DS = 0.35m. What's the electric field at point D?
4
votes
5answers
356 views
The approximate uncertainty in $r$
The volume of a cylinder is given by the expression
$V=\pi r^2 h$
The uncertainties for $V$ and $h$ are as shown below
$\begin{align}
V&\pm 7\%\\
h&\pm 3\%
\end{align}$
What is the ...
3
votes
1answer
156 views
Wick Order and Radial Ordering in CFT
I am not so much familiar with the computations tools of conformal field theory, and I just run into an exercise asking to demonstrate the following formula (related to the bosonic field case):
...
2
votes
1answer
106 views
Two masses attached to a spring
I'm trying to understand the solution of the following problem.
Two masses $m_{1}$ and $m_{2}$ slide freely in a horizontal frictionless track and are connected by a spring whose force constant is ...
2
votes
2answers
378 views
Homework about spinning top
I have a top of unknown mass that has a moment of inertia $I=4\times 10^{-7} kg \cdot m^2$. A string is wrapped around the top and pulls it so that its tension is kept at 5.57 N for a distance of .8 ...
1
vote
2answers
385 views
Why does the light bulb's brightness decrease?
The book said that the brightness of R1 would increase and R2 would decrease. I don't understand this at all.
Why does adding a wire from b to c change anything to R1?
Shouldn't charge still flow ...
1
vote
1answer
690 views
Trajectory of projectile thrown downhill
I'm teaching myself mechanics, and set out to solve a problem determining the optimum angle to throw a projectile when standing on a hill, for maximum range. My answer seems almost plausible, except ...
1
vote
1answer
83 views
How to find the value of the parameter $a$ in this transfer function?
I am given a transfer function of a second-order system as:
$$G(s)=\frac{a}{s^{2}+4s+a}$$
and I need to find the value of the parameter $a$ that will make the damping coefficient $\zeta=.7$. I am not ...
1
vote
2answers
587 views
What is a Free Body Diagram?
I have this problem:
A 10 kg box is resting on a frictionless table and someone drags it by a string at a 30 degree angle to the horizontal with a force of 40 Newtons. Find the acceleration of the ...
1
vote
3answers
553 views
Maxiumum weight a buoy can float [closed]
I have a buoy that can hold 2943.02 cubic millimeters of non-pressurized air, I need and equation to work out the maximum weight that the buoy can float.
0
votes
0answers
47 views
Trigonometry in the plane mirror [closed]
I was trying to solve a problem taken from an Physics Olympiad when I came across a curious and complex mathematical expression. I can not prove with what I know so far about mathematics, does could ...
0
votes
1answer
238 views
Mass points of a Mass-spring model
Let's say I have a mass spring model like the one in the picture below:
So, there are 3 parts of the spring joined together in an equilateral triangular manner. Each of the joints has a mass of ...
0
votes
1answer
318 views
Elastic Collision And Momentum
The question I am working on is, "Two blocks are free to slide along the friction-less wooden track shown below. The block of mass $m_1 = 4.98~kg$ is released from the position shown, at height $h = ...
0
votes
2answers
232 views
Force applied off center on an object
Assume there is a rigid body in deep space with mass $m$ and moment of inertia $I$. A force that varies with time, $F(t)$, is applied to the body off-center at a distance $r$ from its center of mass. ...
0
votes
1answer
326 views
Calculating torque in a structure
I posted this on math stack exchange but realize it is more a physics question.
I have a structure which is set up as shown in the image. A weight hangs from point A with mass $m$. Joint B is free ...
0
votes
3answers
799 views
Why does salty water heat up quicker than pure water? [duplicate]
Possible Duplicate:
Why does adding solutes to pure water lower the the specific heat?
Why do higher concentration salt solutions heat up more quickly?
0
votes
1answer
114 views
Question about interaction between three objects
I have been thinking of Newton (insert bad words because of frustration) laws for hours and hours today, and I'm not going anywhere. I'm going insane! So, I made up this question.
Object A | ...
-1
votes
1answer
1k views
What affects the damping of a spring?
What variables affect the damping of a spring executing simple harmonic motion?
What are the independent variables, and what variables would need to be controlled in an experiment?
I'm attempting to ...
-4
votes
1answer
181 views
Tricky Conservation of Momentum problem: find the ratio of the carts by mass percentage lost [closed]
A wagon is coasting at a speed $v_A$ along a straight and level road. When 42.5% of the wagon's mass is thrown off the wagon, parallel to the ground and in the forward direction, the wagon is brought ...
6
votes
4answers
120 views
Integrating radial free fall in Newtonian gravity
I thought this would be a simple question, but I'm having trouble figuring it out. Not a homework assignment btw. I am a physics student and am just genuinely interested in physics problems involving ...
4
votes
1answer
733 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 ...
1
vote
0answers
72 views
Showing that the Ricci scalar equals a product of commutators
I have to compute the square of the Dirac operator, $D=\gamma^a e^\mu_a D_\mu$ , in curved space time ($D_\mu\Psi=\partial_\mu \Psi + A_\mu ^{ab}\Sigma_{ab}$ is the covariant derivative of the spinor ...
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 ...
6
votes
2answers
140 views
Radioactive Decay
Problem:Nuclei of a radioactive element $\Bbb X$ having decay constant $\lambda$ , ( decays into another stable nuclei $\Bbb Y$ ) is being produced by some external process at a constant rate ...
5
votes
2answers
284 views
relativistic acceleration equation
A Starship is going to accelerate from 0 to some final four-velocity, but it cannot accelerate faster than $g_M$, otherwise it will crush the astronauts.
what is the appropiate equation to constraint ...
4
votes
4answers
2k views
Infinitely charged wire and Differential form of Gauss' Law
I have tried calculating the potential of a charged wire the direct way. If lambda is the charge density of the wire, then I get
$$\phi(r) = \frac{\lambda}{4 \pi \epsilon_0 r} \int_{-\infty}^\infty ...
3
votes
2answers
572 views
Electric field and electric potential of a point charge in 2D and 1D
in 3D, electric field of a piont charge is inversely proportional to the square of distance while the potential is inversely proportional to distance. We can derive it from Coulomb's law.
however, I ...
3
votes
2answers
551 views
The Energy-Momentum Tensor and the Ward Identity
I have a question regarding a homework problem for my quantum field theory assignment.
For the purposes of the question, we can just assume the Lagrangian is that of a real scalar field:
...
3
votes
3answers
358 views
D'Alembert's Principle: Where does $-Q_j$ come from?
This is a follow-up question to D'Alembert's Principle and the term containing the reversed effective force.
From the second term of Eq. (1.45)
$$\begin{align*}
\sum_i{\dot{\mathbf{p}}_i \cdot ...
2
votes
2answers
276 views
Plotting $\psi$ for finite square well potential
Lets say we have a finite square potential well like below:
This well has a $\psi$ which we can combine with $\psi_I$, $\psi_{II}$ and $\psi_{III}$. I have been playing around and got expressions ...
2
votes
1answer
73 views
Varying an action (cosmological perturbation theory)
I am stuck varying an action, trying to get an equation of motion. (Going from eq. 91 to eq. 92 in the image.)
This is the action
$$S~=~\int d^{4}x \frac{a^{2}(t)}{2}(\dot{h}^{2}-(\nabla h)^2).$$
...
2
votes
3answers
2k views
Finding Lagrangian of a Spring Pendulum
I'm trying to understand Morin's example of a spring pendulum. What I don't get is his expression for $T$. I can understand the $\dot x^2$ term in the brackets. But I don't understand the $(l + ...
1
vote
1answer
258 views
Force exerted on potential wall
A particle bound in an infinite potential wall at $x=0$ will apply a force on the wall. For a plane wave and imagining it as a fluid bouncing off the reflection wall at $x=0$, find the force in terms ...
1
vote
1answer
184 views
Knowing the mass and force acting on a particle, how do we derive the relativistic function for velocity with respect to time?
Use this scenario:
An electron gains speed in the Stanford Linear Accelerator (SLA) across 3000 meters, reaching a final velocity of 0.95c due to a constant force pushing on the electron. Given the ...
0
votes
1answer
140 views
Two Qubit problem
A two-qubit system was originally in the state $ \frac{3}{4}|00\rangle-\frac{\sqrt{5}}{4}|01\rangle+\frac{1}{4}|10\rangle-\frac{1}{4}|11\rangle $ , and then we measured the first qubit to ...
0
votes
3answers
672 views
4th equation of motion
Ive been given the 4 equations of motions The fourth being:
$$s=ut+\frac{1}{2}at^2$$
If rearranged it forms the quadratic equation
$$at^2+2ut-2s=0$$
But that means that t has 2 values.
Will ...
6
votes
3answers
1k views
Charge density in concentric spheres
Question:
If there are two conducting spherical shells and the inner shell is grounded, what will be >the charge density in the inner shell if there is a charge Q placed on the outer shell?
...
4
votes
2answers
332 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 ...
3
votes
1answer
135 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 ...
