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 questions.

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1
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2answers
177 views

Force on carabiner from free fall [closed]

Suppose I weigh about $75~\text{kg}$ and I am attached to a (non-elastic) rope which is attached to a carabiner which can take at least $20~\text{kN}$. Suppose furthermore that I reach terminal ...
2
votes
2answers
76 views

A question about surface term of ghost fields

(skip disclaimer) Hi, I have a question in Polchinski's string theory vol I p 90, after introducing the ghost fields $b_{ab}$ and $c^a$, it is claimed The equations of motion then provide a ...
0
votes
2answers
133 views

Car racing problem [closed]

A speedway turn, with radius of curvature $R$, is banked at an angle $\theta$ above the horizontal. If there is a coefficient of static friction, $\mu_{s}$ between the tires and the track, what are ...
1
vote
1answer
125 views

Applying Lagrange's equations ignoring normal reaction

A small bead is sliding on a smooth vertical circular hoop of radius $a$, which is constrained to rotate with constant angular velocity $\omega$ about its vertical diameter. $\theta$ is an angle ...
0
votes
0answers
35 views

Finding all marked elements using Grover's algorithm

As part of an assignment I am supposed to design a variant of Grover's algorithm that finds all $t$ marked elements. I guess like most people I'm more familiar with classic computation, so the ...
0
votes
1answer
82 views

Question about Cars: Momentum

Car B rests at the bottom of a frictionless inclined plane. In order to travel a height of 0.6m and maintain a speed of 2 m/s at the end of the track it needs to start with 4 m/s. a) If ...
0
votes
1answer
84 views

Uncertainty and wave-trains

My textbook and the following extract from feynman's lectures present the same idea regarding wavetrains and uncertainty in their wavelengths. Why is it that a wavetrain confined to some space has an ...
0
votes
1answer
135 views

Using Thrust from Air Pressure [closed]

Alright, so I've been messing around with turning air pressure into thrust, however I'm only a sophomore in high school, so my physics knowledge is fairly limited. (I've studied higher level physics, ...
1
vote
1answer
50 views

A question about variation of metric under Weyl and coordinate transformations

I have a question about deriving variation of metric under Weyl and coordinate transformations in Polchinski's string theory (3.3.16). Under transformation $$\zeta: g \rightarrow g^{\zeta}, \,\,\, ...
2
votes
2answers
610 views

Hermite polynomials for expected value of harmonic oscillator

This was a problem on my final exam that has been really bugging me. Consider the quantum Harmonic oscillator prepared in an energy eigenstate, $\psi_n$(x). Calculate the expectation value of the ...
2
votes
2answers
146 views

Relative velocity of sound

Relative velocity of sound. As I know that speed of sound in medium is property of medium. And independent of source motion but depend on motion of audience or observer and motion of medium. But it ...
1
vote
1answer
190 views

Particle slightly disturbed radially in stable circular motion under central force

A particle of mass $m$, moves around a central force whose potential is $$V(r) = kmr^3 \; (k>0),$$ then $(c)$ if the particle is slightly disturbed from the circular motion, what is the ...
1
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1answer
179 views

Derivation for 7.14 in Atomic Physics by Foot

I was going thru Ch7 of Foot and trying to fill in the gaps. However I got stuck on (7.14). So Foot was working with a two level system with a small perturbation in the Hamiltonian resulted from an ...
2
votes
0answers
110 views

Can spheres leaking charge be assumed to be in equilibrium?

I am struggling with the following problem (Irodov 3.3): Two small equally charged spheres, each of mass $m$, are suspended from the same point by silk threads of length $l$. The distance between ...
1
vote
1answer
90 views

Surface integral of a line

Suppose I am calculating the (homogenous) electric flux through a straight line. How do I calculate the surface integral of a straight line? Is it simply a line integral, or is it more complex (I ...
3
votes
3answers
2k views

Relationship between torque and angular momentum

I want to know how to deduce the equation $\vec{\tau}=\vec{\omega} \times \vec{L}$, where $\vec{\tau}$ is the moment of force (also known as torque), $\vec{L}$ is the angular momentum, ...
0
votes
2answers
138 views

Conditions for Conservation of energy law

In two-dimensional motion, which conditions are needed to be satisfied so the conservation of energy law holds? (for example, simple pendulum motion)
-1
votes
1answer
253 views

A Rolling Quarter [closed]

A U.S quarter is rolling on the floor without slipping in such a way that it describes a circular path of radius $R=4 \text{cm}$. The plane of the coin is tilted at an angle of $\theta=45^{∘}$ with ...
0
votes
0answers
127 views

Calculating uncertainty $\Delta E$ or $\Delta E_k$ from $\Delta p$

How do we calculate uncertainty in kinetic energy $\Delta E_k$ if we only know that an (a) electron (b) proton is closed in a 1-D box of width $d=10fm$. I first assumed that uncertainty in position ...
1
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3answers
555 views

Overtaking with non-constant acceleration

I have tried to solve this problem by adding the sum of the displacements during acceleration, constant velocity and deceleration, but it does not work out. Question: A car accelerates from ...
1
vote
0answers
79 views

Motion on a smooth surface

A particle of mass $m$ is moving on the inner side of smooth circular cylinder of radius $R$ whose $Oz$-axis is vertical and directed downwards. The particle started its motion from the $x$-axis with ...
0
votes
1answer
112 views

Minimum required to lift a roller up a step

Consider a roller of diameter $0.82m$ which is to be raised up a step measuring $0.32m$. Find the minimum force required to do so. The roller weighs $500N$. The roller could be considered as a ...
1
vote
1answer
177 views

1D Motion under Gravity

I have attempted this question in so many different ways but I am getting nowhere. Could someone point me in the correct direction so I can work it out myself? Here is the question: An object $K$ ...
0
votes
1answer
375 views

Pulley problem with spring balance

Please excuse the crude drawings The question is, if I burn the thread t, how will the reading in the spring balance(S) change? Ie, increase, decrease, or remain the same? My approach: When we ...
0
votes
1answer
266 views

Rotating frame of reference

Can you help me to do this: Two frames of references $S$ and $S'$ have a common origin $O$ and $S'$ rotates with constant angular velocity $\omega$ with respect to $S$. A square hoop $ABCD$ is made ...
1
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1answer
1k views

Minimum height of mirror required to view image

I wanted to know the minimum height of mirror required to be able to view a complete image of a person. I considered the following setup: $HF$ is the person in question. $H$ denotes the head, $F$ ...
2
votes
1answer
156 views

Refraction of light in water

Here's a question in a paper based on refraction of light. I can't seem to solve it for some reason. A man looks down at a fish length of 20 cm. His eye is 2m above the surface of the water ...
2
votes
2answers
182 views

Question on the uncertainty principle

The problem statement: Measurement detects a position of a proton with accuracy of $\pm10pm$. How much is the position uncertainty $1s$ later? Assume the speed of a proton $v\ll c$. What i ...
0
votes
0answers
75 views

Crystalography - hardest homework problem i have seen so far

The problem statement: We are observing an electron diffraction on a "Ni" crystal. We point the narrow beam of electrons on a crystal which can be rotated so that we are changing the angle ...
2
votes
1answer
123 views

About the conserved charge for the ghost number current in $bc$ conformal field theory

(skip disclaimer) I have a question about the conserved charge for the ghost number current in $bc$ conformal field theory in Polchinski's string theory p62. It is said For the ghost number ...
1
vote
1answer
61 views

Problem similar to Davison & Germer eksperiment but with more angles and the mirror reflection

The problem statement: Narrow beam of electrons hits an aluminium plate under an angle $\alpha=30^\circ$ according to the flat surface. We know the distance between two crystal planes in ...
-3
votes
1answer
4k views

Neutral axis of T shaped beam? [closed]

I am not a mechanics or physics student but a computer science student. I came across a question related to find neutral axis of figure but I do not have slightest idea of what it is and how to find ...
2
votes
1answer
799 views

Calculating specific orbital energy, semi-major axis, and orbital period of an orbiting body

Is it possible to calculate the specific orbital energy $ϵ$, the semi-major axis $a$, and the orbital period $T$ (or $P$) without any of them being available to you? The values I do have available to ...
0
votes
1answer
274 views

Gas mixture flow rate equation [closed]

2 gas flows connect and mix in a pipe. The following is known: $\dot{m_1}$; $\dot{m_2}$ - mass flow rate of the inlet gases $(\frac{kg}{s})$ $A_1$; $A_2$; $A_{mix}$ - the cross section of the pipes ...
3
votes
2answers
144 views

Quantum Mechanics Basics: product space

Consider a coupled harmonic oscillator with their position given by $x_1$ and $x_2$. Say the normal coordinates $x_{\pm}={1\over\sqrt{2}} (x_1\pm x_2)$, in which the harmonic oscillators decouple, ...
4
votes
2answers
236 views

Calculating an expression for the trace of generators of two Lie algebra

Suppose we have $$[Q^a,Q^b]=if^c_{ab}Q^c$$ where Q's are generators of a Lie algebra associated a SU(N) group. So Q's are traceless. Also we have $$[P^a,P^b]=0$$ where P's are generators of a Lie ...
1
vote
1answer
51 views

Escape from gravitational field

Why total energy of a spacecraft must be non-negative if we want spacecraft to escape from the Earth's gravitational field?
1
vote
1answer
169 views

Motion of block on wedge

there is some confusion to me in the case of "motion of block on a frictionless wedge" Below is a simple diagram! Let us consider a situation as above in which there is a block of mass $m$ moving ...
0
votes
0answers
77 views

nuclear physics- Energetics and Mechanics of Nuclear Reaction Homework

A sample of $^{24}_{12}\mathrm{Mg}$ is bombarded by a monoenergetic proton. If the resulting nucleus in a $^{24}_{12}\mathrm{Mg}(p,\gamma)$ reaction; $^{24}_{12}\mathrm{Mg}(p,\gamma)$ has its ...
0
votes
1answer
91 views

Velocity of a spacecraft in a circular orbit

What is a formula for velocity of a spacecraft in a circular orbit? Actually, is there a formula or I can find it from equation of motion?
0
votes
1answer
738 views

Potential energy of the dipole-dipole interaction for two parallel dipole moments

I am looking for an equation that gives me the potential energy of the interaction between two parallel dipoles.
1
vote
1answer
237 views

What is the friction between cylinder and wall (ground)?

A hollow cylinder (radius $R$) is rolling against the wall at angular speed $\omega$. The coefficient of friction between the cylinder and the wall(ground) is $\mu$. After how many rotations the ...
1
vote
2answers
272 views

At what starting angle will a billiard ball hit a pocket? [closed]

Let's say you have a square pool table with one pocket. At what angles can you hit the ball to make it hit the pocket? What relation do these angles hare? Constraints: Table is square: $\ell=b$. ...
0
votes
3answers
177 views

Which parameter determines how much the 'recoil' or force applied by an object hurts?

Another question from my textbook, directly following after this question: 2 identical bullets are fired, on from a lighter rifle and one from a heavier one, with the same amount of force. Which ...
6
votes
4answers
567 views

Symmetry in resistor circuits

Given 6 points that are connected with each other with a resistor of resistance $R$, find the resistance between any two points. (Answer: $R/3$) (All the conducting wires have the same ...
1
vote
2answers
103 views

Possibility of determining the mass of the water without a scale?

Possibility of determining the mass of the water by knowing water volume, water temperature and atmospheric pressure. I want to know if I can determine the mass of $V=0.01\,m^3$ of water in ...
2
votes
2answers
378 views

Falling chain with friction

I really need some help with a physics problem, and I guess my doubt is more conceptual, than question based. But still, let me pose it to you: A uniform chain of mass $M$ and length $L$ lies on ...
1
vote
4answers
1k views

Velocity of 2 balls with different masses on a moving train after the train brakes

Question This is from my textbook: There are 2 balls of the same size made of rubber and iron respectively kept on the smooth surface of a moving train. Which ball will move faster when the train ...
1
vote
1answer
152 views

Compute the central charge of $bc$ conformal field theory

I have a s****d question, how to calculate the central charge of $bc$ conformal-field theory in Polchinski's string theory, Eq. (2.5.12)? For a $bc$ CFT given by $$S=\frac{1}{2\pi } \int d^2 z \,\,b ...
0
votes
1answer
103 views

Atom emitts a photon - what is the frequency of the energy of the photon?

Lets say I have an atom of known mass who goes from excited state to the ground state whose energy is $14.4 keV$ lower. I know that the emitted energy $14.4keV$ got converted into an energy of a ...