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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10 views

Mean value of number of molecules of a gas inside a volume

I'm trying to solve this problem in Statistical Mechanics but I'm not sure if my reasoning is right. The problem is as follows: we have a certain region with volume $V_0$ containing $N_0$ molecules of ...
0
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2answers
33 views

What is the pressure of falling water?

Imagine a breaking wave 10m high. The wave breaks onto a beach. What is the maximum pressure created by the wave on the beach, ignoring effects such as might be cause by air?
-4
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1answer
17 views

Frictional force

a box of banana weighing 40N rests on a horizontal surface. the coefficient of static friction between the box and the surface is 0.4 and coefficient of kinetic friction is 0.2. a. if no horizontal ...
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0answers
13 views

Choosing a term's sign — a rolling body question

https://prettygoodphysics.wikispaces.com/file/view/Rollin_and_Slippin.pdf In this link's first problem, top-spin is applied to a ball that slips; while the ball moves forward, it spins clockwise. ...
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0answers
19 views

Precession problem with a rotating cross [on hold]

I'm attempting a homework question about advanced classical physics. This problem describes a light rod between two poles with a cross at the middle rotating, what happens if we remove one of the ...
1
vote
1answer
33 views

Approximating the moment of inertia of a quadcopter

I want to compute an approximated moment of inertia for my quadcopter: my idea is to take the frame and the electronics, approximate it as a sphere in the center of mass $M$ and radius $R$ (with ...
-2
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0answers
18 views

Circular Motion [on hold]

How do I prove the last part? I have no idea how to start.
0
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0answers
31 views

STEP problem (2004 paper II, mechanics)

This question says that the force exerted on a train (by its engine) when it is moving with velocity $v$ is $Pv^{\frac{3}{2}}$ where $P$ is a constant; and at velocity $v$ the resistance is $kv$ where ...
1
vote
1answer
26 views

Velocity of mass colliding with another mass under the influence of gravity

If two spherical objects of mass $m$ and mass $4m$ are in outer space with no other forces other than the gravitational attraction between them and they're separated by distance $d$, what velocity ...
0
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0answers
25 views

Wind blows to surface, how long legs I need? [on hold]

Wind blows onto the surface normally at at speed of 20m/s. The frame (in vertical plane) has two supporting bars in the plane of the ground as shown welded rigidly. What is the length of horizontal ...
0
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0answers
18 views

How do we find the cone angles of the conic sections that are the Newtonian orbits?

In planetary motion, when the eccentricity of a conic section and its semi-latus rectum are known, how do we find out the cone's semi-vertical angle and angle of cutting of the cone by plane?
1
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1answer
28 views

Potential of a sphere induced by a charged sphere

I am trying to solve what seems like a simple problem but something is bothering me: Imagine we have a sphere (1) with a charge $Q_1$ and at a distance $d$ we have another sphere (2) which is a ...
1
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0answers
24 views

Rain falling into a cart on an incline

I have a practise question in which a cart on and incline of angle $\alpha$ and starts initially at velocity $v_ 0$. Just as the cart moves off it starts raining vertically and the mass of the rain ...
-1
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0answers
32 views

How do I maniptulate the Fourier transform of a field using the delta function?

In "Classical covariant fields" by Burgess, kindly tell me how he reached equation 2.54 from equation 2.52. I tried to solve the delta function according to the given instructions but I am making some ...
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0answers
24 views

Exam preperation [on hold]

I really need your help preparing for my upcoming exam. here are my questions:
0
votes
0answers
9 views

Electromagnetics [on hold]

the electric potential V(z) between two transmission lines is 6000KV, calculate the electric field the two transmission lines exert on each other,If the electric field is vertically downwards and V1 ...
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0answers
26 views

Intensity of a sound radiated from a moving source

Let A be a point source (or vibrating sphere small enough - don't worry about the Dirac functions) moving along x-axis with constant speed of v. Determine the intensity $ I=I\left(\vec{r},t\right) $. ...
2
votes
1answer
20 views

How to measure the fineness of a textile fibre by diameter and fibre length?

I measured the diameter (um) and the fibre length (cm) of different (loose) textile fibres and have an average. Now there must be a relation between these two factors that gives an idea of the ...
-1
votes
0answers
35 views

A fly travelling on a Cartesian plane [on hold]

Beginning at point $(-1,0)$, a fly travels across the semicircle defined by $y=\sqrt{1-x^2}$. Its acceleration is given by $a=(t-1)^3+1$. How long does it take the fly to arrive at point $(1,0)$?
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0answers
34 views

Special relativity : time dilatation

A space shuttle is flies away from the earth with $v=0.8c$ . When it has reached a distance of $d=6.66\cdot 10^{11}m $, a light flash is sent from the earth. How long does the signal need to reach ...
-1
votes
0answers
32 views

What is the dimension of the solution for Laplace's equation between two cylinders?

Calculate the $\phi$, satisfying $\nabla^2 \phi=0$ between the two cylinders $r=a$, on which $\phi=0$, and $r=b>a$, on which $\phi=V$. I calculate it and found the solution is ...
2
votes
2answers
86 views

How many hours will be in a day if the radius of Earth increases by 70 m?

I am little confused about the linear momentum and angular momentum, will the linear momentum of earth change due to changing of its radius or it will stay as it was and i know that the moment of ...
2
votes
3answers
192 views

Finding current in the circuit

Hi anyone. I try to de-draw the circuit in the simpler to calculate i1 and i2. But can not. Anyone have any hint to do this question ? . Or just give me the specific name of the way of how to do ...
1
vote
1answer
50 views

Pendulum problem where it hits a peg swings around and lands on top of it

Using energy equation from when it is released to when it reaches its highest point after hitting the peg I get change in kinetic energy = 0 and change in potential due to gravity = 0 Does the string ...
1
vote
1answer
43 views

Evolution operator for “blending” a pair of eigenstates

Consider a measurement operator ("observable") $\hat O$ which has ("a spectrum of") only two distinct eigenvectors; formally $$\hat O |\bullet\rangle := r_{\bullet}~|\bullet\rangle, \qquad \hat O ...
0
votes
3answers
45 views

How much work do I need to convert 300ml of water from 25°C to 3°C? [on hold]

A question on how to apply thermodynamics principles to figure out how much work is needed to hold 300ml water in room temperature at 3°C. So far I have: 1Cal for each degree per g of water. We have ...
0
votes
1answer
58 views

Neutron star: escape velocity

The textbook from which I teach physics at the end of secondary school calculates the escape velocity at the surface of a non-rotating neutron star Newton-style using $\frac{1}{2}\cdot m\cdot ...
0
votes
1answer
32 views

Kinematics - When projectile lands at an angle

Question: You are watching an archery tournament when you start wondering how fast an arrow is shot from the bow. Remembering your physics, you ask one of the archers to shoot an arrow parallel to ...
0
votes
0answers
28 views

Incline plane problem [on hold]

A string carrying a $5\times10^{-2}$kg bob hangs from the roof of a vehicle. It moves down an incline at an angle of $30 °$ with the horizontal with acceleration of $3\mathrm{ms}^{-2}$. What angle ...
0
votes
1answer
51 views

Where does this hyperbolic tangent in Nakahara's text come from?

I don't see why the term with $\tanh$ appears in the equation 1.164 The textbook is the second edition of Geometry, Topology and Physics
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0answers
11 views

How to find the enthalpy if I have the pressure and temperature?

There is a refrigerator with R-134a as the working fluid. The R134a enter the evaporator at P=100kPa, quality =0.2 and it leaves at P=100kPa, T=-26 degree Celsius How can I find out the enthalpy? I ...
0
votes
1answer
21 views

Movement of accelerating points with maximum speed (i.e. running away from tiger)

I'm walking through forrest and suddenly I see a tiger. And now I have a problem. I want to know how much time I have left but I don't know how to calculate it. Or more fomally: Let's say there are ...
0
votes
1answer
50 views

The general equation for a wave packet derivation? [on hold]

On Wikipedia it gives the general equation for a wave packet (and therefore for a wave?) to be: $$u(x,t)=\frac{1}{\sqrt{2\pi}}\int^{\infty}_{-\infty}A(k)e^{i(kx-\omega t)} dk$$ I have been trying to ...
-3
votes
0answers
62 views

Jee Main 2015 Question. Inter-molecular Interaction [on hold]

The Intermolecular interaction that is dependent on the inverse cube of distance between the molecules is: Hydrogen Bond? ion-ion interaction? ion-dipole interaction? London Force?
-2
votes
0answers
133 views

JEE MAIN 2015 Question. Electromagnetism [on hold]

Two coaxial solenoids of different radii carry current I in the same direction. Let F1 be the magnetic force on the inner solenoid due to the outer one and F2 be the magnetic force on the outer ...
2
votes
2answers
67 views

Neutron star: free fall acceleration

The textbook from which I teach physics at the end of secondary school, has a question about a neutron star: $M_{star}=1.4\cdot M_{sun}$, radius 15km. "Calculate the free fall acceleration at the ...
1
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1answer
66 views

Perturbations in linear response theory

I've been working on applications of linear response theory to condensed matter systems, and I've got quite far into the literature on the subject. However, there is an identity which seems to be ...
0
votes
1answer
34 views

A question about velocity and acceleration [on hold]

I'm working on MasteringPhysics and came across this problem. I thought I knew how to solve it but the website says my answers are not quite correct. Please see the image below for the problem and ...
0
votes
2answers
49 views

Explanation on anticommutation relations

Setup Given two states: $|K\rangle=a_i^+a_j^+|\rangle$ and $|L\rangle=a_k^+a_l^+|\rangle$. Evaluating the overlap: $\langle K|L\rangle=\langle|a_ja_ia_k^+a_l^+|\rangle$ Introducing: ...
0
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0answers
55 views

Variation on the “Hole through the Earth” problem

In the news recently was the announcement that the answer to the classic Newtonian mechanics question of the time it takes for a body to fall all the way through the Earth was corrected to account for ...
0
votes
1answer
54 views

The correct integral for Newton's shell theorem

I am trying to prove Newton's Shell Theorem, and the natural integral I think of to use does not seem to give the right answer, so I am trying to find my mistake. For simplicity, I will work in one ...
-1
votes
0answers
19 views

Geometric and algebraic aspects of geometric vectors [on hold]

I'm writing some notes for a honors physics class and I am having some trouble with some proofs. Say $\vec{A}$ and $\vec{B}$ are some geometric vectors. Then we defined the dot product ...
0
votes
0answers
11 views

Problem with Parabolic motion [on hold]

Could someone help me out with this one: A tank will fire a missile with speed of 400m/sec to a target that is at 1500 meters from it and 400 m above it. Which are the angles that a thank will have ...
2
votes
1answer
27 views

Getting the ($hkl$) Miller indices from the angle of incidence of a light ray

I was given an X-ray diffraction lab this week. We measured the count rate for different angles $2\theta$ of the rotation of the detector. From these measurements we plotted a graph of 2theta vs the ...
-2
votes
0answers
36 views

Which quarks will I find in a neutron? [on hold]

A: Three up quarks. B: Two up quarks and a down quark. C: Two down quarks and an up quark. D: One charm quark, one up quark and one down quark. please expain why
2
votes
2answers
50 views

Homework: electrical circuit with 2 voltage sources & 1 switch

I can't figure out how to handle this circuit. I am asked to calculate the total resistance and the current over each resistance (R1-R4) when the switch is open. However I'm not getting anywhere ...
0
votes
2answers
74 views

How is the uniform gravitational field approximation $F_g\approx mg$ near Earth's surface derived from Newton's law $F_g=GMm/r^2$ of gravitation?

I am really bothered about how we can derive the equation of projectile motion. Suppose a point mass will move in the gravitational field of the Earth according to the equation $$\ddot R ...
0
votes
0answers
158 views

The location of a point of destructive interference [on hold]

Two speakers, one at the origin and the other facing it at x = 1.04 m, are driven by the same oscillator at a frequency of 642 Hz. If the speed of sound is 343 m/s, find the following. I know the ...
0
votes
0answers
33 views

What's the quantization of a Hamiltonian? [on hold]

Suppose Hamiltonian of a conservative system in classical mechanics is $$H~=~\omega xp,$$ where $\omega $ is a constant and $x$ and $p$ are the position and momentum respectively. What is the ...
0
votes
2answers
27 views

What the wave function looks of a particle in the infinite square well looks like after collapse for measurements of position and energy

Consider a particle in a the infinite square well from x=0 to x=L. At t=to, I make a measurement of position and get x=L/2. What is the resulting wave function at t=to? My understanding, from reading, ...