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Questions tagged [homework-and-exercises]

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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14 views

Calculating RPM of a motor

I am trying to calculate the optimal RPM of a DC motor in order to move a car. I know that the force exerted on the wheel by the motor is: $$ F=9N $$ I also know the following information ("h" ...
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1answer
39 views

Who will win the race?

There is a wedge which is inclined at an angle X to the horizontal.Supppose two carts of same mass M one having 3 wheels and the other having 4 wheels are released from rest at the top of the incline,...
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1answer
155 views

Transformation of a Lagrangian

$$L(\lambda,\mu,\dot{\lambda},\dot{\mu})=\frac{m}{2}(\lambda^2+\mu^2)(\dot{\lambda}^2+\dot{\mu}^2)-\alpha \lambda^2\mu^2,$$ I'm supposed to express this Lagrangian through $x=\lambda^2-\mu^2$ $y=2\...
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2answers
19 views

Why are there 6 interactions with 4 positive charges?

From a homework assignment, there are 4 spheres spaced 1cm apart. Each of the spheres are charged to +10nC and weigh 1 gram. The question wants us to find the final speed of the charges once they've ...
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1answer
19 views

If the cross-sectional area of a wire doubles shouldn't the current also double?

In this question the answer is B but I don't understand how that could be possible considering the area has doubled.
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2answers
651 views

Express Laplace transform of voltage across a capacitor in terms of charge

In Charge Tunneling Rates in Ultrasmall Junctions section 2.1, the authors consider the problem of charge relaxation in a simple circuit shown in Figure A. The implicitly use an assumption made about ...
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0answers
30 views

How to calculate and express $*d*F$ in index notation?

Gravitation by Charles W. Misner, Kip Throne and John Wheeler page 120 Box 4.4 Duality Plus Exterior Differentiation, related exercise 3.13, 3.17 I want to calculate the component of $*d*F$ in index ...
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0answers
15 views

How to find the ideal intersection between constant object and controllable object?

So I'm an engineer who's a bit out of practice, but have taken up a side project for entertainment purposes. I'm trying to teach myself to code and find myself struggling to come up with a solution ...
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9 views

Physics Superposition

If the width of both slits is increased without changing their separation of a. State the effect of the brightness of the light fringes and dark fringe. The brightness of the light fringe will ...
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3answers
320 views

Equation of motion - choosing directions for the coordinate axis

I'm not sure this equation is the right one if we invert the direction of the positive $y$-axis. Am I right if we assume the positive direction of the $y$-axis downwards, that the correct equation of ...
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0answers
16 views

The acceleration needed to stop a car in time [on hold]

Say you are driving at the speed of 30 m/s, when suddenly a car in front of you, which was previously traveling at the same speed, brakes. If you press your brakes 0.455 s later, what would the ...
4
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2answers
97 views

A bead on the wire loop

In my physics book there is a question which stated is as follows: A thin circular loop of radius $R$ rotates about its vertical diameter with an angular frequency $\omega$. Show that a small bead ...
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1answer
29 views

Finding the maximum shear value acting on a beam [on hold]

Here is the setup if the beam: As I cut the beam at 4m from the left(fixed wall). It gave a shear force of zero. If I cut it 3m from the right I get a shesr force of 5kN. Well I am not totally sure ...
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1answer
39 views

What are the energy eigenstates for a modified quantum harmonic oscillator?

Imagine a particle obeying Schrodinger's Equation with an harmonic oscillator potential modified with an additional linear potential and cut off with an infinite potential barrier at $x=0$. That is, $$...
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0answers
28 views

Magnetic field inside hollow cylinder [on hold]

A hollow thin walled conducting long cylinder of radius $R$ is carrying current I along its length. A thin slit of width $h$ is cut along the length of cylinder. What is the magnetic field just inside ...
8
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2answers
484 views

Existence proof of Lorentz transformation from lightlike to lightlike vectors

This is a question that might have quiet an easy answer and intuitively has an easy solution, but I struggle a bit with the strict mathematical proof. The statement is relatively simple: Let $t^\mu,...
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2answers
421 views

Perturbation theory with infinite potential

I'm trying to solve an excercise that involves first order perturbation theory and an infinite potential. To ease the problem, I tried to consider an easier one dimensional model. Consider an infinite ...
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1answer
59 views

Theory that we cannot observe everything

If I remember physics class correctly, and if not feel free to correct me, there is a theory that shows that we can not observe everything. I think the example given was energy and space. So either we ...
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0answers
11 views

Problem in Sequential Digital circuit [on hold]

i want to know that for this circuit do we have to put values of x from least significant digit and also as i have done my calculation can anyone verify the value of z. These are the values i had ...
1
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1answer
42 views

Prove that the Quantum Fourier Transform satisfies $F_N(\sum_x |xr \rangle) \simeq \sum_y |y N/r \rangle $ when $r$ divides $N$

Before I start, this is a homework question, but I have spent the last 7 hours on it and gotten nowhere. I will mention what I've tried and noticed, but it's not much. At this point, I'm wondering if ...
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0answers
18 views

Terminal velocity and distance traveled of a sphere in a liquid

I have actually done this question, but I think it's wrong. The terminal velocity I got was 6000 m/s and a distance traveled of approximately 4000 km. The question is: A sphere with radius R = 5.0 cm ...
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0answers
30 views

How to derive the group velocity in a dispersive medium and how to interpret it in momentum space?

:) I had this homework where I was supposed to derive the group velocity of a given wave packet $$\psi (x,t) = \frac{1}{\sqrt{2\pi}}\int_{-\infty }^{\infty}dk g(k)e^{i(\omega t-kx)}$$ where $$g(k)=e^{-...
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0answers
30 views

How do you determine whether answer is A or D? [on hold]

How do you tell the difference between graphs A/D from t=0 to their maximum postions given this eqaution.
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0answers
38 views

Write Equations of motion and a phase portrait of a point particle [on hold]

A point particle with mass m = 1 moves along the X axis under the the force $f (x) = x^3 + x^4$. Write the equation of motion of the point and draw a phase portrait. What I have so far: So, $f = m\...
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0answers
48 views

Frame of Reference in Kinematics

If we have two frames of reference, say S1 and S2, and the acceleration of a particle as seen from both the frames has equal magnitude, say 4 m/s^2, then shouldn't the frames have no relative ...
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1answer
601 views

Tension in horizontal circular motion of a non-uniform object (friction negligible)

A rod of total mass $M$ and length $x$ has its tip attached to a string of negligible mass, so it spins in a circular motion horizontally free from friction with constant tangential velocity. I have ...
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2answers
261 views

Variation of a field action

The field action in flat spacetime is $$ S = \int d^4x\, \mathcal{L}(\phi,\partial_\mu\phi).\tag{1} $$ The variation in $S$ leads to $$ \delta S = \int d^4x\, \delta \mathcal{L}. \tag{2} $$ ...
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1answer
148 views

Isotropy subgroup of a non-zero vector in 3D space

Recentely i got an assignment for a course of group theory applied to physics, in which i had to find the isotropy group of a vector $v=(1,0,0)^T$ under the euclidean group $E=\mathbb{R^3}\ltimes O(3)$...
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0answers
17 views

Physics for resistors in parallel which resisto is the warmest [on hold]

for all resistors in parallel, which of the three became warmest when a voltage is applied? Explain your reasoning. Rhigh 9930 ohms, Rm 501 ohms, and Rlow is 17 ohms
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1answer
56 views

Finding two masses as one rotates about an axis, no givens

My Physics C class was given a lab today with little explanation, and many of us are having issues determining a solution. We are given this apparatus: We must find the masses of both objects m and ...
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2answers
38 views

Calculating the probability of finding particle in a certain excited state of energy in a 1D box

Assume that the wave function of a particle in a one-dimensional rigid box of length L $(V=0$ between $x=0$ and $x=L$ and $V=+\infty$ elsewhere) at time $t=0$, is given by $$\Psi(x, 0) = \left\{ \...
2
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1answer
247 views

Why can't you calculate the absolute magnitude of cepheid variables from apparent magnitude?

I'm currently working on an assignment related to Cepheid Variable stars, and I feel like I've basically gotten all the relevant concepts down pat. However, in the course of my work, I've discovered ...
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0answers
36 views

Find gravitational force of a point that moves along an ellipse [on hold]

A point with a mass of 20 g, that moves under the force of gravity to a fixed center according to Newton’s law of gravity, moving along a complete ellipse with 10cm and 8cm axes for 50 seconds. Find ...
2
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1answer
39 views

Write Equations of motion of a point particle that moves on the surface of a paraboloid [on hold]

I'm not a physics major student and I call myself a total noob in physics, however, I have this issue to solve. The problem is that I don't even know where to start from. One little thing to add: this ...
2
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1answer
312 views

Deriving Rabi oscillations using the Heisenberg picture of QM

The semiclassical treatment of an simple two level atom in a resonant electromagnetic field is usually done in the Schrodinger/Interaction picture of QM, by assuming that the wavefunction of the atom ...
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0answers
56 views

Lagrangian density of Electromagnetism

In this paper (journals.aps.org/prd/abstract/10.1103/PhysRevD.38.478) the author claims that if one uses FRW metric $$ds^2=\sigma^2[-N^2(t)dt^2+a^2(t)d\Omega_3^2]$$ With straightforward manipulation ...
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1answer
22 views

Center of $SU(3)$

I assumed a 3x3 matrix of the form $$A= \begin{pmatrix} a & b & c\\ d & e & f\\ k & l & m \end{pmatrix}$$ Then, since we know that the center is always an Abelian invariant ...
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0answers
18 views

A particle is observed from two frames $S_1$ and $S_2$

The frame $S_2$ moves with respect to $S_1$ with an acceleration a. The $F_1$ and $F_2$ be the pseudo force on the particle when seen from $S_1$ and $S_2$ respectively. My problem is, can such a ...
0
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1answer
29 views

How can I prove Poisson's equation from direct substitution

I’ve got this question where I should take the Laplacian of electric potentials main formula, then it should hold the Poisson’s equation. I’ve somehow gotten close to doing this but I just can’t get ...
3
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2answers
49 views

Differentiation of the determinant $g$

Let $g$ be the determinant of the metric tensor. I want to derive the following equation $g_{,\nu}=gg^{\lambda \mu}g_{\lambda \mu,\nu}$. It is said that $gg^{\lambda \mu}$ is a cofactor, but I can't ...
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0answers
19 views

What would the capacitance of a capacitor be in this scenario? [on hold]

The capacitor consists of two cubes. One big cube with the length of its edge as $3a$. Inside this cube there's a smaller cube with the length of its edge as $a$. The smaller cube is in the ...
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0answers
69 views

Converting velocity vector formula from Cartesian coordinate system to polar coordinate system

I have a little question about converting Velocity formula that is derived as, $$\vec{V}=\frac{d\vec{r}}{dt}=\frac{dx}{dt}\hat{x}+\frac{dy}{dt}\hat{y}+\frac{dz}{dt}\hat{z}$$ in Cartesian Coordinate ...
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0answers
59 views

Proving that the sum of an operator and it's adjoint is Hermitian [on hold]

I can solve the two questions using matrices in math, but we are expected to show it in form of the property of Hermitian operators:$$\int_{-\infty}^{\infty}\Psi^*O\Psi=\int_{-\infty}^{\infty}(O\Psi)^*...
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1answer
202 views

Temperature change of water and the copper can

The question is: "0.38kg of a liquid at 12 degrees C is heated in a copper can weighing 0.9kg by an electrical heater of power 20W for 3 minutes. It reaches a temperature of 17C. Assuming no heat loss ...
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1answer
62 views

How do i calculate the speed of the ball? [on hold]

A spring pushes a 15 grams ball with 10 m/s in a bent pipe. What is the formula to calculate the speed of the ball in point A and point B. The pipe is horizontal so there is no g-force and the pipe ...
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0answers
15 views

A doubt and different approach regarding “A hypothetical magnetic field existing…”

A popular question from the book HC Verma's Concepts of Physics goes like this:- "A hypothetical magnetic field existing in a region is given by $$ \vec{B}=B_{0} \vec{e}_{r} $$ where er denotes the ...
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0answers
41 views

Question is in description [on hold]

If a volleyball gets served at a height of 2ft at 15m/s from one side of a 60ft court over a 7ft net, what angles will make the ball travel the whole court exactly?
0
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0answers
17 views

How to find acceleration with angles included? [on hold]

[![enter preformatted text here][1]][1]How do I find acceleration of an object if I have force, mass, friction, and there is 2 given angles (object and force)? I ...
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0answers
38 views

Average acceleration [on hold]

The space shuttle Discovery has made 30 flights. The space shuttle is approximately 37 meters long and has a wing width of 24 meters. The three main engines of the space shuttle are used when ...
1
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1answer
102 views

Polchinski, Making a good gauge choice

I was reading Polchinski book and there he tries to make a gauge choice to fix the redundacies in the Polyakov action. In order to make this choice, he fixes $$\tau=X^{+}\tag{1.3.8a}$$ and then he ...