Tagged Questions

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
10 views

laplace equation

Consider u ( x,y ) satisfying Laplace’s equation inside a rectangle 0 ≤ x ≤ L , 0 ≤ y ≤ H subject to the boundary conditions ∂u ∂x (0 ,y ) = 0 , ∂u ∂x ( L,y ) = g ( y ) , ∂u ∂y ( x, 0) = 0 , ∂u ∂y ( ...
0
votes
0answers
13 views

Boundary value problem

Consider the boundary value problem \begin{align} \frac{du}{dt}&= \frac{d^2u}{dx^2} , \\ u(0,t)&=0 \\ u(L,t)&=0 \\ u(x,0)&=f(x) \end{align} I know how to solve it using ...
0
votes
0answers
21 views

Help with 4-velocity [on hold]

The 4-velocity $\overline{U}=(U^0,U^1,U^2,U^3)$ corresponds to the 3-velocity, $\underline{v}$, in the sense that $\overline{U}=\gamma(v)(1,\underline{v})$. Express (a) $U^0$ in terms of ...
0
votes
1answer
20 views

Vertical motion initial velocity given max height

I'm having a hard time finding a formula that calculates the initial vertical velocity for a projectile that should reach a certain height. I'm using this formula to calculate both vertical and ...
0
votes
0answers
5 views

Minimum deviation [on hold]

A ray of light experiences a minimum deviation when passed symmetrically through an equilateral triangular prism. Calculate the angle of incidence of the ray when the refractive index of the glass is ...
0
votes
0answers
18 views

Forces at each point for a simple truss [on hold]

I have a truss here: It is given that the forces at point 2 are horizontally and vertically. Positive values for lowercase f forces indicate tension, and negative ones indicate compression. I ...
1
vote
1answer
32 views

Work done in case of an accelerating object [on hold]

How do you calculate work when opposing force is less than applied force. So a 5 N static friction force is overcome by 15 N manual force for say 5 meter. What is the work done by manual force?
2
votes
2answers
53 views

Calculating vector to aim for moving asteroid (3D asteroid game)

Considering we're in a 3D system of coordinates: our ship is at point A, motionless our ship can shoot bullets, the speed of which is known the asteroid is at point B the asteroid is moving on known ...
-1
votes
0answers
18 views

Physics question dealing with friction and something else [on hold]

I have 2 questions A 10 kg block is placed on top of a 20 kg block. The top block is then pulled toward the right with a force of 60 n. IF the coefficient of f kinetic friction bewtween the floor ...
0
votes
1answer
47 views

Verifying a solution to Einstein's vacuum field equations

I need to verify a solution to Einsteins vacuum field equations. I have the solution as follows $$ds^2=a\,dt^2+b\,dr^2+\cdots$$ Is the following the right approach? Einsteins equation reduces to ...
-4
votes
0answers
39 views

What can be a differential solution for $\frac{dy}{dt}= -Cy$? [on hold]

which of the following can be a differential solution for $\frac{dy}{dt}= -Cy$ a) $y(t)=2cos(Ct)$ b) $y(t)=5e^{Ct}$ c) $y(t)=5sin(Ct) + 2cos(Ct)$ d) $y(t)=5e^{-Ct}$ e) $y(t)=4sin(Ct)$
0
votes
2answers
37 views

Slope With Varying Kinetic Friction [on hold]

I am stuck on the following problem; On a winter’s day in Maine, a warehouse worker is shoving boxes up a rough plank inclined at an angle α above the horizontal. The plank is partially covered ...
0
votes
1answer
53 views

Is this expression for the kinetic energy of a spinning disk revolving about a second axis correct?

My question is motivated from a question from another user. You can see the configuration of the rotating system here: One disk/ring in double rotation and sum of energy. I am not interested in all ...
0
votes
0answers
15 views

General relativity, expressing a distance in terms of the metric tensor [on hold]

My starting point is: I've been told in the question. The interval between events which belong to the same world line of light equals zero, so I think that means ds = 0. So I now have: From ...
0
votes
3answers
36 views

Derviation of group velocity

I am working thru a derivation of the group velocity formula and I get to this stage: $$y=2A\cos(x\frac{\Delta K}{2} -t\frac{\Delta \omega}{2})\sin( \bar k x-\bar \omega t)$$ Then all the derivations ...
2
votes
2answers
27 views

Bungee jumping - find spring constant

A person of mass $m$ is attached to a spring that its loose length is $l_0$. He is standing on a bridge and jumps off it, so he travels down a distance $2 l_0$ when measured from the bridge up to the ...
0
votes
2answers
34 views

Since no current flows, the voltage is 0?

I didn't understand that , "Since no current flows in the ammeter the potentioal difference is 0 " How did they conclude that? I made this example So i made this example in paints , (R square ...
0
votes
0answers
13 views

Light angles measured in moving reference fame (SR, homework)

I've managed to get through all of this question without trouble until part d). The full question is given here: I've calculated the "true" angles of Star A and Star B as 71.57 degrees and 45 ...
1
vote
1answer
37 views

Circular orbit in Schwarzschild coordinates [on hold]

This was an example in a general relativity textbook which I've been trying to work through myself. A spaceship uses its rocket engine to maintain a circular orbit around a Schwarzschild black hole ...
0
votes
2answers
48 views

What is the jerk due to gravity with ascent? [on hold]

Acceleration is defined as the rate of change of velocity with time. Jerk is defined as the rate of change of acceleration with time. What is the jerk due to gravity with ascent?
0
votes
1answer
48 views

Magnetic levitation frog example

I'm trying to figure out the strength of the magnetic field needed to levitate the frog, assuming it's spherical, has mass $5$kg and diameter $20$cm. I also assume $\vec{B}$ is uniform. Since it's ...
2
votes
0answers
68 views

Bell's Spaceship Paradox

Consider two spaceships a distance $\left|L\right|$ apart, that in the lab frame start from rest at $t = 0$ and then both accelerate with proper constant acceleration $g$. There is a string ...
0
votes
3answers
72 views

How much further can I throw a baseball (148 grams) than a grenade (453 grams)?

Question: Over a flat plane, how much further can I throw a Major League baseball than I can an M67 grenade? Major League baseball: 148 grams M67 frag grenade: 453 grams For a baseline of how ...
0
votes
1answer
25 views

Problem Boltzmann distribution [on hold]

I am trying to solve a problem about boltzmann distribution. If(A)=0.74 Uf(V)=0.037 Ia(A)=0.130 I have to find ...
0
votes
1answer
20 views

Solving kinetic energy exercise through electrical work formula instead of voltage formula?

While studying electric potential, I run into an issue on exercise 43 of Chapter 24 of Fundamentals of Physics 8th ed. Vol. 3 (Halliday et al.). The exercise states the following, paraphrased: A ...
0
votes
0answers
22 views

Boltzmann Distribution [duplicate]

I am trying to solve a problem about boltzmann distribution. If(A)=0.74 Uf(V)=0.037 Ia(A)=0.130 I have to find ...
-1
votes
0answers
30 views

conservation of mechanical energy in presence of an impulse [on hold]

A block of mass $m$ is tied to a string, passing through a pulley of moment of inertia $I$, rests on a support so that the string is slack.The block is lifted to a height $H$ and the support is ...
4
votes
1answer
48 views

Distribution of current of a rotating cone

If I have a hollow cone (surface with no bottom cover ) as the one in the picture. The cone has surface charged density $\sigma$. It rotates around the symmetry axis with an angular velocity ...
0
votes
0answers
42 views

Tensors and Minkowski space time [on hold]

I have a vector field $A^i$ in Minkowski space time whereby I need to determine the following: $$\nu_\mu \frac{\partial \nu^\mu}{\partial t}$$ $$\nu_\nu \nabla \nu^\mu$$ In terms of the gamma ...
0
votes
4answers
33 views

How to find total force on an object?

If there are are a multitude of forces on an object, acting on it in different directions, how do you find the TOTAL force? I know you add up the x components and the y components individually, but ...
1
vote
0answers
34 views

Electric field at a point [on hold]

We consider a thin disk whose center is $O$ and radius is $R$, which has an electric surface charge density of $\sigma$. We want to find the expression of the electric field $\vec{E}$ at a point ...
-1
votes
1answer
21 views

Is the probability density of a finite potential well independent of time or not? [on hold]

The probability of the first excited state of a finite potential well is time independent or not? Why?
2
votes
1answer
43 views

Fermion propagator decomposition

I've seen the following decomposition for the fermion propagator for a fermion with momenta $p-k$, and where both $p-k$ and $p$ have a mass of $m$: $$\frac{(\not p-\not k)+m}{(p-k)^2-m^2}\gamma_\mu= ...
-2
votes
0answers
30 views

Two masses attached by a spring (differential equation) [on hold]

Consider two masses $m_1$ and $m_2$, connected by a spring of spring constant $k$ and an uncompressed length $L$. The masses are on a frictionless surface. Initially, $m_2$ compresses the spring to $L ...
1
vote
2answers
50 views

Determining Acceleration Based On Graph

I understand how to solve this problem, but I am unsure how to generate an equation for the graph (below). My current attempt involves using the mass provided along with the derivative of the line ...
-2
votes
0answers
30 views

Finding the angle of fuzzy dice in a car when deccelerating [on hold]

You are driving at 25m/s when you see Bambi in the road! The coefficient of friction between your tires and the ground is .8. You slam on your brakes praying friction is great enough for Bambi to ...
0
votes
0answers
21 views

Tension - Force Question [on hold]

A 72kg sled is pulled forward from rest by a snowmobile and accelerates at 2.0m/s^2 for 5.0s. The force of friction acting on the sled is 120N [backwards]. The total mass of snowmobile and driver is ...
-5
votes
0answers
24 views

Two blocks on a pulley [on hold]

A block of mass $m_1 = 41.3 \,\rm kg$ on a horizontal surface is connected to a mass $m_2 = 13.2\,\rm kg$ that hangs vertically as shown in the figure below. The two blocks are connected by a string ...
0
votes
0answers
31 views

Angular Momentum Operators confusion [on hold]

I asked a question earlier that now realize sort of made no sense, but that is only because I was combining a few things in my head....this is the question I meant to ask. How would you calculate ...
-4
votes
1answer
155 views

One disk/ring in double rotation and sum of energy

Before start Before $t=0$, I launch grey disk at $w1$ around blue axis (clockwise) and at $w2$ around green axis (anticlockwise). This need energy. Energy of disk is $1/2md²w1²+1/2mr²(w1-w2)²$, ...
0
votes
1answer
18 views

How do I determine the electric charge on two (initially discharged) spheres after an impact with a charged sphere?

This is the first part of a problem on my book: Two identical conducting spheres, initially discharged, of mass $ m = 0.5 \ kg $ come in contact, in consecutive moments, with another sphere, ...
0
votes
3answers
71 views

In this pulley system, if the mass was on the table greater than the handing mass, would tension equal Fg of the hanging mass? [on hold]

If the mass on the table was greater than the hanging mass, would the the tension force equal the force of gravity on the hanging mass?
0
votes
1answer
19 views

Friction and Conservation of Angular Momentum [on hold]

suppose we have 2 discs placed on a smooth horizontal surface.... one of the discs say A is rotating with W angular velocity and is slowly brought near the other disc kept fixed about its centre but ...
0
votes
0answers
38 views

Asymmetrical Newton's Cradle [on hold]

If we have a Newtons Cradle with decreasing ball size how do the small ones behave when hit by the larger ones? Do more balls move with the same velocity, or do a smaller number of balls move with a ...
-3
votes
0answers
28 views

Forces in parallel and perpendicular components [on hold]

Why is this slope 3/4 (written in green) is upside down in a force (written in blue).
0
votes
0answers
33 views

Derivation of wave-velocity from a wave-equation [on hold]

I am asking for a derivation of the wave-velocity $c$ in terms of the compression modulus $K$ and the density $\rho$. The basis for the derivation should be the acoustic 2-way wave-equation in a ...
0
votes
0answers
30 views

Why to write the Navier-Stokes equation with dimensionless quantities?

The Navier-Stokes equation is $$\rho \dfrac{D\mathbf{u}}{Dt} = -\nabla p+(\lambda+\mu)\nabla(\nabla\cdot\mathbf{u})+\mu\nabla^2\mathbf{u}$$ Then if the flow is incomprresible, and the fluid is ...
2
votes
2answers
841 views

How many atoms exist within a continuum body?

Materials, such as solids, liquids and gases, are composed of molecules separated by "empty" space. On a microscopic scale, materials have cracks and discontinuities. However, certain physical ...
2
votes
1answer
26 views

Conservation of energy and inclines

I have question which I'm unsure of whether or not I am thinking about it right. If an object is sliding down frictionless incline and it then comes to the "foot" of the incline where it encounters ...
0
votes
0answers
43 views

Newtonian gravity in 3D space, with 2 point masses, calculus, no approximations [on hold]

I have been trying to find this information all over the net and can't find a straight answer. Here is what I am looking for. How do I take Newton's law of gravity to get final position, velocity, ...