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
12 views

A question about a small block sliding down a larger one, involving conservation of momentum and energy

I have been working on a problem for some time but haven't found a solution yet. I am mainly looking for a hint that will help me find a breakthrough and not a full solution. Here is the statement of ...
2
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4answers
120 views

Special Relativity - Simultaneity of events

For the discussion below, consider only 1-space dimension, $x$ Let $E_1$ and $E_2$ be two physical "events". Let $S$ ans $S'$ be two IRFs. Let the coordinate of $E_1$ be $(x_1,t_1)$ w.r.t to $S$ and ...
2
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3answers
4k views

Visualizing Tension in FBD — Rope With Mass

Asked to find the tension of a point that is halfway up a rope (where the rope has mass), given the system: The solution of the problem (FBD included) is as follows. In this diagram (Tm = tension at ...
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0answers
17 views

Trace distance expressed with maximization of probabilities

I'm studying chapter 9 of Nielsen and Chuang, where trace distance is defined by equation (9.1) $D(p_x,q_x)=\dfrac{1}{2}\sum_{x}|p_x-q_x|$ and later on the next page (also asked to be proven in ...
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0answers
32 views

Coefficient of friction in pulley system

EDIT: Picture doesn't show the mass of $\mathrm A$. It is $60\ \mathrm{kg}$. My attempt: I placed my coordinate system with $x$-axis parallel to the ramp surface and $y$-axis parallel to the normal ...
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0answers
36 views

How can you get distance traveled from time when acceleration is a function of distance?

Let's say object $A$ has an initial velocity of 0 m/s and is $x_i$ meters away from object $B$. Object $A$ is accelerating towards object $B$ at $a=x^{-2}$ where x is the distance between the two ...
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0answers
8 views

Conformal mappings in electrostatics applied to an infinite conducting fence in a uniform electric field

A long straight fence made of conducting wire mesh separates two fields and stands one metre high. Sometimes, on fine days, there is a vertical electric field over flat open countryside. Well away ...
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0answers
17 views

Heisenberg uncertainty principle and minimum Energy of a system

A body has the total energy $E=p^2/2m+mgx$ if $x>0$ and it has the total energy $E=p^2/2m+1/2*kx^2$ if $x<0$ .Find the minimum Energy of the system? This is my two different attempts of solving ...
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2answers
113 views

Compute the Legendre transform for a singular Lagrangian

I'm given the lagrangian: $$ L(q,\dot{q}) = \frac{1}{2}(\dot{q_1}^2+\dot{q_2}^2+2\dot{q_1}\dot{q_2})-\frac{k}{2}(q_1^4+q_2^4). $$ I have to compute the Legendre transformation associated to it. The ...
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0answers
21 views

I'm looking for the solution for this question regarding Schrödinger equation [on hold]

I'm studing for my quantum mechanics exam and I'd appreciate any help regarding this question
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0answers
39 views

Physics help please [on hold]

Calculate the distance an arrow will land from a hunter if he releases the arrow with a speed of 20m/s at a 60 degree angle above the horizontal. The tip of the arrow is 1.5 m above the ...
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1answer
273 views

Linear momentum rod hit by bullet

I am trying to solve the question which I have attached. My attempt at a solution is that the angular momentum is conserved thereby we have $$ mva = \frac{ML^2\Omega}{3} + a^2m\Omega $$ My problem ...
3
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1answer
844 views

Finding eccentricity of orbit given speed, altitude and polar angle

Problem For a certain satellite the observed velocity and radius at v = 90° is observed to be 45,000 ft/sec and 4,000 n mi, respectively. Find the eccentricity of the orbit. (Answer: e = 1....
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0answers
20 views

Find equivalent resistance [on hold]

The equivalent resistance between A and B is
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3answers
280 views

Understanding Derivation of Euler-Lagrange

I am trying to understand the derivation of the Euler-Lagrange equation. I drew a graph below. So, according to the graph, $$ \int_{t_1}^{t_2} L(x+\delta{x},\dot{x}+\delta\dot{x}\,t) dt - \int_{...
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0answers
16 views

Find electron-hole recombination rate?

In an n-type semiconductor, the thermal equilibrium electron concentration (n0=1015/cm3) and the intrinsic carrier concentration (ni=1010/cm3). The mean life time of minority carriers is 1us. The ...
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2answers
124 views

Feynman Lectures. Derivation of efficiency law from logic

Question is about Feynman Lectures. Lecture 44. Ch.44-4 The efficiency of an ideal engine [feynmanlectures.caltech.edu/I_44.html] In this chapter Feynman derives the formula $\frac{Q_1}{T_1} = \frac{...
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0answers
14 views

Electric field of a charge distribution on an arbitrary curve

I want to calculate the electric field intensity at a point $P(x,y,0)$ for charge distribution lying on an arbitrary curve such as in the figure. We know the coordinates of some points on curve. What ...
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0answers
26 views

How to calculate the wavelength with just the frequency? [on hold]

there is a training question in my study and I've completely lost how to work this out. Can you give me any tips or which steps I have to go to calculate this? Using the relationship between ...
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4answers
87 views

Why is Lorentz force linear in velocity?

I am working on the derivation of the Lorentz force. (I know that Lorentz force is in some sense definition of fields, but still, there is nontrivial dependence on velocity) I want to derive that ...
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1answer
376 views

I've a problem understanding Absolute Gravitational Potential Energy?

My current concept about potential energy is that 'If work is done on a body when it is at a point (taken as zero P.E), it covers some distance. Afterward, when it is allowed to move freely it does ...
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0answers
25 views

Longitudinal stress [on hold]

A uniform wire is subjected to longitudinal tensile stress of $5 \times 10^7 Nm^{-2}$. If the overall volume change in the wire is $0.02\%$, then the fractional decrease in the radius of the wire is?
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2answers
196 views

Is there a formula to calculate the deviated angle of a ray passing through a prism?

I'm assuming the angle of incidence to be the number of degrees from a perpendicular line on the side of a prism the ray starts passing through... I'm trying to figure out the angle the ray would be ...
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0answers
34 views

What is the meaning of $\tau$=$Gb/l$

What is the physical meaning or intuition of the Shear Stress vs Shear Modulus and Burger Vector over length relation($\tau$=$Gb/l$). I have seen this formula being used to determine length in case ...
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0answers
32 views

To show that any central force field is conservative

Denote the radial and cross-radial components of a conservative force field by: $F_r$ and $F_\theta$ respectively. [The the central orbit is planar, i.e. a plane curve]. We need to show that $\nabla \...
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1answer
116 views

Mass-Spring system on an accelerating jet

Imagine a perfect mass spring system. If it's put on an accelerating plane, how will the motion change? Is the plane's acceleration like a driving/damping force, where: $$F_{\text{driving}} = \text{...
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1answer
19 views

Measuring small changes in length using the micrometer screw gauge

Which of the following method/instrument cannot be used to measure small changes of the order of a millimeter occurring in a length of about 50cm. 1)spherometer 2)travelling microscope 3)meter ...
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1answer
30 views

Electrostatics and Conservation of angular momentum [on hold]

A charge $+Q$ is fixed. Another charge $+2q$ and mass $M$ is projected with a velocity $V$ with an angle of 30 degees from a distance $R$ from the fixed charge. Minimum separation between the two ...
1
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1answer
113 views

How to calculate the potential energy of an $H_2$ molecule

From left to right, electron $e_1$, $e_2$ and proton $p_1$, $p_2$. $r_0=0.529nm$ The total energy is sum of energy require to bring each particle to its place. Take the place of $e_1$ is zero ...
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1answer
141 views

What exactly is average power?

This question arises due to a problem I'd been solving recently: The question was to find the average power supplied by the water over the six hour period. Now I have read this post which ...
3
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1answer
101 views
+50

Importance of an extra total derivative term in Liouville theory

In this paper on boundary Liouville theory, the authors have introduced an extra term, $-\partial_{\sigma}^2\phi$, (the last term in the equation below) in defining the stress tensor of the Liouville ...
1
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1answer
53 views

Free fall time after being accelerated

An elevator car whose floor-to-ceiling distance is equal to $2.7m$ starts ascending with constant acceleration $1.2 m/s^2$; $2.0 s$ after the start a bolt begins falling from the ceiling of the car. ...
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0answers
33 views

A charge accelerating while confined to an equipotential surface

Suppose I had the following in Cartesian $(x,y,z)$ space, where $x$ is horizontal, $y$ is vertical, and $z$ is depth: A negligibly thin conductive rod extending from $(0,1,0)$ to $(0,+\infty,0)$. An ...
2
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1answer
66 views

Kinetic Energy of a Symmetric Dumb-Bell

The problem is as follows: Consider the system consisting of the symmetric dumb-bell (two particles each of mass $m$ connected by a light, inextensible rod of length $l$) moving in the plane. Use ...
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1answer
38 views

How can we see the microstructure of steel samples at room temperature?

When we intend to see the microstructure of a steel sample at a temperature say 950 $^\circ$C they say we quench it in order to freeze the microstructure. However, if we quench it are we not going to ...
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1answer
33 views

What is the relation between Earth's orbit and orbit of another planet around Sun? [on hold]

Lately, I come across the following problem, "The maximum and minimum speeds of a comet that orbits the Sun are 80 and 10 km/s respectively. The ratio of the aphelion distance of the comet to the ...
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1answer
56 views

A Question About $4$-Spinor Contractions

Let $f_{abc}$ be a constant which is totally anti-symmetric with respect to indices $a$, $b$ and $c$. Let $\psi^{a}$, $\psi^{b}$, $\psi^{c}$ and $\epsilon$ be Grassmann-valued Majorana fermions. How ...
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1answer
29 views

How to express the $Q$-value for alpha decay in terms of binding energy of the relevant species?

I have 2 questions regarding the solution to the following question: The solution states that: Mathematically, I understand how the end result $$Q=BE\left({}^{A-4}_{Z-2}\mathrm{Y}\right)+...
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0answers
22 views

Young's Experiement [on hold]

My question is about 2 slits where there is an monochromatic source of light and: 100 lambda - distance between slits, wide of slits is 20lambda and 40lambda L is the distance to the screen. ...
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0answers
35 views

Is my method correct? [on hold]

Suppose we have two point charges, one negative and one positive separated by a distance d and we want to find the potential of the dipole a point $P$ where the distance of the positive charge to $P$ ...
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1answer
30 views

Scale factor for flat universe filled with radiation and cosmological constant

I am trying to solve problem 1.20 in the book 'Physical Foundations of Cosmology' from Mukhanov. In the problem, we should show that the scale factor $a$ for a flat universe filled with cosmological ...
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0answers
19 views

How do I calculate the moment of inertia of a regular tetrahedron spinning around it's centroid? [on hold]

I know it's $Ms^2/20$, but I just want to see how it's done.
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1answer
25 views

Avarage Speed conventional formula VS Mean Value of Multiple Speeds [on hold]

Here the basic problem: The motion of an extraordinarily jubilant bird flying straight up and down is given by the graph below, which shows the vertical position y as a function of time t. What was ...
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1answer
155 views

Stagnation points of combination of line sink and line source

I've got the complex potential of a flow consisting of a line sink at $z=-a$ and a line source at $z=a$ both of equal strength. I want to find the stagnation points but when I differentiate I get no ...
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0answers
24 views

The physics behind amusement park ride [on hold]

i want to know about the physics behind tora-tora ride. How do tora-tora ride work? The physics theory in tora-tora ride?
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0answers
18 views

Physics 2 Wave question 3rd harmonic frequency [on hold]

A string vibrates at its third-harmonic frequency. The amplitude at a point 35 cm from one end is half the maximum amplitude. How long is the string? I cannot find the correct answer anywhere online ...
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0answers
22 views

Very simple vector question (solid state) [on hold]

Problem Statement Graphene is a 2D crystal made entirely of carbon atoms. The crystal lattice can defined by the lattice vectors a and b, where the magnitudes |a|=|b|= 2.46 Å and the angle between ...
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3answers
31 views

Why we add acceleration of celestial body with acceleration of spaceship to determine force exerted by space ship on astronaut?

The acceleration due to gravity acts in the downward direction, and a spaceship accelerates in an upward direction. Therefore, why do we add the accelerations together instead of subtracting them?
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1answer
29 views

If a rectangular loop is present between two wires carrying current in same direction but fluctuating, what will be the direction of current in loop [on hold]

The current in $I_1$ is continuously decreasing and the current in $I_2$ is continuously increasing, then what will be the direction of current in the loop? Here's what I know, the wires carrying ...
0
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1answer
396 views

On the Bogoliubov transformation in the BCS

I have a question regarding the diagonalization of the BCS-Hamiltonian using the Bogoliubov-DeGennes-transformation. I hope someone can help me, so I start with the following Hamiltonian, it is ...