# 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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18 views

### What are the hermitian operators in cylinderical coordinates?

In cylindrical coordinates which of the below are hermitian. 1/i*(ծ/ծs) 1/i*(ծ/ծs+1/s) 1/i*(ծ/ծs+1/2s) (ծ/ծs+1/s) the answer says "3" but i don't understand how. The difference in option &...
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### How fast would you need to go for five years to become roughly half an hour in relative time?

I'm trying to write a story involving time dilation (came up with it before Lightyear）and one plot point is that an important character was missing for five years, but to them, only a little while had ...
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### Translational Equilibrium

The diagram shows two possible choices for a multiple choice question that I used from a test bank. My gut tells me that the answer is B, but I am unable to come up with an explanation as to why ...
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### Angular velocity of a physical pendulum [closed]

I'm having trouble solving this problem: "A pendulum consists of a rod of length 2 m and mass 3 kg connected to the bottom of a solid sphere of mass 1 kg and radius 0.3 m. The axis of rotation ...
47 views

### Acceleration due to gravity having non uniform density of a solid sphere [closed]

A solid sphere of mass M and radius R has a non uniform density in such a way that the density is proportional to r² at a distance r from the centre. Find acceleration due to gravity at a distance r ...
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### Electrostatic force between two Non-conducting spheres that are close [closed]

I wanted to find the electrostatic force between two spheres with radius $R_1$, $R_2$, and charge density $\rho_1$, $\rho_2$, and the distance between them is $R$. Also, $R_1$, $R_2$, and $R$ are all ...
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### Poynting vector on a rotating cylinder [closed]

A long cylinder of radius $R$ has a surface charge density of $\sigma$. An externally applied torque causes the cylinder to rotate with a constant angular acceleration: $\omega = \alpha t$ about the ...
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### What will happen if we change all the resistances in an infinite resistance ladder to their double value? [closed]

Let's have a circuit as below: If the equivalent resistance of above circuit is $Z$ and if we have another circuit as below: And if the equivalent resistance of above circuit is $R$. Now my question ...
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### What happens when we integrate angular displacement $θ$ (question picture given below)? [closed]

Here I was trying to Solve this question and I thought what if we integrate both $\alpha$ (angular acceleration) and the value we get when we equate torque with Iα. If we do so We get the desired ...
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### Electric flux through spherical gaussian surface due to a charged non conducting cylinder [closed]

I tried integrating the Electric field component and the elemental ring differential area in 2 parts . First when the Elemental ring radius is greater than R and once when Its less the R because the ...
24 views

### Fresnel's Equation - Area Terms in Power Expression

I have a lecture note about Fresnel Equations and related power ratios of the reflected and refracted rays. But I cannot understand how the area on the boundary is related to the areas of the rays, e....
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### Calculating the capacitance [closed]

Two charged identical spherical objects are hung by two threads in a medium from a point and they become balanced at a linear distance from each other. In another medium, their positions remain all ...
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### How can I calculate the approximate air volume to obtain neutral buoyancy per 1 kilo of weight in Salt water? [closed]

I have a solid round object that weighs 5 kilos. I want to calculate how many liters of air in a plastic bottle is needed for neutral buoyancy in sea water.
1 vote
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### Problem regarding open systems and a compression [closed]

Say we have a cylinder of height $h_0$ and transversal area $S_\perp$ filled with water vapour and covered by diathermic walls, as well as a mobile diathermic piston on the top, which is in contact ...
1 vote
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### External leg correction to 3-point QED Green's function

I am trying to calculate the following diagram to solve the Callan-Symanzik equation for the three-point Green's function (two massless fermions and a photon). The counterterm to the photon ...
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### Projection of an object towards a planet [closed]

An object (of mass m) is projected from outer space with a velocity v, making an angle $\theta$ with the position vector from the centre of a planet of mass M and radius R. The object is at a distance ...
36 views

### Momentum and Impulse Sample Question [closed]

Jonathan and Jane are sitting in a sleigh that is at rest on frictionless ice. Jonathan's weight is 800 N, Jane's weight is 600 N, and that of the sleigh is 1000 N. They see a poisonous spider on the ...
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### What's the amount of force needed to push a object on a inclined surface with a particular constant velocity? [closed]

Suppose there is no friction and the angle of inclination is $\theta$. We first need to apply a force of $g \sin(theta)$. I know that this will cause it to start moving forward with no acceleration as ...
1 vote
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### Finding error in Choi matrix calculation which suggests a CP map is not CP

The question Consider the following linear, trace preserving quantum map over $d \times d$ quantum states $\sigma$, $$\Phi(\sigma) = \Pi_R \sigma \Pi_R + (1 - \text{Tr}(\Pi_R \sigma)) \frac{\Pi_R}{R}$$...
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### Circular motion string at highest point [closed]

So why is the tension in the string at the top most point of a circular motion equal to zero ?
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### What is the moment of inertia of an I-beam? [closed]

A thin rod of length L and mass m, perpendicular to the axis of rotation, rotating about its center. Its moment of inertia would be $I=(1/12)mL^2$ (see picture). If I want to use an I-beam instead of ...
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### Problem of optic [closed]

i need help with a problem and idk how do it, please if someone can help me gonna be great, i want learn how do this problem Certainly, here's the translation of the problem into English: Problem of ...
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1 vote
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### Small Oscillations aproximation for gravitational potential in cartesian coordinates [closed]

Hi I have a question concerning the following problem, where masses 1 and 2 slide frictionless on the x axis, while mass 3 hangs from two springs connected with the other two (only the horizontal ...
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### Calculating eigenvalues symbolically [closed]

I'm reviewing this paper and towards the end I came across this problem and I have no idea how to solve it properly. I constructed the matrix $M$ and calculated the eigenvalues as; \begin{equation} M =...
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### How to do this question? [closed]

From a solid sphere of mass $M$ and radius $R$, a smaller solid sphere is cut with its diameter as the radius of the bigger sphere. Calculate gravitational field at the centre of hole due to remaining ...
Problem: I am currently tackling a problem involving the derivation of the electric field $\mathbf{E}$ at location $(0,y)$ induced by an infinite homogeneous line charge $\lambda$ (refer to the ...