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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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1answer
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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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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0answers
31 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
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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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 ...
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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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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
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 ...
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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
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
31 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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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
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
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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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
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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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 ...
1
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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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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 ...
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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\{ \...
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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 ...
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 ...
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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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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
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
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 ...
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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 ...
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
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?
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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
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
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
34 views

Calculate new hight of a moving object being hit from underneath [on hold]

I have an old exam-question I have truoble understanding. A skeet with mass M = 0.25 kg is fired away with initial speed 25 m/s and 28 degrees. When the skeet has ...
0
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0answers
16 views

Projectile landing at different elevation [on hold]

A ball is kicked at an angle of 30 degrees from the horizontal from point A and lands at Point B which is located 15 ft horizontally ahead and -9 ft vertically. There is also an equation provided, $y=-...
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0answers
48 views

Magnetism question from REE (Roorkee Entrance Examination) 1991 [on hold]

A short magnet of magnetic moment 6 A.m2 is lying in a horizontal plane with its North Pole pointing 60 degrees east of north.Find the net horizontal magnetic field at a point on the axis of the ...
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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
36 views

Line charge inside a conducting sphere? [on hold]

I could not find any example like below. How can we solve this question? I tried to solve like sum of the point charges along the z axes but I am not sure if I am wrong or right. I think the shape of ...
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0answers
37 views

What is the best angle to throw a rock across a distance? [on hold]

If i were to throw a piece of rock, across a river of x metres wide, what would be the best angle of elevation i should throw the rock with, so that the rock can land x metres away with minimum force ...
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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
43 views

In the circuit the switch was in position 1 for a long time and at t=0, it is thrown to position 3. Find current as function of time t after switching

I started solving the problem by finding the steady state current. Then as the switch was shifted to position 3 the current in the inductor would decay. But my teacher told me that the current ...
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0answers
35 views

Lagrangian mechanics a cylinder on a cylinder barrel

This is a typical problem to be solved using the Lagrangian mechanics, but I have a problem in understanding the rotational energy. The task is to formulate the Lagrangian equation for the system of ...
0
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1answer
19 views

Finding the average speed plus a break [on hold]

I have the following equation when the driver drove 2.5h at 120 km/h and then stopped for 30 minutes. Then he drove for 20 minutes at 25 m/s. The result of the equation is to determine the average ...
1
vote
1answer
33 views

Direction of acceleration vector at the apex of a curve

(Test Your Understanding of Section 3.2) The question wants me to choose the correct acceleration vector, from the given 9 options where 9th option is zero acceleration. Ok, first I thought option ...
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0answers
9 views

Electric potential of a charged spherical conductor using laplace equation

Reviewing a course of electromagnetism I encountered the problem of a spherical conductor with a charge of $Q$ in a uniform electric field. The problem of a conductor in a uniform electric field (a ...
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0answers
13 views

Do the oil and mercury in this manometer affect the pressure in A and B?

One of my friends came to me with this homework question: We've worked out that the height difference between A and B is 50 mm. That makes the difference in pressure between them $$ \Delta P = \rho ...
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0answers
23 views

Problem with relative magnetic reference frames [on hold]

I came across this problem a while ago and I'd like to solve it, but I'm not even sure where to start. I know I need to use the magnetic reference frame to its angular rate of change, then use it to ...
1
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0answers
15 views

Problem with relative reference frames [on hold]

I came across this problem a while ago and I'd like to solve it, but I'm not even sure where to start. How would I start this problem and what formulas do I need? I'm not asking for a solution, but ...