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. Please READ THE GUIDANCE IN META before asking homework-like questions.

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0
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0answers
14 views

Motion of particle in T seconds [on hold]

A particle starts moving from rest position ($v = 0$) under a constant acceleration $a$. If it travels a distance $\Delta{x}$ in time $T$, what distance will it travel in the next time $T$?
-2
votes
0answers
21 views

Density and Pressure [on hold]

Suppose there is a U-shaped tube on a table: In which right arm is shorter than the left arm.The open end of the right arm is 10cm above the table.The radius throughout the tube is 1.5cm. Water is ...
0
votes
0answers
20 views

What is the capacity of this circuit? [duplicate]

I simply can't figure out what the capacity of this circuit is. I can do the math myself, I just need a hint how to create an equivalent circuit where it is obvious what is parallel and what is in ...
2
votes
1answer
50 views

Mode operators in the Virasoro algebra

This questions concerns Exercise 2.11 in Polchinski. We are asked to compute the commutator $$L_{m}(L_{-m}|0;0\rangle) - L_{-m}(L_{m} |0;0\rangle)$$ By plugging the mode expansions, we use the ...
0
votes
1answer
39 views

Solution of the Radial Part of the Schroedinger Equation

The general Schroedinger Equation is: $$\left[-\frac{\hbar^2}{2m}\triangle +V(r,\vartheta,\varphi)\right]\psi_{nlm}=E\psi_{nlm}$$ When considering free waves, i.e. $V(r,\vartheta,\varphi)=0$ and a ...
0
votes
0answers
36 views

Calculation of OPE in Polchinski

Consider Exercise 2.8 in Polchinski's String Theory book. We are asked to compute the weight of $$f_{\mu \nu}:\partial X^{\mu} \bar{\partial}X^{\nu}e^{ik\cdot X}:$$ I have carried out the usual ...
-3
votes
1answer
21 views

Vectors and projectile [on hold]

Two particles A and B starts moving from a high point O at t=0 in the opposite direction with horizontal velocities 9 3 m/s and 4 m/s respectively. Due to earth's gravitational field the two particles ...
1
vote
0answers
42 views

Probability flux

I was reading a text on Quantum Mechanics in which it said that $$\int{d^3 x \, j(x,t)} = \frac{\langle p\rangle}{m},$$ where $\langle p\rangle$ is the expectation value of the momentum operator at ...
-3
votes
3answers
60 views

Derivative of kinetic energy

I read that the derivative of kinetic energy=$F\cdot v$. I tried to differentiate (1/2) mv^2 with respect to time but each time I am getting $m*v$ and not $m*a*v$ which solves to $F*v$. My efforts are ...
3
votes
1answer
53 views

Help understanding work

I have this problem: An object with 800kg mass is lifted up 2.4m by a force $F$. How much work does the Force do on the object (gravity is the only other force acting on the object)? From what ...
-1
votes
0answers
14 views

Projections of a vector [on hold]

The projections of a vector on x-y plane, the y-z plane and the z-x plane are √13,√40,√45 respectively. The vector lies on the first octant. It's approximate magnitude is
0
votes
1answer
46 views

What is the algebraic form of the momentum eigenstate?

I'm asking this in the context of trying to verify the equation $a^{\dagger}_{p} \vert 0 \rangle = \frac{1}{\sqrt{2\omega_p}} \vert p \rangle$. So far I have calculated $\vert 0 \rangle = ...
0
votes
0answers
39 views

Proving $-\frac{\partial P}{\partial V}|_{T,N}\geq0$

I have obtained the following condition from the entropy maximum principle: $$\left.\frac{\partial^2U}{\partial V^2}\right|_{S,N}\left.\frac{\partial^2U}{\partial ...
0
votes
1answer
31 views

Calculating time and speed change on downhill incline

I am trying to calculate the time and final speed if lets say a bike with constant speed of 20km/h of goes in this incline (downhill), what would be its speed after say $M$ meter ? I remember the ...
0
votes
0answers
40 views

Electron energy in magnetic field

For a problem early in a book I'm reading (Quantum Mechanics by Albert Messiah), I'm asked the following: Consider an electron following a circular trajectory in a constant magnetic field $H$ (I'm ...
0
votes
0answers
34 views

Mass point on a circle - lagrange [on hold]

A mass point of mass m moves on the circle $x^2+y^2=R^2$ and $z=0$. No external forces are acting. Solve the equation of motions and determine the constraint force with the lagrange equations of ...
-2
votes
1answer
26 views

Mechanics Problem: The Acceleration of A Slider [on hold]

The acceleration of a slider A is defined by the relation $a = -2k\sqrt{k^{2}-v^{2}}$ where $k$ is a constant. The system starts at time $t=0$ with $x=0.5$ m and $v=0$. Knowing that $x=0.4$ m when ...
-1
votes
0answers
24 views

Oscillation - atoms [on hold]

A homogeneous, spherical electron cloud describes an atom (radius $a_0$ and total charge $^-e$ and positive point charge $^+e$ as the nucleus. An external electric field stimulates the electron ...
1
vote
1answer
54 views

How is the formula for the capacitance of a parallel plate capacitor derived?

I have seen in I.E. Irodov that if the permittivity is given as $\epsilon$ then we can find the capacitance as $$C = \frac{\epsilon A}{d}$$ but I wonder whether it is dimensionally correct or not and ...
0
votes
1answer
28 views

Coaxial waveguide - wave impedance

Determine the capacity $C$ per $m$, the inductance $L$ per m and the wave impedance $Z_0$ for a coaxial waveguide with inner radius $a$ and outer radius $b$. How large would $b$ have to be so ...
0
votes
0answers
23 views

Resonance - electron cloud - oscillation [on hold]

A homogeneous, spherical electron cloud describes an atom (radius $a_0$ and total charge $^-e$ and positive point charge $^+e$ as the nucleus. An external electric field stimulates the electron ...
-1
votes
0answers
23 views

Electric potential of a uniform rod at coordinate beginning [on hold]

I was doing a problem in electrostatics and stumbled on an interesting potential question. The problem is stated as: Suppose there is a thin metal rod of length $2a$ placed in the origin as shown ...
0
votes
0answers
10 views

Exact inductance calculation per unit mass and per unit volume

How to exactly calculate how many joules is induced on 1 $cm^3$ aluminum or 1 mol copper when applied flux density changes from 0 to 1 tesla
0
votes
1answer
35 views

Estimate the amount of atoms in the smallest speck of matter you can see with the naked eye [on hold]

I am currently working through a physics text book at my own pace on Newtonian mechanics. I came across this question in the the chapter 1 problems and had a bit of trouble with it. My biggest issue ...
0
votes
0answers
23 views

Angle to move in to cross river w/ a current? [on hold]

so I have a hw problem given that a swimmer is swimming 1.2 m/s relative to the river and they swim across trying to get the a point directly opposite the 74 m wide river but they end up 33 m ...
-2
votes
0answers
44 views

How much wind power is needed to lift someone up? [duplicate]

I'm talking about fan wind power. A range would be sensible from about 8yrs-50yrs as not all human beings are the same weight with the added weight of a 5kg engine and average skateboard.
2
votes
1answer
62 views

Operator product expansion energy momentum tensor

We have the following equation from Polchinski (2.4.6) $$ T(z)X^{\mu}(0) \sim \frac{1}{z}\partial X^{\mu}(0) , \tag{2.4.6} $$ where $T(z)$ is defined as $T(z) = -\frac{1}{\alpha'} :\partial X^{\mu} ...
2
votes
1answer
115 views

How do you know when you need to use distributions to represent charge densities?

I tried to solve a problem using Gauss' law in the following way. Let's assume we have a spherical shell of radius $R$ with a charge $Q$ being homogenously distributed on its surface. I am trying to ...
1
vote
1answer
65 views

How to prove $\frac{\mathrm{d}^3\vec{p}}{E}$ is Lorentz invariant? [duplicate]

To prove $\frac{\mathrm{d}^3\vec{p}}{E}$ is Lorentz invariant is to prove $$\frac{\mathrm{d}^3\vec{p}}{E} = \frac{\mathrm{d}^3\vec{p}'}{E'} \qquad(\mathrm{d}^3\vec{p} := \mathrm{d}p_x \mathrm{d}p_y ...
2
votes
1answer
59 views

Analytical mechanics with SR

Is there an analytical mechanics with SR? Of course you can write down the Lagrangian and Hamiltonian of a free particle. What about non-free? Are there any problems? To be specific: what would the ...
0
votes
0answers
47 views

Quantum Mechanics - Induction Methods [on hold]

Let $a$ be a lowering operator and $a^\dagger$ be a raising operator. Prove that $$a(a^\dagger)^n = n (a^\dagger)^{(n-1)} \, .$$ My professor suggested to use induction method with formula: $$\left( ...
0
votes
1answer
22 views

How do I incoporate gyro information in my position and velocity calculation? [on hold]

The velocity in 1 direction can be calculated as : $V_t=V_0+at$ And position could be calculated as $P_t=V_0t+1/2at^2$. Besides time, acceleration, I was also given information from 3 axis ...
0
votes
1answer
17 views

Which features effect on the Reflection of things?

I'm working on something and I need to find a wire that doesn't reflect well. so, I know one feature is the color of the wire to decrease the reflection. But what other features effect on the ...
0
votes
0answers
25 views

Calculating center of mass after movement [on hold]

For this problem there is no change in External Forces so change in momentum to the system is 0. We have a massless boat of length $L$ with a person on either end of masses $m_1$ and $m_2$ Person 1 ...
0
votes
0answers
43 views

Resistance of a metal sphere

How would you go about calculating the electrical resistance of a sphere when you only know it's radius and conductivity? I know with cables you can use $$ R = L/(g \cdot S),$$ but I don't know how ...
-2
votes
1answer
39 views

Simple question about using determinant to find eigenvalues of $\hat{S_x} + \hat{S_y}$ [on hold]

The problem is: Find the eigenvalues and eigenfunctions of the operator $\hat{S_x} + \hat{S_y}$ where $\hat{S_i}$ is the spin operator in the i direction (i = x,y,z). The first step of their ...
0
votes
1answer
27 views

Inertia of a disk - axis of rotation through the x-axis [on hold]

In general one computes the inertia by the formula of: $$ I = \int r^2 dm $$ However in this case it does not work, as the mass is not equally distanced from the axis. The disk in the xy - plane ...
0
votes
0answers
22 views

Finding the amplitude of a pendulum [on hold]

I'm tring to simulate the behaviour of a pendulum. I have it in the equilibrium position, then I apply on it an initial velocity $\vec{v_{0}}$ Knowing $\vec{v_{0}}$ and its mass m, how can I find the ...
0
votes
1answer
57 views

Cylindrical capacitor in an electric circuit

I've come across a tricky question and would appreciate some hints or explanations as to why the given solution is the way it is. The question reads as follows: A coaxial cable consists of a wire ...
4
votes
1answer
90 views

Quantum Mechanics - Lowering Operator [on hold]

Let $a$ be a lowering operator. Show that $a$ is a derivative respects to raising operator, $a^\dagger$, $$a = \frac{\textrm{d}}{\textrm{d}a^\dagger}$$ Can someone please explain how to prove the ...
0
votes
0answers
16 views

Bead and Disc with a String [on hold]

I want to know about the path the bead follows, and whether tension increases , decreases or remains constant and work done by tension (along with analysis and reasoning).
0
votes
0answers
9 views

Electrical potential help? [on hold]

The electric potential increases from 72 V to 686 V from the bottom plate to the top parallel plate. What is the magnitude of the change in potential energy of a -3 X 10^-6 C charge from top to ...
0
votes
0answers
21 views

Problem on Conservation of Energy in Pulley System [on hold]

See the masses in Figure (1) which start out at rest. (i) Find the velocity of the 14 kg mass just before it hits the ground. (ii) Find the maximum height reached by the 8 kg (and don’t ...
0
votes
1answer
73 views

How to write the Lagrangian in terms of a projection

We know that $$ L=\frac{1}{2}\left(\partial_{\mu} A_{\nu} \partial^{\mu} A^{\nu}-\partial_{\mu} A_{\nu} \partial^{\nu} A^{\mu}\right) $$ But how do we write the Lagrangian in the following way: ...
1
vote
1answer
32 views

Particle hitting particles attached with springs [on hold]

In classical mechanics if you have a particle moving in two dimensions and it hits a particle at rest although that particle is attached to a spring that is in turn attached to a third particle. ...
2
votes
1answer
42 views

Cylinder inside a cylinder - moment of inertia

A homogeneous cylinder with radius a and mass m rolls in a hollow cylinder with radius R. Determine the kinetic energy of the cylinder as function of $\dot{\theta}$. I'm sorry for ...
0
votes
0answers
18 views

Cylinder rotating inside another cylinder. [duplicate]

A homogeneous cylinder with radius a and mass m rolls in a hollow cylinder with radius R. Determine the kinetic energy of the cylinder as function of $\dot{\theta}$. I'm sorry for ...
0
votes
0answers
18 views

Time in terms of various other variables [on hold]

I have a quick question and I just need two equations as the answer. So I know that: d=(vi*t)+(0.5*a*t^2) and d=(vf*t)-(0.5*a*t^2) So, I just need both of those equations in terms of T!
-1
votes
0answers
23 views

What's the equivalent capacitance and how? [on hold]

Hey guys! Could you tell me how to find the equivalent capacitance?
1
vote
1answer
42 views

How to calculate distance travelled while car is rolling, given start and end speed?

I'm trying to calculate the distance travelled by my Formula Student racecar if it starts at a certain speed, goes into Neutral (no acceleration, no brakes, just rolling on its wheels), and ends at ...