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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-1
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0answers
23 views

What is the work being done a girl carrying a bag walking down the street? [on hold]

What is the work being done a girl carrying a bag walking down the street?
0
votes
0answers
28 views

Doubts taking the second functional derivative of the Klein Gordon action

I have very little background with functional derivatives and I would like to clarify some issues. I am trying to compute the second functional derivative of the Klein Gordon action expressed in real ...
-1
votes
0answers
28 views

I tried to solve circuit analysis problem but it failed [on hold]

I tried to solve circuit analysis problem in Hayt's Circuit Analysis 7th. But my answer is wrong and I don't know why I failed to solve this. Here's the question. I have to find what the value of v3 ...
-7
votes
0answers
16 views

Need help please [on hold]

An electron's speed is doubled from $0.2~c$ to $0.4~c$. By what ratio does its momentum increase? What happens to the momentum ratio when the electron's speed is doubled again from $0.4~c$ to $0.8~c$? ...
0
votes
1answer
17 views

Symmetry axis and products of inertia

So if we have an object that has a symmetry axis let us say $z$-axis is a symmetry axis does this mean that the product of inertia $I_{zx} = I_{zy} = 0$? And if that is true why is it true ...
0
votes
0answers
19 views

Flow Rate Calculation from a drain tank through a pipe [duplicate]

I want to know how to calculate the flowrate (Q) of a drain tank flowing through a pipe (see the figure below) All I know is that Q=Velocity(V) x Area (A) V= sqrt(2*g*H)=sqrt(2*9.81*10)=14m/s. I ...
-2
votes
0answers
18 views

its tricky circuit to given output voltage find without source [on hold]

solve it two quation in that pic,1) find Vout voltage and 2) no source given and find v1,v2,v3.
0
votes
1answer
31 views

Deriving Gravitational Potential Energy using Vectors

Here is my attempt at derivation: First you must find a vector function for the gravitational force. By the inverse square law, the magnitude of gravitational force between two bodies of mass $m$ ...
-5
votes
0answers
19 views

Free fall motion of a body [on hold]

A ball is projected vertically upwards with an initial velocity of 100m/s. Find the speed of the ball at its maximum height Take g=10metre per second square.
-2
votes
0answers
13 views

Average velocity of a clock [on hold]

The minute hand opf a clock is 4 cm long Find the average velocity of the tip of the minute hand between 11:00 a.m to 11:30a.m and 11:00 a.m to 11:30 p.m
-2
votes
0answers
19 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 $T$?
0
votes
0answers
17 views

Shear flow in J section type beam

What does the distribution of shear flow look like in a J section type beam. I'm only interested in a qualitative picture of it. I'm not interested in the calculations themselves. It is the top of the ...
-2
votes
0answers
22 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
21 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 ...
3
votes
1answer
67 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
42 views

Solution of the Radial Part of the Schroedinger Equation [on hold]

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
42 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
22 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
47 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
62 views

Derivative of kinetic energy [on hold]

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
55 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
47 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 $-\frac{\partial P}{\partial V}|_{T,N}\geq0$ [on hold]

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
0answers
55 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
116 views

How do you know when you need to use distributions to represent charge densities? [on hold]

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
18 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
44 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 ...