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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9 views

How can I prove poisons 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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22 views

What is the value of a unit of energy in an Einstein solid, given the temperature?

Given N = 6000 and q = 30000, we can find the multiplicity of an Einstein solid by: $\Omega = \frac{(q+N-1)!}{(N-1)! q!} = \frac{35999!}{5999!30000!} $ Using Sterling's approximation, $ln(N!) = NlnN-...
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20 views

Property of the Quantum Fourier Transform

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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1answer
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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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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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16 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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12 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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37 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 ...
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1answer
31 views

Calculate new hight of a moving object being hit from underneath

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 ...
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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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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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1answer
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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
29 views

Line charge inside a conducting sphere?

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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35 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
58 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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41 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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34 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 ...
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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 ...
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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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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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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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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 ...
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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 ...
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1answer
72 views

Conserved current from the three $SU(2)$ transformations

We are asked to show that the following Lagrangian is invariant under the three $SU(2)$ transformations $\Phi \rightarrow \exp{({\frac{i}{2}{\alpha_j\sigma^j}}) \Phi}$, where $\Phi$ is a doublet ...
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19 views

What downward forces will a ladder exert as it is lifted to be parallel with a wall from flat? [on hold]

Assume you have a ladder with uniform weight of 100 lbs and length of 20 ft. It is flat on the ground and one end is against an immovable wall. And a person with arms straight up has a foot to hand ...
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40 views

Help With a Projectile Motion Problem [on hold]

So I have what seems to be a fairly easy projectile motion problem that I just can't solve, I believe I am using the wrong idea and am requesting help! The problem is as follows: A boy on the edge ...
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47 views

Electricity cost of a kettle [on hold]

A 2.5kW electric kettle has an efficiency of 95%. (95% of the electrical energy supplied is converted to heat in the water.) Electricity is charged at 22 cents per kWh. How much money would the ...
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22 views

A ball of mass $m$ is dropped from a height $h$ on to a rigid floor. If $e$ is the coefficient of restitution [on hold]

A ball of mass 'm' is dropped from a height 'h' on to a rigid floor. If e is the coefficient of restitution, the time required for the complete bounce is? After finding the initial and final velocity ...
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22 views
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Molecular dynamics - Fermi Pasta Ulam Tsingou

I'm taking a course on computational physics, but still fairly new to physics at this level, and because of that, I'm looking for some help getting a better understanding of the following. Working on ...
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30 views

Is the incompressibility the only condition I could use for this problem? [on hold]

The problem is In cylindrical coordinates, the velocity is $\mathbf{u}=u_{r} \hat{\mathbf{e}}_{r}+u_{\theta} \hat{\mathbf{e}}_{\theta}+u_{z} \hat{\mathbf{e}}_{z}$. Conside an incompressible flow ...
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37 views

Maximum velocity and period of the earth around the sun [on hold]

The Earth has an orbit with eccentricity e=0.0167 around the sun. If the minimum distance from the Earth to the sun is 146*10^6 km, what is the velocity of the Earth at this distance? Additionally, ...
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Susskind exercise on quantum mechanics

On Susskind book, exercise 3.1 say to prove the following: "If a vector space is $N$-dimensional, an orthonormal basis of $N$ vectors can be constructed from the eigenvectors of a Hermitian ...
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Two questions regarding a Lorentz Transformation related to catching a bullet on camera

I am trying to resolve a question about Relativity Theory, with an answer key at hand. The answer given creates more confusion than it resolves, and I am looking for some insights and answers to three ...
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1answer
37 views

Electric potential due to two spheres

The setup is as below: The two conducting spheres, of radii $r_1$ and $r_2$ have charges $Q_1$ and $Q_2$. The distance between their centers is AB. BC is perpendicular to AB. The problem asks to ...
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Confirmation on an answer [on hold]

I would like to know how the pressure at C is greater than pressure at D? I didnt understand the reason given. Plz help me Asap
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34 views

Several issues related to the collapse of a radioactive nucleus, interpreted based on Relativity Theory

I am trying to resolve a question about Relativity Theory, with an answer key at hand. The answer given creates more confusion than it resolves, and I am looking for some insights and answers to very ...
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42 views

How does vectorization affect $\nabla$?

The homework and exercise was to prove $\nabla \times {A}$ transform as a vector, and I've solved it thorough hard algebra. However, something occurred to my mind and I have a hard time to resolve it....
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1answer
20 views

How to find the work of each force acting on a block?

A push of 200 N moves a 100 N block up at 30º inclined plane. The coefficient of kinetic friction is 0.25 and the length of the plane is 12 meters. a. Find the work done by each force that acts on ...
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Uniform circular motion /centripetal motion [on hold]

A particle starts moving along a circle of radius (40/π) m with a constant tangential acceleration. If the velocity of the particle is 120 m/S at the end of the second revolution after the motion has ...
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19 views

What is the temperature at which water will boil at an altitude where pressure drops by 10%? [on hold]

If there is a 10% decrease in the atmospheric pressure at a hill compared to the pressure at sea level, then the change in the boiling point of water is_____ °C. (Take latent heat of vaporisation of ...
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35 views

Infinity problem in basic classical mechanics problem

My question arises from a classical mechanics problem from a Hong Kong physics training programme: This is not a homework question as I am not asking about how to solve the problems in the image. As ...
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1answer
58 views

Work done by spring force

Consider a spring connected to blocks on it's ends lying on smooth horizontal table. Now let the right end block be displaced $x_1$ and left end be displaced $x_2$ from the mean position such ...
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1answer
18 views

How to find the acceleration of two blocks? [on hold]

Help I'm really lost. I'm not even sure what forumla im supposed to be using on this problem A 15 kg block is placed on top of a 25 kg block (see figure below). A force of 387 N is applied to the ...
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39 views

Deriving the equations of motion of a real vector field [duplicate]

I'm trying to calculate the euler-lagrange equations of the following Lagrangian density. $$\mathcal{L} = -\frac{1}{2}(\partial_\alpha A_\beta)(\partial^\alpha A^\beta)+\frac{1}{2}(\partial_\alpha A^\...
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1answer
63 views

Path integral XY model

I am given the Hamiltonian for the XY model of spins $$H[\theta] = \frac{K}{2} \int |\nabla \theta|^2dxdy,$$ where $K = J/k_B T$ with $J$ the interaction energy between neighbouring spins. I ...
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2answers
39 views

What am I supposed to look for in the question “What is the total resistance between $X$ and $Y$?” [on hold]

I'm not really concerned with answering the question but more so what I'm trying to look for. The question is "Three resistors are connected as shown. What is the total resistance between $X$ and $...
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1answer
29 views

How to calculate this electric field? [on hold]

I can't solve this question. Please help. Q: Solve using Multiple integrals (Gauss is not allowed). A hemisphere of radius $R$ has a uniform distributed surface charge density $ρ_s$. Calculate ...
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10 views

What is the moment of inertia for two identical masses that lies not symmetrical to each other INSIDE a cylinder? [on hold]

Suppose I have a cylinder with r = 5.2cm, two masses are both ~200g, radius of the mass is 2cm each. If both of them are on the same line as the diameter of the cylinder (mass 1, center of cylinder ...
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0answers
14 views

Finding initial velocity of balls [on hold]

Ball A is released from rest at height h above the ground. At the same time, ball B is projected vertically upward with initial velocity u. After some time, two balls are at the same height above the ...