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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0
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0answers
10 views

Finding final velocity of neutron [on hold]

A neutron moves with velocity $u$. It collides head-on with a stationary nitrogen nucleus. The collision is elastic. The mass of a nitrogen nucleus is 14 times that of a neutron. After collision ...
0
votes
1answer
11 views

A massive slider starts from rest from a point S (which is at the same height as a point T at the top of the track)

This is a question from previous physics aptitude test. A massive slider starts from rest from a point S (which is at the same height as a point T at the top of the track) and slides along a ...
0
votes
2answers
25 views

Problem in understanding Newtonian 2nd law

In Resnick, Halliday, Walker s' Principle of Physics , I found this question : Two forces are acting on a block on a frictionless floor. On the left of the block $3~\text{N}$ & on the right of ...
5
votes
1answer
48 views

How to use Euler-Lagrange when Lagrangian is $L=\sqrt{t}\sqrt{1+(dy/dt)^2}$

In this Lagrangian problem, action is $$S = \int_{t_1}^{t_2} \sqrt{t}\sqrt{1+\dot{y}^2} \,\,dt$$ where $\dot{y} = dy/dt$ and $t_1$ and $t_2$ are some fixed points. I tried to solve this problem using ...
0
votes
1answer
27 views

Covariant derivative tensor [on hold]

Given that $T^{αβ}{_γ}^δ{_σ}$ is a tensor, I worked the covariant derivative as: $T^{αβ}{_γ}^δ{_σ} + Γ^{α}{_{ρσ}}T^{ρβ}{_γ}^δ + Γ^{β}{_{ρσ}}T^{αρ}{_γ}^δ - Γ^{ρ}{_{γσ}}T^{αβ}{_ρ}^δ + ...
-1
votes
0answers
22 views

Screening in semiconductor

how can we get the following solution for the screened potential of a point charge Q: by using the following Thomas-Fermi equation as: where:
0
votes
1answer
38 views

Get angle to hit (x,y) with vertex [on hold]

In a projectile trajectory, I'm using the following equation to get the angle required to hit a target in x,y coordinates: This works great, however, it implies that the target is reached by one of ...
0
votes
2answers
31 views

Using the force law to obtain total energy of an electron as a function of its radius

I am working on a problem which starts saying determine the total energy of a hydrogen atom with an electron moving with momentum $p$ at a radius $r$. For that part I got: $E = \frac{p^2}{2m_e} - ...
0
votes
0answers
26 views

Boundary problem with forced vibration [on hold]

Consider the forced vibration of a string governed by $$\frac{\partial^2 u}{\partial t^2} = \frac{\partial^2 u}{\partial x^2} + Q(x,t) $$ subject to the boundary conditions $$u(0,t) = A(t)$$ $$ ...
1
vote
1answer
18 views

Are muon energies quantized?

I'm working on a problem that asks: For a central charge $Ze$, obtain an expression for the radius $r_n$ of the $n$th muonic orbit. Express this as a multiple of the radius $a_o$ of the first Bohr ...
0
votes
1answer
34 views

Force on ball going around loop [on hold]

Suppose I have a ball of mass m going around a loop. It is radius $r$, and rolling without slipping. Half way up the loop, what is the force in the $x$ an $y$ direction if it is at velocity $V$? I ...
2
votes
1answer
55 views

Representations of Galilei group

Show that the operator $U(\alpha, \beta) = e^{i(\alpha \hat{x}^2 + \beta \hat{p}_{x}^2)}$ can represent the space reflection of the 1D Galilei group: $x \to -x; t \to t$. I don't really know ...
0
votes
1answer
28 views

Parallel Resistors (special cases formula)

1) A car's rear window defroster uses $n=15$ strips of resistive wire in a parallel arrangement. If the total resistance is 1.4 ohms, what is the resistance of one wire? Solution: I rearranged ...
3
votes
1answer
61 views

Bosonic Schrödinger field [on hold]

When second quantizating the Schrödinger field $$\psi(r,t) = \sum_i \phi_i(r)b_i(t),\quad\mbox{and}\quad \psi^{\dagger}(r,t) = \sum_i \phi_{i}(r)^* b_i^{\dagger}(t),$$ we have the commutation ...
1
vote
1answer
29 views

Work energy and laws of motion and friction problem?

I am having a confusion in the following question- Q-To stimulate car accidents auto manufacturers study the collision of moving cars with mounted springs of different spring constants. consider a ...
0
votes
2answers
47 views

What is a CCD structure?

In a exam paper for entry into Oxford University for physics, I came accross something which I hadn't met before, and I have no idea what the question is asking - none of my teachers could explain it ...
-1
votes
1answer
33 views

Question on kinetic energy, mass and velocity [on hold]

Two bodies have kinetic energies in the ratio of 4:1 They are moving with equal linear momentum. What is the ratio of their masses. My answer : I get $$\frac{m_1}{m_2}=\frac{4v^{2}_2}{v^{2}_1}$$
0
votes
1answer
27 views

Power - 1/2 Force x velocity? [on hold]

A pump, taking water from a large reservoir, is used to spray a jet of water with speed 20m/s. The nozzle of the pipe has radius 0.05m, and is level with the surface of the reservoir (no change in ...
-2
votes
0answers
23 views

laplace equation [on hold]

Consider $u(x,y)$ satisfying Laplace’s equation inside a rectangle $R=\{0\leq x \leq L, 0\leq y\leq H\}$ subject to the boundary conditions $$\begin{array}{rclcccrcl} \frac{\partial u}{\partial ...
0
votes
1answer
50 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
2answers
54 views

Nondimensionalization of a simple system [on hold]

A damped spring mass system is modelled below: $$m\frac{d^2y}{dt^2}=F_s+F_d\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space t>0$$ ...
0
votes
0answers
23 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
25 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
33 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
3answers
65 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
22 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
79 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
40 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
38 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 ...
1
vote
5answers
152 views
+200

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
39 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
28 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
15 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
40 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
49 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
51 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 ...
0
votes
3answers
75 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
29 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
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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
31 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
55 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
43 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
35 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
22 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?