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1answer
464 views

Calculating torque with gears and wheel-axles

For my homework in engineering, I was given a question like this: There is an axle attached to a gear, which is meshed with another gear, which has an axle attached to it. I am given the diameter of ...
2
votes
2answers
305 views

Sum of all forces

Let us glue up these two images, where we get closed loop thrust of water. Force $F_3$ has direction $-x$ and force $F_2$ has $x$ direction. What is the sum of all forces? Can it be more than zero? ...
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0answers
55 views

First-order pertubation theory

I'm having some trouble figuring this out, so I was hoping someone could help. I need to show that the first-order pertubation of the ground state energy is not changed by the pertubation $H'$, given ...
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1answer
28 views

Averaging energy density

Let we define energy density by, $$E= a^2 \cos2 t +b^2 \sin2 t+ c^2 \cos 3t$$ where, $E=$ total energy, $a,b,c =$constants, and $t$ is the time. I'm confused from someone's statement that ` There ...
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3answers
424 views

waves on water generated by a falling object

Let an object of mass $m$ and volume $v$ be dropped in water from height $h$, and $a$ be the amplitude of the wave generated. What is the relation between $a$ and $h$. How many waves are generated? ...
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2answers
356 views

How to find $\tau$ in this circuit?

1) 2) I know, that $$ τ = \frac{L}{R} $$ but what is $R$ in this formula? It seems to be the total resistance, but how to find it in 1) and 2)?
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1answer
260 views

Electromagnetic Tensor in Cylindrical Coordinates

I understand that the Electromagnetic Tensor is given by $$F^{\mu\nu}\mapsto\begin{pmatrix}0 & -E_{x} & -E_{y} & -E_{z}\\ E_{x} & 0 & -B_{z} & B_{y}\\ E_{y} & B_{z} & ...
0
votes
1answer
50 views

Total current of two sources in series? [closed]

Here is a question that a friend asked me. He had to an experiment in school and do some calculations afterwards. Those calculations require maximal current that the DC source can produce. He has ...
0
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1answer
202 views

Vectors on Different Coordinate Systems? [closed]

Consider the product of vectors coordinated relative to a given coordinate frame, defined by $$\vec{a}\square\vec{b}=((a_{1},b_{1})\square(a_{2},b_{2})):=(a_{1}b_{1},a_{2}b_{2})$$ Explain why ...
1
vote
1answer
207 views

Curvature tensor of 2-sphere using exterior differential forms (tetrads)

$ds^2= r^2 (d\theta^2 + \sin^2{\theta}d\phi^2)$ The following is the tetrad basis $e^{\theta}=r d{\theta} \,\,\,\,\,\,\,\,\,\, e^{\phi}=r \sin{\theta} d{\phi}$ Hence, $de^{\theta}=0 ...
-1
votes
1answer
302 views

Newtons Cannon ball problem [closed]

Let's I have a Newton's Cannon ball of 15 kg and want to fire so it will round the earth and back to me. My query is which law can evaluate the speed that cannon ball will revolve round the sun ...
5
votes
3answers
1k views

Moment of Inertia (triangular plate)

I want to generalize the formula for the MOI of a triangular plate (sides $a,b,c$) about an axis passing through mid point of one sides and perpendicular to it's plane . The mass of plate $M$ is ...
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3answers
301 views

Which quantity gives the resistance of a component?

In a current vs potential difference graph, we can obtain the value of the resistance of the component. There are books that say gradient-inverse is the resistance and also books that say the value of ...
1
vote
1answer
142 views

B-field and Magnetic forces, speed of a particle

Let's say I have a particle moving perpendicular to a uniform magnetic field of magnitude $x \ T$, and it moves in circle with a fixed radius. How do I find the speed of this electron? Initially I ...
-1
votes
1answer
88 views

Vectors and motion in a plane [closed]

A particle travels with speed $50 m/s$ from the point $(3,-7)$ in the direction $7i-24j$ . Find its positional vector after 3 seconds. My approach: It has travelled a distance of 150m in the ...
8
votes
4answers
5k views

Weight on carpet vs. hard floor

Will the scale say I weigh more, less, or the same on a carpet as compared to a hard floor? You can assume the scale works via spring mechanism. Free body diagrams encouraged!
3
votes
1answer
154 views

What is the direction of an accelerated particle due to a variation of magnetic field?

Let an ion of charge $q$ and mass $m$ traverse a circular orbit of radius $R$ under the influence of a uniform magnetic field $\mathbf{B}=B(r)\,\hat{\boldsymbol{z}}$, where $r$ is the distance from ...
0
votes
1answer
64 views

Einstein-de Sitter - confusing constant of integration

I'm struggling to understand this derivation of the Einstein-de Sitter universe model It starts with $$\int R^{1/2}dR=\int H_{0}R_{0}^{3/2}dt.$$ Evaluating this integral ...
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votes
2answers
1k views

Electric Field Lines between two non parallel plates [closed]

A potential difference is applied between two metal plates that are not parallel. Which diagram shows the electric field between the plates? A is the answer BUT WHY explanation needed !
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2answers
248 views

A small oscillations of a rod on the cylinder

Let's have the next case. A rod (with mass $m$, length $L$ and a momentum of inertia $I$) at the initial time is located on a cylinder (with radius $R$) surface so that it's (rod's) center of mass ...
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2answers
149 views

Is my physics correct or torque correct?

I would like to put this into a differential equation. This is what I have. $$r \times F = I \ddot\theta + \mu \dot\theta + k \theta$$ What I need verified: $\text{Torque} = I\ddot\theta + \mu ...
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2answers
159 views

Field lines question

A friend of mine asked me this question, that is asked in an entrance examination. It shouldn't be that difficult, but I fail to find a rigorous answer for it. The figure shows three charges, that ...
0
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0answers
436 views

Density of states of a photon gas in volume V and temperature T

I have a question on the density of states for a photon gas: Suppose I have a photon gas in a box of volume $V$ at temperature $T$. If I enumerate the total number of states accessible to the system ...
1
vote
3answers
496 views

Maximum angular velocity to stop in one rotation with a known torque

I have an object I can rotate with a given torque. I would like to stop applying torque once I've reached a defined maximum rotational speed. The maximum rotational speed should be defined so that ...
1
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3answers
144 views

Doubt in Kinematics

I know that this isn't the place for such basic questions, but I didn't find the answer to this anywhere else. It's pretty simple: some particle moves in straight line under constant acceleration from ...
1
vote
1answer
271 views

Infinite Potential Well Energy for Piece-wise Constant Wave Function

I'm trying to compute the expectation value of energy for a certain state in an infinite potential well but I'm getting contradictory answers. The well has potential \begin{align} V(x) = \left\{ ...
1
vote
1answer
260 views

What is a “lateral fringe displacement”?

I encountered the term "lateral fridge displacement" in my optics homework for a problem about inserting a thin plate of glass over one of Young's double splits. So what does "lateral fringe ...
-2
votes
1answer
131 views

Simple pendulum and planet mass [closed]

A simple pendulum, $20cm$ in length, has the period of $2.7s$ on a certain planet. Find the mass of the planet if its diameter is $18000km$. $G=6.67\times10^{-11}Nm^2/kg^2 $ I have no idea how to get ...
5
votes
2answers
149 views

Generalizing a relativistic kinematics formula for spatial-acceleration dependence

I'm starting from this expression $$ \alpha dt = \gamma^3 dv $$ where $\alpha$ is proper acceleration of a point particle, $dv$ and $dt$ are coordinate differentials of velocity and time, and ...
1
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2answers
186 views

Velocity of an object undergoing homogenous acceleration

So I was considering the following problem within the context of Special Relativity: Given an object O, with initial velocity v, undergoing constant acceleration at a rate of a, I want to express the ...
1
vote
0answers
78 views

A quicker way to verify that a function is a constant of motion?

I have three particles that we can indicate with $\alpha$ ($\alpha$=0,1,2), they are identified by the $r^i_\alpha$ coordinates and $p^\beta_j$ conjugata momenta ($\beta=0,1,2$ and $i,j=1,2,3$). I ...
3
votes
1answer
504 views

Problem with calculating the curvature tensor of the $n$ dimensional sphere

I am calculating the Riemann curvature tensor, Ricci curvature tensor, and Ricci scalar of the $n$ sphere $$x_0^2 + x_1^2 + ....+x_n^2=R^2,$$ whose metric is $$ds^2=R^2(d\phi_1^2 + \sin{\phi_1}^2 ...
0
votes
1answer
470 views

Bouncing ball simulation computer science

In my Computer science class I was given a problem where I have to simulate a bouncing ball using "real physics". I have been trying to find a equation that will simulate the height of the bounce ...
1
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2answers
442 views

1 dimensional Ising model

How to solve the Ising model in 1D by low temperature, and high temperature expansion, and by change of variable method? Can you please give me some reference links?
4
votes
1answer
245 views

Firing machine question

Suppose we have a firing machine on a frictionless surface at point $x=0$. It fires a bullet of mass $m$ every $T$ seconds. Each bullet has the same constant velocity $v_0$. There's a body of mass ...
1
vote
0answers
52 views

Hamiltonian system: match transformations and constants of motion

I have a problem about the interpretation of an exercise. Given the following Hamiltonian $$H=\frac{\mathbf{p_0}^2}{2m}+\frac{\mathbf{p_1}^2}{2m}+\frac{\mathbf{p_2}^2}{2m}-2V(\mathbf{r_1}- ...
3
votes
1answer
114 views

Two masses on rope spinning around

Two balls of the same mass $m$ are connected to each other with rope of length $l$. One of the balls is also connected to the ceiling with a rope of the same length $l$. The balls are spinning ...
0
votes
1answer
604 views

How to find constant of motion for Hamiltonian system?

I have to find a constant of motion associated to this Hamiltonian but I don't know how to proceed. $$H=\frac{\mathbf{p_0}^2}{2m}+\frac{\mathbf{p_1}^2}{2m}+\frac{\mathbf{p_2}^2}{2m}-2V(\mathbf{r_1}- ...
3
votes
1answer
91 views

Interaction of an electromagnetic wave with a two level system in the domain of quantum field theory

Suppose I shine an electromagnetic wave on a two-level system. I need to describe how the system evolves in context of quantum field theory i.e. using a quantized EM field in the problem. The first ...
1
vote
1answer
402 views

Electric field of a flat metal plate and a point particle

I'm currently studying electric potential, and I'm having trouble with one of the problems on my homework: A) A point particle with charge $+q$ is on the x-axis at a distance $d$ from the origin, ...
6
votes
3answers
278 views

Induced current using a reference system bound with a moving charge

Suppose we have a charge moving at velocity $\mathbf{v}$ in the same plane of a square wire. If I sit in a reference frame where the square wire is still, since the charge is moving with velocity ...
1
vote
1answer
201 views

A block, a string and Newton's third law

So this is a general force diagram of the system shown. My question is, according to the third law, if the block is exerting a force of magnitude mg on the thread in the down direction, then the ...
2
votes
2answers
2k views

Dimensional Analysis: Buckingham Pi Theorem

I am studying for a fluids quiz and I am having a few problems relating to dimensional analysis but for the time being fundamentally I have a problem selecting the repeating variables. Like does ...
0
votes
2answers
111 views

Hamiltonian equations: can I divide a solution of motion for a constant?

I'm solving an exercise about Hamiltonian equations. I have followed the proceeding below. The results given by the book are different to mine because its first result is the half of mine (and the ...
1
vote
1answer
161 views

Bose–Einstein statistics exercise

I've a basic Bose–Einstein statistics exercise. I've tried to solve it in two ways, but each way gives a different result. We have $n$ identical bosons without interactions at temperature $T$. There ...
2
votes
0answers
228 views

Trouble with calculating Christoffel symbols of FLRW metric using Lagrangian method

The FLRW metric which I am using is $$ds^2 = dt^2 - \frac{a(t)^2}{c^2} \left( dx^2 + dy^2 + dz^2 \right)$$ where $a(t)$ is the so-called 'scale factor'. I did not want to calculate the Christoffel ...
3
votes
2answers
209 views

Frictional Forces

In the figure, blocks A and B have weights of 45 N and 23 N, respectively. (a) Determine the minimum weight of block C to keep A from sliding if μs between A and the table is 0.21. (b) Block C ...
3
votes
0answers
201 views

Finding the terminal velocity of a magnet dropped in a solenoid

We have to find proportionality of the terminal velocity with the factors of the system: Plot: a small dipole(mass $m$) with dipole moment $\mu$ is dropped in a long solenoid (radius $r$, ...
3
votes
1answer
792 views

Electric field a distance $z$ above the center of a circular loop. The Hard way [closed]

Problem 2.5: Find the electric field a distance $z$ above the center of a circular loop of radius $r$ which carries a uniform line charge $\lambda$. This problem is in refereced here (with ...
0
votes
2answers
94 views

3D Vector Rotation of Point Mass

Triangle defined by points OA, OB and OC : (-i,3 j,-4 k), (i,2 j,2 k) and (3 i,7 j,- k) where i, j, k are unit vectors along x,y,z axis. Point mass is placed at OA. Triangle rotates with angular ...