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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2answers
22 views

Working out velocity (including air resistance) [duplicate]

For my computing project, I am creating a projectile simulator but I cant seem to get my head round the air resistance. I tried by working out the horizontal and vertical velocity by solving it ...
0
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0answers
17 views

How to calculate explicitly the *external leg correction* diagram

I tried to calculate the Amplitude of the external leg correction of figure 6.1 in Peskin&Schroeder. I focus on the diagram with one incomming charged fermion $f^-$ (with momentum $p'$), then a ...
0
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0answers
39 views

What is relation between Fourier transform and basis function?

I want to know that how any signal can be represented as sum of basis vector and what basis function actually means?Also is there any relation between Fourier transform of signal and its Basis ...
0
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2answers
46 views

How does the object at constant speed pass the accelerating object?

A car starts from rest and move with constant acceleration of $4\frac{m}{s^2}$ and at the same time a motorcyclist moving with a constant speed of $36\frac{km}{hr}$ overtakes and passes the car. ...
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0answers
20 views

How will a stick against the frictionless corner move? [on hold]

As it shows, a stick is lying against the corner (right angle), and now assume the angle between the vertical wall and the stick is $\theta$, (this mainly because that as it moves the angle will ...
0
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0answers
27 views

Thermodynamics - Efficiency of an engine [on hold]

I am taking a mathematical physics course and got stuck on this rather simple problem Consider a thermodynamic cycle of monoatomic gas () I want to calculate efficiency of this cycle I know that ...
-5
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0answers
24 views

assume that air resistance is proportional to speed [on hold]

Assume that air resistance is proportional to speed. A particle is thrown directly downwards with initial speed equal twice it's terminal velocity. Obtain a fomula for the time it takes for its speed ...
0
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1answer
23 views

Derivation for certain non uniformly accelerated movements [on hold]

A sphere of $m_1$ is thrown at a speed of $v_i$ perpendicular to the surface from the surface into the atmosphere.. Are there equations for position $x(t)$ and the speed $ v(t) $ of the sphere at a ...
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2answers
23 views

If the force of friction is 20 N for a moving body does acceleration increase with the increase of the mass?

A body is moving and the only force acting on it is the force of friction. If we put another body on this moving body somehow the force of friction will increase; does its acceleration change?
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1answer
53 views

Light reflecting inside a sphere [on hold]

I'm having a bit of trouble cracking this problem. The diagram is given below. My job is to find $\phi$ as a function of $\theta$ and the index of refraction $n$. For this part of the question, I ...
0
votes
1answer
57 views

I need help with divergence and gradient? [on hold]

$$A_z = \mu{\frac{e^{-jBr}}{4\pi r}}∫I(z')e^{jBz'\cos\theta}dz'$$ Midway into my question, I want to compute: $$-j\left( \frac{\nabla(\nabla\cdot A) }{w\mu\varepsilon} \right).$$ Symbols like $ w, ...
0
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1answer
24 views

What are the support reactions at these points? [on hold]

How do I find the horizontal and vertical support reactions at A and C? I've tried balancing the moments and the forces in the x and y directions, and I can't seem to get it. Every time I try, I ...
0
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0answers
11 views

Statics- Ball and Joint Analysis [on hold]

I'm having trouble dealing with this problem. I know as much that there are reaction forces at point and tension forces in the direction of the cables, but I don't know how to obtain these forces ...
21
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4answers
2k views

Freefall into snow

In the movie Frozen, the following dialogue takes place: Anna: "It's a hundred-foot drop." Kristoff: "It's two hundred." Anna: "Okay, what if we fall?" Kristoff: "There's 20 feet of ...
-2
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0answers
25 views

Electricity. I'm finding difficulty answering this question [on hold]

Consider 3 identcal spheres, A,B,C. A carries a +5q charge. B carries a -q charge. C carries no net charge. Spheres A and B are touches together and then seperated. Sphere C is then touched to Sphere ...
0
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0answers
24 views

Mass-velocity-relation for tachyon

Suppose that $m = \gamma m_0$ with $$\gamma = \left(1-\frac{u^2}{c^2}\right)^{-\frac{1}{2}}$$ is valid for tachyons, where $m$ is the mass of the tachyon and $u$ its velocity. Since $u>c$, the rest ...
1
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1answer
28 views

Neutrino Reaction: Is the Following Reaction Allowed?

Is the following reaction allowed and why? $$ \nu_e \to e^- + \mu^+ + \nu_{\mu} $$ I would say it is allowed since individual lepton number and charge are conserved.
1
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3answers
42 views

How to find stopping distance of a car? [on hold]

I am trying to calculate minimum stopping distance of a car once the brakes are applied. I know the velocity at braking instant, i also know the distance from the obstacle. I found out that $$ d = ...
1
vote
1answer
33 views

Optics problem: Determining the new focal length when changing from film to CCD detector [on hold]

An older camera has a lens with a focal length of 50mm and uses 34-mm-wide film to record its images. Using this camera, a photographer takes a picture of the Golden Gate Bridge that completely ...
0
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0answers
60 views

In every glass of water there are water molecules that came through Hitler's urinary bladder? [on hold]

I was told that in every glass of water there are water molecules that came through Hitler's urinary bladder. How much it is true probabilistically?
2
votes
1answer
66 views

Why is it difficult to cook food on mountains?

Why is it difficult to cook food on mountains? Is it because on increasing pressure boiling point of substance increase and on mountains there is less pressure so less boiling point. Then shouldn't ...
1
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1answer
24 views

Mass Moment of Inertia of combined objects

I have a small circular ring placed on a circular disc. The outer radius of the ring is same as the radius of the disc. I have worked out the difference of Moment of Inertia of the combined shape ...
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0answers
17 views

Flow through a packed bed

I'm given a situation where a horizontal tube is filled with metal oxide particles and a reducing gas is passed through it. I'm given all the parameters of the system, including the maximum pressure ...
2
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1answer
92 views

Missing a factor of $\sqrt{\frac{\hbar}{m}}$ in a QFT Practice Problem. Can someone explain why?

I am doing problem 2.3 on page 27 of Quantum Field Theory for the Gifted Amateur. Use eqns 2.46 and 2.62 to show that \begin{equation} \hat{x}_j = \frac{1}{\sqrt{N}} ...
0
votes
0answers
51 views

Precession due to mountain on Earth [on hold]

I've been stuck on this homework problem, and I could really use some help. My thought process is that when the mountain is added, the Earth will "tip" down in such a way so that the axis of ...
1
vote
1answer
35 views

Moment of Inertia: uniform rigid rod on smooth plane [on hold]

Consider a rod of length $b$ and mass $m$ on a smooth horizontal plane. A force is applied to one end of the rod. What is the acceleration $a$ and angular acceleration $\alpha$ of the other end of ...
0
votes
1answer
43 views

Fermions in a well

I have two identical fermions in an infinite potential well. They are non-interacting. How should I show that the first excited state is four-fold degenerate? Is the wavefunction just the ...
1
vote
2answers
44 views

Moment of inertia for a triangle? [on hold]

First, I fully admit this is one of the physics problems that we have as a homework. However, I am totally clueless how I am supposed to solve this. I do have an answer that I am supposed to get, but ...
2
votes
1answer
37 views

Computing the gravitational force on a planet in a particular system [on hold]

I have a system of four planets moving in a 2D plane. I'm trying to write some code (C++) to find the positions of these planets at time t=3. I'm probably going to attempt this via a leapfrog ...
0
votes
1answer
13 views

How to calculate surface density of conducting plates, given zero potential condition and another conducting plate [on hold]

Suppose there are three conducting plates at $z=6,3,0$(m) surface. The surface at $z=3$ has static surface-charge of $2C/m^2$, and the surfaces at $z=6$ and $z=0$ have zero electrostatic potential. ...
0
votes
1answer
29 views

Optics (rainbows) [on hold]

A passenger in an aeroplane shall (a) never see a rainbow. (b) may see a primary and a secondary rainbow as concentric circles. (c) may see a primary and a secondary rainbow as concentric arcs. ...
1
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2answers
32 views

Uniformly Accelerated Motion question

A ballast bag is dropped from a balloon that is 300 m above the ground and rising at 13 m/s. For the bag, find the maximum height reached. Now the book gives me this answer - $V_i$ = initial ...
0
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2answers
18 views

How can I use volume charge density to find charge density constant knowing charge Q

I have an insulating sphere of radius R that has a total charge Q. It is distributed as $\rho$= $\rho_0*r^4$. I know that the charge is equal to Q and I'm trying to solve for $\rho_0$. I've tried to ...
0
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0answers
28 views

Rolling w/o slipping - does a friction force acts in this scenario?

Assume the following scenario of rolling without slipping: A small disk starts rolling w/o slipping at $\vec{r}_{CM}=-9R\hat{x}$ (scenario A) and reaches the bottom of the large hollow disk ...
0
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0answers
21 views

Energy required to move air through 50x50x50 hanger at 25mph

I've been trying to calculate how much energy it would take (without losses) to continually move a 50x50x50 meter blob of air through a hanger of the same dimensions. I came up with 620 watts which is ...
1
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2answers
91 views

Problem with an exercise: position and velocity of a thrown sphere with non uniform acceleration

You stand at the earth's surface, have an object, for example a sphere, of mass $ m_1 $ and throw it with a velocity $ v_1 $ into the atmosphere, perpendicular to the surface. Let $g$ at the surface ...
0
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0answers
24 views

Change in effective gravity due to a B-field

I have a bar magnet being suspended in a B-field at an angle $\frac{\pi}{2}$ (45 degrees). If I change the frame of reference such that the bar magnet is at 0 degrees I intuitively know that the new ...
0
votes
1answer
72 views

How do I do this loop integral?

I'm having some trouble doing a loop integral. It is the loop integral that gives neutrinos their mass in the famous Zee model (http://dx.doi.org/10.1016/0370-2693(80)90349-4). The integral is $$ ...
1
vote
1answer
51 views

Indexed Gradient operator on trigonometric functions

$$\nabla_{i}\nabla_{j}\Big(\frac{\sin(kR)}{R}\Big)$$ Where $R$ is the distance between particle $i,j$. And $k$ is a constant I took $\nabla_{i}=\frac{\partial}{\partial R_{i}}$ and ...
1
vote
1answer
47 views

Given potentials, how does one find conserved quantities using Noether's theorem?

I've been asked to find the conserved quantities of the following 3D potentials: $U(\vec{r}) = U(x^2)$, $U(\vec{r}) = U(x^2 + y^2)$ and $U(\vec{r}) = U(x^2 + y^2 + z^2)$. For the first one, ...
0
votes
1answer
37 views

Lasers : Threshold Pump Power for Laser Oscillation [on hold]

I was working my way through some basic laser problems , when I cam across this one : Consider the ruby laser for which we have the following values of the various parameters: $N =$ $1.6$ x ...
0
votes
2answers
49 views

How to analyse this mass-spring system

I'm trying to analyze this mass-spring system -- i.e. write down the differential equation governing it. As you can see, there is a block of mass $m_1$ attached to a wall by an ideal spring of ...
0
votes
1answer
34 views

Potential Energy of Interaction Between a Sphere and a Particle Formula Derivation [on hold]

A sphere of radius R has density described by ρ=ρ(r). Derive equation for pontetial energy of interaction between the sphere and some point particle of mass m which is at distance r from the center of ...
0
votes
0answers
21 views

Tank draining with piston

I am trying to solve a problem which is close to the tank draining (as here Fuel tank draining) but with a piston a the top of the tank which means $p_A$ different of $p_B$. I search the velocity ...
1
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2answers
73 views

Variation of a term in the Lagrangian

I don't understand why $$\frac{\delta}{\delta\phi}\left(\frac12\partial^\mu\phi\partial_\mu\phi\right)~=~\partial^\mu\partial_\mu\phi.\tag{1}$$ If we use integration by parts, there should be a minus ...
0
votes
1answer
69 views

Second Quantisation, Fourier Transform, minus sign [on hold]

I want to expand a field \begin{equation} \Phi (x) = \int \frac{d^3 p}{(2 \pi)^3} e^{ipx} \end{equation} in terms of the second quantisation \begin{equation} \Phi = \frac{1}{\sqrt{2 E}} (a + ...
1
vote
1answer
57 views

Can two bodies move with equal acceleration if forces acting on both are unequal?

In my textbook there is a case given that is to find acceleration and tension produced in a string passed over a frictionless pulley and attached with bodies of different masses at its ends. Now below ...
0
votes
0answers
19 views

rotational motion of a rigid body [on hold]

Find the ratio of radius of gyration of a ring about an axis passing through the ring center, and that of a disc about an axis passing through the disk center, if both the ring and the disc have same ...
0
votes
0answers
23 views

About acceleration in cylindrical coordinates [on hold]

The question is attached as a pic I can't seem to find $\ddot r$ and $\ddot\theta$. Assuming I already got $\dot r$ and $\dot\theta$ (the answers are written after the question). The idea I tried ...