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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19 views

Root mean square of $N$ random Walkers

Lets say we have $N$ (independent) random walkers where root mean square of one of the walkers is $x_{rms}$ . What is the relation between $xN_{rms}$ and $x_{rms}$ where $xN$ is the average of all the ...
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Help with this particle graph

for 1. I said that it is attractive because 2^1/6 is 1.12 so r is less than 1.15 for 2. I said 2^1/6 for 3. I said that it is repulsive
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2answers
55 views

How do I find Kinetic Energy when $T$ is not given?

I'm confused about a question given which is briefly: ... full of Cola, places it on the counter top and gives the 2.6-kg pitcher a 8.8 N forward push over a distance of 48cm to send it to a customer ...
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12 views

How determine flow rate of air into a sealed vessel with increasing pressure

I am able to measure a target pressure inside the vessel or rate of increasing pressure, temperature and know th volume of the vessel (1000L). I would like to determine flow rate of air from a ...
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1answer
28 views

Calculating travel time of an object on the inclined plane

Let's consider we have frictionless inclined plane and we pushed small object on the plane with initial velocity $v_0$. Travel time while going up can be calculated from following formula: $l = v_0 ...
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9 views

How to find the density lapse rate linked with the Brunt-Vaisala frequency?

I get an exercise about atmospheric thermodynamics. It is asked to prove density lapse rate satisfies the equation but I am confused which law should I start with. Show that the "density lapse ...
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15 views

What is the exit velocity of a charged mass travelling parallel to field lines in an electric field (between two plates)?

I did some pretty basic calculations and I got the exit velocity as $sqrt(2qUm)/m$ where $q$ is the charge, $U$ is the voltage across the plates, and $m$ is the mass. The reason I'm sceptical is ...
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1answer
34 views

Forces applied on a planet

Planets go around the sun in elliptical orbits. The highly exaggerated diagram below shows a portion of such an orbit and the force on the planet at one position along that orbit. The planet is moving ...
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17 views

We're supposed to find potential function of the mass two spring system [closed]

I know we want to find the second derivative and evaluate for when Xo is X=0 which should give us harmonic oscillation but I think I'm not taking the derivatives correctly since I'm not arriving at ...
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39 views

Pressure in a fluid at rest

I hope this is the right place to post such a question. I'm studying Continuum Mechanics from Gurtin's book, and so far in my class we've seen Cauchy's the about the existence of stress, and nothing ...
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1answer
38 views

Distance calculation without knowing time

I’m studying physics and am struggling to work out the correct formula for the below example question. Any help and explanation would be amazing. A tourist being chased by a bear is running in a ...
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11 views

Radiation energy intensity [closed]

A hot black body emits the energy at the rate of 16Jm^-2 s^-1 and its most intense radiation corresponds to 20,000À. When the temperature of this body is further increased and it is must intense ...
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1answer
49 views

Graviton Scalar interaction - Feynman rules

Having the interaction lagrangian for scattering between two scalars and the graviton $h$ \begin{equation} \mathcal{L}_{\phi \phi h}=\frac{\kappa}{2}\left(-\frac{1}{2} h_{\mu}^{\mu} \phi^{2} m^{2}+\...
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1answer
34 views

Natural frequency mass-spring system on inclined plane

I have to find $\omega$ for this system using the forces. I have a disc $radius = R, mass = M$ By using $F=ma$, I get $mg\sin\theta - kx = m\ddot{x}$ then $$-g\sin\theta + \frac{kx}{m} + \ddot{x} = 0$...
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1answer
75 views

$φ^3$ theory: three-point, one-loop diagram ($D=4$)

I am attempting to calculate the one-loop diagram for the $φ^3$ three-point function, which is equal to (dropping a factor of $-i$): $$g^3 \int_E \frac{1}{(2 \pi )^4} \frac{1}{(k+q)^2+m^2} \frac{1}{(k-...
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12 views

Energy density in a toroid

Is the energy density of a toroid of rectangular cross section uniform? I've watched this video (https://www.youtube.com/watch?v=p7MAABxuZLk) but I've done the maths by myself and did not get a ...
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29 views

What is the answer of it? [closed]

A surprising demonstration involves dropping an egg from a third-floor window to land on a foam-rubber pad 2 in (5 cm) thick without breaking. If a 56-gram egg falls 12 m, and the foam pad stops the ...
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31 views

How to solve the following exercise? [closed]

On the slope with a height of $2.0 m$ and the horizontal side of the slope of $5.0 m$, a sled descends, they accelerate evenly to the foot of the slope. Calculate the acceleration and final velocity ...
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1answer
85 views

How we can prove this vector identity?

I was trying to rederive the formula of the angular momentum of electromagnetic field, and all the steps are clear for me except this one which I took from "Photons and Atoms: Introduction to ...
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1answer
17 views

Why is the difference between 2 streamlines is the flow rate per width?

So is there any intuitive explanation why is the difference of a stream function at two different points is the flow rate per width at the same time? Kinda confusing..
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3answers
51 views

How can you show that the Moon's gravity is one sixth of the Earth's? [closed]

I've tried using \begin{equation} F_s = \frac{GMm}{d^2} \end{equation} but I got this answer: $1.8704 \times 10^{20} $ I'm not sure what I'm doing wrong?
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13 views

Radiation energy density [closed]

Assuming that the sun is a spherical black body with a radius of $7\times 10^8m$. Calculate the sun's temperature and radiation energy density within it. The intensity of the sun's radiation at the ...
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33 views

How to compute the cone angle? [closed]

This is question 4.84 from Optics, 5th Edition, Eugene Hecht A fish looking straight up toward the smooth surface of a pond receives a cone of rays and sees a circle of light filled with the images ...
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1answer
28 views

Tension pulley problem [closed]

A block of mass M rests on a block of mass M1 of 5kg which is on a tabletop. A light string passes over a massless, frictionless pulley and connects the blocks. The coefficient of kinetic friction μ k ...
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1answer
56 views

Missing factor 1/2 when using generalized Stokes theorem

I'm doing the following homework question: By invoking Stokes' theorem, according to which the integral of a vector field (which equals the field strength) over any two-dimension surface S that is ...
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27 views

A cell is connected in two kinds of circuit, one with only 1 load(a bulb) other with 2 bulbs in which case would it be used up faster and why? [closed]

Case 1- Cell is connected to 1 bulb Case 2- Cell is connected to 2 bulbs In which case cell would be used up faster?
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1answer
34 views

Angular velocity as a function of angle in simple pendulum [closed]

Say i've got this simple pendulum with it's corresponding newton equations in polar coordinates. equation in $\hat{r}$ axis is actually $mg\cos(\theta)-T = -mL\dot{\theta}$ for clarity, the original ...
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22 views

Latitude calculation using Day duration [closed]

I am currently studying for my exam in advanced photovoltaics. There is a question in a previous exam that states the following: Consider the day duration to be $10.8$ hours, the tilt angle to be $30$...
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1answer
24 views

Can you determine the speed needed to make a corner with using centripetal as well as centrifugal force?

A car weighing 800kg is driving on a motorway. It accelerates from 0 to 100 km / h in 3.5 s; this acceleration is only limited by the grip of the tires. a)How fast can it go in a curve with a radius ...
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2answers
26 views

How does the buoyant force on an object change as its being lowered into water?

Buoyancy I for one cannot find any answer to my question and myself am quite confused.Google isn't very much help for the question I have.I researched "how does it change as it being submerged ...
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1answer
21 views

Capillary action on capillary joints

There isn't much to say. In this question $B$ & $C$ are correct answers, and I can see how. However, I don't know how $D$'s validity could be determined. The solutions claimed that water will not ...
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32 views

What is the total degeneracy for the orbital motion for the case where the principal quantum number $n=4$? [closed]

I assumed the answer would be 9, as the angular number $l = n$ and the degeneracy is given by $2l+1$. However, my professor says my answer is wrong. I cannot spot the mistake.
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1answer
82 views

General Relativity and Tensors [closed]

One-form $\tilde{p}=(1,1,1,1)$ is given along with its basis $\tilde{\lambda}^{\alpha}$. How can I find the $l_{\alpha}$ $\tilde{p}=l_{\alpha}\tilde{\lambda}^{\alpha}$ Do I have to lower the index of $...
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2answers
75 views

Taylor Series Expansion of unknown, fraction function

I am learning about deformation, and the deformed state between two points can be defined as $$E(x) = \frac{(f(x+dx) - f(x))^2 - (dx)^2}{2(dx)^2}$$ My textbook says When $dx \to 0$ we can use a ...
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1answer
32 views

How to calculate overall temperature of two systems after pressure drop in one

If I have two closed systems (with a valve in between and assuming total insulation from environment) with the first being, Dry air, T1 = 321K, P1 = 101325 Pa, and V1 = 1 m3 and the second being, Dry ...
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2answers
46 views

Hamiltonian for multidimensional dissipative system

I am trying to find hamiltonian for system described by EOM $$ \ddot{x}(t) + \beta \dot{x}^2(t)\sqrt{\dot{x}^2(t)+\dot{y}^2(t)} = 0, \\ \ddot{y}(t) + \beta \dot{y}^2(t)\sqrt{\dot{x}^2(t)+\dot{y}^2(t)} ...
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44 views

This is a question from renormalized perturbation theory that appeared in my exam. I have very little idea about renormalized perturbation theory [closed]

Derive the one-loop renormalized $ψ$ selfenergy to O($h^{2})$ in the theory $L_{int} = −h|ψ|^{2}φ$. For what kinematical region does this self energy become complex? Do you have any physical ...
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1answer
22 views

How would I find the height that the ball rebounds? I found impulse, but I don't know what to do with it [closed]

A ball (1.5kg) falls from rest from a height of 2 meters. When the ball hits the ground, it experiences a 1500 N force upwards for 0.01 seconds. To what height will the ball rebound? (How high will ...
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23 views

How to get a vs X from G vs X?

Typically for this diagram(also taken from the book Porter and Easterling) I really do not understand how is activity of A vs mole fraction of A been derived.I really do not understand how they ...
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29 views

Kleppner and Kolenkow errata?

I’m self-studying “An Introduction to Mechanics” by Kleppner and Kolenkow, 2nd edition. Is there a published errata (list of corrections) for the book and more importantly the solutions manual? I’ve ...
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1answer
25 views

Does car push a broken down car exert a force on it if both are travelling at the same crusiing speed [closed]

If both bodies are travelling at a constant cruising speed, does that mean by Newton's First Law, they are not exerting a force on each other? Only when they're accelerating do they exert a force on ...
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17 views

Optical phenomena due to temperature gradient in the atmosphere

What kind of phenomena can we observe due to the deviation of the light in the atmosphere, for example when the reflection index vary with the altitude. I would say that the orange lights in the ...
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17 views

Using Binding Energies to Prove Whether Decay Occurs [closed]

I am to use a table of binding energies to show that: a) a C-12 nucleus won't decay into 3 alpha particles and b) a Li-7 nucleus won't decay into one alpha particle and a tritium (H-3) particle. I ...
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26 views

Group Theory Commutation Relations [duplicate]

Can someone help me prove these commutation relations? $$[M_{ij},M_{kl}]=\delta_{jk}M_{il}+\delta_{il}M_{jk}-\delta_{ik}M_{jl}-\delta_{jl}M_{ik}$$ where $i, j, k, l$ run from $1$ to $3$. And: $$[M_{\...
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1answer
34 views

Change in temperature of overflowing container

Mixing of identical fluids at different temperatures is simple, as per here: https://physics.stackexchange.com/a/24433/290018 We have a slightly different situation in that the container is ...
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1answer
58 views

Expansion coefficient for an ideal gas at constant pressure is $α = 1/Τ$. Explain this relation

How the coefficient of expansion for an ideal gas is given by $α=1/Τ$, (at constant pressure). How could it be inversely proportional to it. Doesn't gas expand more on higher temperature, but this ...
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15 views

Some questions about sub atomic decays

How do I argue that the following process can or can't occur? $K^{+} \rightarrow \pi^{0}+\mu^{+}$ Q lhs: 1 Q rhs: 1 $L_{\mu}$ = 0 LHs and 1 RHS is it sufficient to argue that the conservation of ...
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2answers
35 views

Why is it necessary for the block to not move only when the wedge accelerates?

Angel of inclination = $37^{\circ}$ Here , there a wedge of mass 5m and a constant force is being applied on the wedge . Now , it says in book that for the mass m (block) to not move , the wedge must ...
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15 views

Derivation of the collision frequency of gas with the walls of cylinder container [closed]

A cylinder with a diameter of 1 m contains 4 kg of helium at a temperature of 300 K What is the frequency of collisions of each molecule with the wall?

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