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.

learn more… | top users | synonyms (3)

0
votes
0answers
24 views

Momentum operator in position basis-expansion

Starting with the translation operator acting on the state \begin{split} \mathcal{T}(\Delta x)|\alpha\rangle=exp\Big(\frac{-ip\Delta x}{\hbar}\Big)|\alpha\rangle \approx \Big( 1-\frac{ip\Delta ...
0
votes
1answer
12 views

The image of a wall clock is to be obtained on the opposite wall 2m away by the means of a convex lens. What is the minimum focal length required?

I'm in 10th grade and this question came in my physics test. Nobody was able to answer this question correctly except my physics teacher who says that the answer is 2m. My answer is that there should ...
0
votes
0answers
26 views

In a $\phi^4$ theory, does the single vertex Feynman diagram (with no internal propagators) contribute to the scattering amplitudes?

If I have an interaction term in the Lagrangian of the form: $$ L_{int}=g\frac{1}{4}\phi^4 + g'\phi^3\chi +... $$ How does the trivial diagram (i.e. just a cross with a vertex at the center) ...
-1
votes
1answer
21 views

Phase differences due to reflection etc

http://www.physicsgalaxy.com/lectures/1/30/1749/Solved-Example-15#42 see this link(only question) and see 1:21. Phase difference is given as $2\mu t$ but shouldn't it be $2\mu ...
0
votes
1answer
40 views

How to get $a(t)$ and $v(t)$ from $a(x)$ without knowing $x(t)$?

Suppose to know, for a one dimensional motion of a particle, the acceleration as a function of the position $a(x)$. Using $\frac{dv}{dx} v=a(x)$ I can get $v(x)$ as a function of the position. Is ...
-1
votes
1answer
13 views

Mechanics of Rotational Motion

Well, I faced a problem in my Physics Examination Problem: A Circular Disc of Mass $M$ and radius $R$ is set rolling on a table. If $\omega$is angular velocity, ST total energy ...
0
votes
1answer
36 views

What effect would GW150914 have had over a much shorter distance? [duplicate]

In the wake of the recent news of the confirmation of the existence on gravity waves, I was discussing gravitational waves with a friend, and he asked whether this could potentially lead to some kind ...
0
votes
0answers
18 views

Electromagnetic Wave Propagation

!http://i.imgur.com/xADz3GB.png Hello I was doing this long problem linked above and I am unsure about certain parts. Thanks for anyone who is willing to help! From my calculations: It is a good ...
0
votes
1answer
30 views

Exercise 18b in Schutz's First course in GR

The question is as follows: Show that a timelike vector and a null vector cannot be orthogonal. So we have a timelike vector $\vec{A}$, s.t $\vec{A}^2<0$; and a null vector, $\vec{B}$ s.t ...
0
votes
0answers
22 views

Question on Law of Conservation of energy? [on hold]

A body of mass $m$ is thrown upwards with a velocity $u$. At a point $x$ metre above the ground, the ratio between kinetic energy and potential energy is 1:5. What will be the new ratio at $x$, if the ...
1
vote
1answer
31 views

$F\cos\theta$ is not proportional to $d\cos\theta$?

If I’m working in two dimensions and I wanted to calculate the net electrostatic force on charge C, where charges A and B are stationary, of the same charge, and placed on the y-axis equidistant ...
0
votes
1answer
28 views

Reference point for U tube question

Referring to the following diagram, I am expected to pick out the one case where the fluid in the tube is not in static equilibrium. They are in the other three cases. The answer is that situation 2 ...
-1
votes
0answers
27 views

Physics specific heat question

The instructions below outline the procedure for a demonstration. Materials: four 100 g metal blocks, each of a different metal, four polystyrene foam cups, each containing 150 g of 10°C water ...
0
votes
2answers
27 views

Proving charge on outer surface of parallel plate capacitor must be zero

If we have two conducting plates, with charge $Q$ and $-Q$, why is the charge on the outer surfaces of each conductor zero? I've been trying to wrap my head around the problem. Firstly, don't excess ...
-1
votes
0answers
14 views

Capacitor charge [on hold]

It takes 14.8 J of energy to move a 13.0-mC charge from one plate of a 17.0-μF capacitor to the other. How much charge is on each plate? Assume constant voltage.
1
vote
0answers
24 views

Acceleration 4-vector in Schwarzschild metric [on hold]

I am working on A note of a paradoxical property of the Schwarzschild solution by Abramowicz M. A. and Lasota J-P It is a note on surprising cases in the context of the standard Schwarzschild ...
1
vote
0answers
26 views

Two Dimensional Momentum [on hold]

[2] []3> A tennis player receives a ball of mass $60\ \mathrm g$ travelling horizontally at $55\ \mathrm{ms}^{-1}$ perpendicular to the net, and returns it at $28\ \mathrm{ms}^{-1}$ in a direction at ...
-1
votes
0answers
10 views

Find total elongation problem? [on hold]

The main question is like this The answer is total elongation = 4FL/AY But in the middle part wouldn't there be a compression . I think there will be a compression . Please clear my confusion
1
vote
1answer
18 views

Speed of pinewood derby car

If I drop a pinewood derby car from 4 feet I know it hits the ground in half a second at about 10.6 mph. If it rolls down the track (starting at 4 feet) it takes longer, but is it possible for it to ...
0
votes
0answers
13 views

Stokes law percentage error

I am supposed to find the % error in t , r and D this is for Stokes law to find the viscosity.I have tried to find the % error in t we used a stopwatch so I found the mean from my values for t and ...
0
votes
0answers
19 views

Charge distribution on a line

Given a conducting line segment from -a to a, with a total charge Q distributed by the continuous charge density $\lambda(x)$. I can write this system of integrals. Since the charge must total Q: ...
1
vote
4answers
36 views

How can I calculate how high an arrow goes when all I know is its initial speed?

I'm not familiar with English physics terms so bear with me. If I shot an arrow straight up and it went off with a speed of 21 m/s, how high up would it go? (air resistance is insignificant). My ...
0
votes
2answers
43 views

How to find the current source Is and the voltage V1 of this circuit schematic?

I have Vs= 1.5V, R1=1 Ohm, R2 =6 Ohms, I got the equivalent resistance to be 7 Ohms. Then I set equal Req to Vs for 7I=1.5 and got my current source to be 0.214A. Since it is only Vs and Req in the ...
0
votes
0answers
16 views

induction of voltage from power lines to telephone lines [on hold]

A pair of isolated parallel power lines of 1 cm radius with a separation of 90 cm have a voltage difference of 440 V. These power lines are 60 cm above a pair of telephone lines. The telephone lines ...
0
votes
1answer
58 views

Sakurai Exercise 2.17 (Harmonic Oscillator, Ladder operators) [on hold]

I the field of the harmonic oscillator and ladder operators I am trying to solve exercise 2.17 from Sakurai and want to proof the following relation $$ \langle x^{2n} \rangle = (2n - 1)!! \langle ...
-1
votes
0answers
10 views

Compton Scattering problem verification [on hold]

A photon emergre in an angle $\theta=\frac{\pi}{4}$ in a Compton scattering the kinetic energy of the photon is $K_i=2K_f$. I must find $K_i$ in $MeV$. $\lambda_f -\lambda_i= \lambda_c$. Then: ...
0
votes
1answer
32 views

Confusion in a problem

This is a very basic homework type question. I guess there is some intermixing of concepts in my head. Help. There was a question I came across - A ball of mass 0.2 kg is thrown vertically upwards ...
0
votes
0answers
22 views

Number of impacts nitrogen molecules make with the palm of a hand [on hold]

What is the number of impacts one mole air makes with the palm of your hand assuming air is entirely made up of nitrogen?
-1
votes
0answers
23 views

Calculus Based Physics Problem with Particle Motion [on hold]

Suppose the position vector for a particle is given as a function of time by $$\hat{r}(t) = x(t)\hat{ i} + y(t)\hat{j}$$ while $$x(t)=at+b \text{ and }y(t)=c(t^2)+d,$$ where $a=1.00\, m/s$, $b=1.00\, ...
0
votes
1answer
18 views

If an object launches off of a table at a given velocity, calculate angle below the horizontal when it hits the ground?

So this is a basic kinematics question that I'm having trouble understanding. Here is the question verbatim: A hockey puck slides off the edge of a table with an initial velocity of 28.0 m/s and ...
-3
votes
0answers
47 views

Physics Centripetal Force problem [on hold]

At what rate a space station 200m in diameter would have to rotate to create gravity equal to 0.7 that at the surface of earth. How fast does it spin, and how long would it take to make a complete ...
0
votes
1answer
18 views

Estimating the characteristic energy of the neutron

I'm trying to do a question which is as follows: "Applying the uncertainty principle on the smallest scales, estimate the characteristic energy of the three relativistic quarks confined within ...
0
votes
1answer
26 views

Helmholtz energy in terms of grand partition function

According to my source of notes $$A = kT \left ( N \ln z-\ln Z(z, V, T)\right) $$ where $z=e^{\beta \mu}$, $\mu$ being chemical potential, $Z(z,V,T)$ is partition function for grand canonical ...
0
votes
0answers
21 views

Force required for balancing a three-bodied frictionless and gravityless system [on hold]

Hello everyone, I made calculations over calculations but I think my free body diagrams were wrong. There are three bodies here: on two of them (the 2 and 3) are applied two forces (F2 and F3) ...
0
votes
0answers
29 views

Question about a solution of a partial differential equation by separation of variables

I'm trying to understand this text: http://www.ekayasolutions.com/UCDMath/HeatCondSphere.pdf But I'm having problem with this part: Whe have to solve: \begin{equation} \dfrac{\partial ...
-2
votes
0answers
23 views

Find the period of a pendulum system [on hold]

How to solve the following problem, A mathematical pendulum is formed by a cable with length $l$ and a sphere at it's end with mass $m$. While the pendulum swings at it's position of ...
-4
votes
0answers
37 views

The question is related to quantum mechanics [on hold]

The energy gap in germanium is 0.72 eV. Find the maximum wavelength of electromagnetic radiation which can cause transition from the valance band to the conduction band.
-1
votes
1answer
24 views

Calculate the pressure in a container given moles and volume [on hold]

Say there are two containers with the same volume. The first has a temperature T and pressure P and contains 1 mole of $O_2$. The second has a temperature 2T and contains 1 mole of $He_2$. I'd like to ...
-2
votes
0answers
20 views

Electromagnetic field tensor [on hold]

How will we derive the component of electromagnetic field tensor by using the electromagnetic field tensor formula; what is the procedure? Can anyone tell me about how I would derive $F_{00}, ...
-2
votes
0answers
17 views

What is the mass defect of stable nucleus? [on hold]

What is mass defect of stable nucleus? A-positive B-zero C-negative D-less than unstable nucleus
0
votes
2answers
18 views

How to determine the friction constants

We know that the friction is formulated in this form $$F=av+bv^2+Nμ$$ I'm working with an object and surface and I want to find $ a, b, μ $ for them. Can you please give me an experimental method ?
0
votes
2answers
35 views

Free falling object with air resistance, what's the velocity as a function of time?

So, I'm looking into how velocity can be represented as a function of time when the air resistance is also function of velocity. The force of drag is $$ F_d = C||\vec{v}||\vec{v} $$ where $C$ is an ...
0
votes
1answer
31 views

Calculating stopping distance of a car on a slope

I'm having trouble setting up a homework problem properly. We are given the following: A given car can brake to a stop from a speed of 60.0 mi/h in a distance of 123 ft on a level roadway. What ...
-1
votes
0answers
30 views

Motion question [on hold]

I have been struggling to understand how to do this question so some guidance would be great. A racing car of mass 941kg accelerates from 28km/h to 102km/h over a distance of 297m. Frictional forces ...
0
votes
0answers
16 views

How to find frequency from wave equation [on hold]

I have wave equation for vibrating string with damping $M\ddot{x}+C\dot{x}+Kx=0.$ I used this formulae for finding (natural)frequency $\sqrt{\frac{K}{M}-(\frac{C}{2M})^2},$ is it ok?
-3
votes
0answers
26 views

complex waveforms [on hold]

just wondering if anyone can help me out with a expression I need to find out the phase angle of any harmonic components present ( relative to the fundamental) of this expression $v=\sin(100\pi t)$ ...
0
votes
0answers
37 views

Electric Filed on a point on a sphere [on hold]

I really want to understand the concept of those problems. Thanks.
1
vote
1answer
16 views

Does the stretching of a spring depend on its rest lenght?

I don't understand if the stretching of a spring depends on its rest lenght besides its force constant $k$. I'll make an example to show my doubt. Consider a vertical spring and a mass $m$ ...
-1
votes
0answers
22 views

Torque Problem (tension and thrust) [on hold]

A crane boom which weighs 200 lb and which ia 10 ft long is supported so that it makes an angle 50° with the supporting wall and so that the rope makes an angle of 70° with the wall. If load of 700 lb ...
-2
votes
0answers
17 views

Torque (Distance × MomentArm) [on hold]

A bar 2.0 m long makes an angle of 30° with the horizontal. A vertical force of 40N is applied 0.4 m from the upper end. Calculate the torque due to this force about each end.