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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-3
votes
1answer
15 views

SHM by using cos equation

A particle start motion according to equation $x=a\cos \left(\omega t+ \frac{\pi}{6}\right)$. Find time required for it to reach mid point. The displacement according to it must be 0 but how?
0
votes
0answers
33 views

Two apparent contradictions in SR involving time dilation and length contraction

So I've encountered the site "Alternative Physics" – a website proposing "alternative" theories to theories in modern physics. Of course, the site claims that both SR and GR are wrong. To show "why" ...
0
votes
1answer
23 views

Circle of light by a laser and wire

I've asked a question before about this phenomenon, see it here Diffraction in Circle of light? or here Laser, wire, and a circle of light But there are some fringes in that circle of light and I ...
0
votes
1answer
21 views

Intersection of two moving objects

There are two objects. The first object moves with speed U1 from known point A to known point B. The second object has speed U2 and starts from known point C. What is the direction the second object ...
-5
votes
0answers
19 views

newtonian-mechanics

A 1000Kg helicopter hovers over a forest fire and starts to squirt water downward with a speed of 10m/s at the rate of 40Kg/s. What is the initial upward acceleration on the helicopter when it first ...
-2
votes
0answers
42 views

Connecting a mathematical solution to a differential equation with it's physical solution

I have seen this question in a neuroscience course: It is given after the lecture with these and these slides. I have no background in physics. However, I do know how to solve a differential ...
1
vote
1answer
45 views

Reduction of Earth's Rotation due to increase in humidity

I came to know that, moon is moving away from earth, resulting reduction of Earth's rotation. Similarly, if Earth's humidity increases, water vapor will be move up to the air from water sources, will ...
-1
votes
0answers
7 views

Kinematic Motors in series to flywheel calculation problem

Motor 1 drives a platform at 15 RPM clockwise steady state always 15 RPM. Call the axis of rotation Y. Motor 2 is mounted on this 15 RPM platform and axis aligned thus also Y axis (parallel Axis). A ...
2
votes
1answer
41 views

Symmetry of extrinsic curvature tensor

I am trying to solve following problem: In a spacetime of signature (+, −, −, −), let $$ u^au_a = 1, \quad A_{ab} = \nabla_cu_dh^c_{\; a}h^d_{\; b}, \quad h_{ab} = g_{ab} - u_au_b $$ Show that ...
2
votes
1answer
35 views

Raychauduri equation for Milne universe

I am trying to solve this kind of problem: Consider the Milne model, i.e., the empty $ \kappa = −1 $ Friedmann model. Verify by a direct calculation that the expansion $ \Theta $ of the unit ...
0
votes
0answers
36 views

Kinetic Energy and reference frame

Two space ships A and B moving through space in the same direction. Space ship A moves 2m/s faster than B, its weight is 1kg and it has a 2 Watt engine. It is accelerating with full power for one ...
-1
votes
0answers
15 views

Mixture of Gases in kg-mol; Find Temperature [on hold]

A one cubic container contains a mixture of gases composed of 0.02 kg-mol of oxygen and 0.04 kg-mol of helium at a pressure of 220 kPa. What is the temperature of this ideal gas mixture in degrees ...
-1
votes
0answers
30 views

Are there any good places to learn physics without going to university or college? [on hold]

I'm potentially interested in learning physics for my own knowledge, and not to get a degree. I want to start some engineering / technology companies in the future, and having a knowledge in physics ...
0
votes
1answer
19 views

What is the work done in isobaric expansion?

I'm really confused now... I thought the equation was $ W = \int PdV = P\Delta V $ since P is a constant and can be taken outside of the integral. But on the other hand, $ P = \frac{NkT}{V} $, so if I ...
0
votes
3answers
107 views

Gravitational time dilatation calculation

I'm proving an example case from Kip Thorne's book, Science of Interstellar for time dilatation at a Kerr black hole with the following data: \begin{align} a & = 0.998, \\ r & = ...
9
votes
0answers
67 views

Degenaracy in mass of $8$ and $27$ reps of $SU(3)$ in Coleman's Aspects of Symmetry

In Coleman's Aspect of symmetry he proposes an amusing problem in the first chapter. It asks us to consider a set of eight pseudo-scalar fields transforming in the adjoint representation of $SU(3)$. ...
0
votes
0answers
64 views

Fourier transformation and commutators

Sorry as this is a rather trivial question, but I'm stuck with a certain implication. I'm working on exercise 1.7 from Polchinski where we are given an open string with boundary conditions ...
0
votes
0answers
35 views

Astronomy - Hubble Law and Universe age [on hold]

We know that the approximated age of the Universe is calculated as 1/H0 where H0 is expressed like (km/s/Mpc). If I try to calculate the dimensional equation I obtain: (10^3 m/(s* 10^6*10^16 m))^-1, ...
2
votes
2answers
88 views

Integration by parts to derive $d\langle x \rangle / dt$

I am reading "Introduction to Quantum Mechanics" by David Griffiths and I am having trouble understanding part of a derivation of $\frac{d\langle x\rangle }{dt}$ in section 1.5 - Momentum - of the ...
0
votes
0answers
19 views

Capacitor problem [on hold]

A charge of 200 micro coulomb is imparted to each of two identical capacitors connected in parallel. At time t=0 the plates of both the capacitors are 0.1 metre apart. The plates of first capacitor ...
-1
votes
0answers
53 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
37 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
29 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
22 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 $0.8~c$? ...
1
vote
1answer
24 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
23 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
25 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
51 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 $m$ ...
-4
votes
0answers
22 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.
-1
votes
0answers
16 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
-1
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
19 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
23 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
22 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
0answers
84 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 ...
1
vote
1answer
53 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
47 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
54 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
68 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
58 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
15 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
2answers
67 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
40 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
32 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
43 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
36 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
26 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
57 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 ...