Stack Exchange Network

Stack Exchange network consists of 174 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

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.

-1
votes
1answer
15 views

Electric field of two equal charges of the same sign

Two equal positive charges are at distance d, -d from the origin on the y axis. What is the distance on the x axis beyond which a small perturbation in y will move a particle away from the x axis? ...
-4
votes
0answers
13 views
1
vote
0answers
17 views

conventional matrix notation for distance interval

Why matrix notation for distance interval is represented by this? $$g_{\mu \nu}\Delta X^{\mu}\Delta X^{\nu}=(\Delta X)^Tg (\Delta X)=\Delta X^{\mu}\Delta X^{\nu}g_{\mu \nu}$$ Could you explain ...
0
votes
1answer
6 views

Rolling without slipping around a fixed axis (practice exam) [on hold]

Everything is pretty much given in the image. We had to find the relation between omega-p and omega-s The answer is omega-p * (r+a*cos(alpha)) = omega-s * a It says that the in the inertial frame ...
0
votes
0answers
19 views

Variable mass pendulum (water filled in Bob)?

There is pendulum having Bob made of hollow sphere, having water filled in it. Now at t=0 , a small hole is made on bottom of sphere ,at the same time pendulum is given oscillation. (Assume no damping)...
-2
votes
0answers
30 views

Number of collisions with block [on hold]

The problem is given in David Morin's Classical mechanics. Here is the screenshot: Now, I jumped to solve part b of question. To find the number of bounces,we note that mass losses momentum of -2mV ...
0
votes
0answers
23 views

Dirac matrices in 1+1 dimensions

Given $\gamma^\mu$ in 1+3 dimensions with signature $(+,-,-,-)$, how can I obtain Dirac matrices in 1+1 dimensions expressed in terms of the $\gamma^\mu$?
0
votes
0answers
58 views

Can’t we use ‘vector product’ to find the angle between two vectors? [migrated]

There are two vectors : $A = (\hat i + j + k)$ and $B = (\hat i - \hat j - \hat j)$, where $\hat i$, $\hat j$, and $\hat k$ are unit vectors along $x$, $y$, and $z$ axis respectively. We have to find ...
0
votes
1answer
38 views

How fast would an object at cruising altitude (around 10,600 m) need to travel to stay on the opposite side of the Earth to the Moon? [on hold]

I am trying to figure out how fast an object will have to travel (at a height around 10,600 m) to never be in the presence of the moon. Meaning the object will always be on the opposite side of the ...
0
votes
0answers
17 views

Why is the electric current is a scalar quantity? [duplicate]

Please help me because I didn't know why. If you know please help me!!
0
votes
0answers
27 views

Ground state of two fermions coupled with one boson

I have an Hamiltonian of the form: \begin{align} \begin{aligned} H _ { 1 } & = f _ { 1 } \otimes b ^ { \dagger } + f _ { 1 } ^ { \dagger } \otimes b \\ H _ { 2 } & = f _ { 2 } \otimes b ^ { \...
-2
votes
0answers
40 views

QFT dirac notation calculation [on hold]

I am trying to calculate $ \langle0|{\alpha_{p}^{\dagger}}{\alpha_{q}^{\dagger}}|0\rangle$ using the definitions below: $|p\rangle = \sqrt{2\omega_p} {\alpha_p}^\dagger |0\rangle$ and $|q\rangle = \...
1
vote
2answers
27 views

Can't see any traces of particles in my Wilson chamber without radioactive material

I am doing an essay about particle physics and I need some help. I am comparing the results in a Wilsonchamber with and without a radioactive material present. I were able to see a lot of traces from ...
1
vote
1answer
36 views

Real gas heat calculation

For the Van der Waals gas we get a cycle consisting of 2 isobaric and 2 isenthalpic processes. The state equations are given as: $p = \frac {RT}{v-b} - \frac {a}{v^2}$ $\;$ and $\;$ $u = cRT ...
0
votes
1answer
15 views

Problem on force components [on hold]

In this problem, is the direction of tension(725N) shown in the free body diagram correct? Wouldn't such a direction lead to compression instead of tension.
0
votes
0answers
19 views

Flow generated by vortex near a rigid cylinder

I have a problem with complex potential and can't figure out how to approach it properly. The problem: Find flow around cylinder of radius $a$ generated by vortex of cicrculation $\Gamma$ at point $b$...
0
votes
0answers
20 views

Kirchoff's rule DC circuits [on hold]

I understand how to solve for the unknown variables but I have trouble labeling the currents and finding the current and junction equations
-1
votes
0answers
16 views

Determine Fuel Flow for the piston engine [on hold]

A piston engine produces 400 kW of power. Its sfc is 75 μg/J. Determine the fuel flow in kg/hr.
-1
votes
1answer
9 views

Finding the carrier concentration of semiconductor

Silicon (4 valence electrons) can be doped with arsenic (5 valence electrons) to make an n-type semiconductor. Assuming 1 in every 15 million silicon atoms is replaced with arsenic, calculate the ...
-1
votes
1answer
45 views

Vector addition of electric fields [on hold]

I have calculated the electric fields of three points in a right triangle, the fields being $E_1 = 2.7\times10^4 ~\mathrm{N}\cdot\mathrm{C}^{-1}$, $E_2 = -9\times10^3 ~\mathrm{N}\cdot\mathrm{C}^{-1}$, ...
-1
votes
0answers
14 views

How to determine amount of heat absorbed by evaporator, power consumed by the compressor? [on hold]

A refrigerator having NH3 as refrigerant .Superheated vapor at 0c and 300 kpa enters the commpressor and compress isentropically till 2000kpa .water cooled condenser rejects heat from vapor till it is ...
-2
votes
0answers
30 views

Problem about fluid flow in a pipe [on hold]

Water at 15°C flows by gravity from one deposit to another, which has its water level 20 meters lower than the first one, on a smooth pipe with a 50 mm diameter and a 100 meters length. Calculate the ...
0
votes
2answers
38 views

Solving elastic 2-dimensional collision problems where the resultant angle is not provided [on hold]

Having real trouble with the question below that is part of my engineering course. I've sussed question 6 but making no headway with Q7. A ball of mass 0.2 kg collides elastically with a heavier ball ...
-1
votes
1answer
48 views

From where to start such a question? [duplicate]

13 equal charges, q , are placed at the corners of a 13 sided regular polygon. What is the force on a test charge Q at the centre? I couldn't figure out from where to start solving it. It would be ...
-2
votes
0answers
24 views

What is the possible value for $A$ and Stress? [on hold]

So I'm trying to attempt this question, I've a paper tomorrow so I came here for a quicker answer, what I've tried already is considering that it can withstand 4kN from both sides but then total ...
-2
votes
0answers
27 views
0
votes
0answers
15 views

Frequency modes of the rectangular shell

The shapes of three natural modes having the frequencies $w_1$, $w_2$, $w_3$ of the rectangular shell are presented in the figure. The exciting pressure $p(t)$ applied uniformly all over the one side ...
-1
votes
0answers
13 views

Derive conservation laws for the energy momentum tensor, $T μν$ in coordinates $(τ,x,y,η)$

How can i derive this relation? i need simulation Transverse momentum spectra of π+ and K+ , but i do not know what discretized this tensor coupled partial differential equations`]1
0
votes
1answer
35 views

Calculate temperature decrease of an evaporating droplet

I have a small droplet of Cyclohexane which evaporates at room temperature and ambient pressure. I measured the volume at two different times ($\Delta V=1.5\times10^{-10}$ m$^3$). I would like to ...
0
votes
3answers
59 views

Can a wave's Poynting vector be in the opposite direction compared to its direction of propagation?

Can a wave's Poynting vector be in the opposite direction compared to its direction of propagation, and if so, what physical implications does it have? As I understand, the poynting vector s can be ...
0
votes
1answer
28 views

Total energy, total momentum of two different balls [on hold]

I have a ball out of plasticine thrown horizontally with speed $v$ on a steel ball that is attached from top to the ground on flexible straps in the middle (see picture). The mass of plasticine is $m$,...
-2
votes
1answer
31 views

How can I calculate the energy or force required to make an object a certain density? [on hold]

I have an object with the mass of 1 gram, the volume of $0.0002192~\rm {cm}^3$ and the density of $4.56 ~\rm g/{cm}^3$ and I need to calculate the force/energy required to make it have a density of $7....
0
votes
0answers
66 views

Derivation of $\frac{\cos(\theta)dA}{r^2} = d\omega$ [on hold]

Note: I asked the same question on the mathematics stackexchange, but was advised to ask it here. It also seems to arise quite often in physics. I've been looking for a (formal) derivation of the ...
-2
votes
1answer
46 views

Simple Kinematics question [on hold]

So this is a pretty simple kinematics question I was working on but I completely skipped over a derivation. Here's what I did to solve for the first part: I'm wondering if there is anyway to solve ...
0
votes
0answers
10 views

Finding resistance and how many appliances can be connected

A household electricity system can be modelled as a fuse in series with appliances, all of which are in parallel across the 240V mains supply. In one household there is a 16A fuse in series with the ...
-4
votes
1answer
37 views

Electric field at center of a uniformly charged hemispherical shell [on hold]

My attempt : I divided the shell into charged rings by whom I know the electric field at the center by the following relation : $\hskip2in$ Integrating the electric fields by all the rings I get ...
0
votes
1answer
21 views

Evaluating a trace with two factors of $\gamma^5$

In the process of calculating a spin-averaged square amplitude in QFT, I came across the following expression: $$ \text{Tr}\left[\gamma^\mu\gamma^5\gamma^\alpha\gamma^\nu\gamma^5\gamma^\beta\right] $$ ...
0
votes
1answer
28 views

Physical explanation of Dirac-Born-Infeld (DBI) inflation

I am studying the Dirac-Born-Infeld (DBI) inflation model and came across this question in a past exam paper from Cambridge that considered the following Lagrangian: $$\mathcal{L}=\sqrt{-g}A(X,\phi)$$...
-6
votes
0answers
30 views

I'm clueless, need help [on hold]

Easy problem but I'm clueless about my approach
-5
votes
0answers
19 views

How is the graph of density-temperature in an isochoric process [on hold]

What will the graph between density and Temperature be like in an isochoric process?
-1
votes
0answers
23 views

Rocket triggering problem [on hold]

If a rocket is to be sent to a body moving with a velocity of 10 m/s from a body at rest at a distance of 1km, if the acceleration of the rocket is neglected and it is moving at a velocity of 50 m/s, ...
-2
votes
0answers
24 views

Finding resistance when given amperage and voltage [on hold]

Question: A household electricity system can be modelled as a fuse in series with appliances, all of which are in parallel across the 240V mains supply. In one household system, there is a 16A fuse ...
-2
votes
0answers
35 views

*NEED HELP* How Many Helium Balloons (radius=0.15 m) to Lift a 50kg Person? (Using Archimedes' Principle)? [on hold]

So I stated the relationship that: B(air)+B(helium)=W(person)+W(ballons) According to Archimedes' principle. Then I substituted values for the density of air (1.225), helium (0.179), and the mass of ...
0
votes
0answers
46 views

Demonstration of simple harmonic motion [on hold]

I have been asked to demostrate that the following object has a simple harmonic motion It is a rigid body and we know its radius ($R$), its mass ($M$) and its moment of inertia $I =MR^2/2$ as it is a ...
-1
votes
0answers
35 views

Two parallel wires [on hold]

We've got a to parallel wires. We know that on the wire 1 is potential $v_{1}$ beetween its ends, conductivity $σ$, current $I_{1}$, $S$ is cross-section area and $d$ is distance beetween wires, $L$ ...
0
votes
1answer
28 views

Why is the entropy of a subsystem density matrix equal to that of the other subsystem?

If we have some system $\rho_{AB}$ we can find the entanglement entropy as $S(\rho_A)=S(\text{Tr}_B(\rho_{AB}))=S(\text{Tr}_A(\rho_{AB}))$ But why are the entropies of the subsystems $\rho_A$ and $\...
-1
votes
1answer
48 views

Lagrangian Mechanics - Charged particle in magnetic field

A particle of mass $m$ and charge $Q$ moves in the equatorial plane ($θ=π/2$) of a magnetic dipole, where the vector potential of the dipole is given by $$\mathbf{A} = \dfrac{\mu \sin \theta }{4 \...
-1
votes
1answer
52 views

Age of empty universe vs actual universe [on hold]

Can anyone please explain the following question for me? I know how to get to the formula to find age from Friedmann equation but I am not sure how to answer this question.
-3
votes
0answers
23 views

I was doing the question in the picture and I don't know why the tension is different [on hold]

I tried solving the question assuming both the tensions same hence my answer was a= (M-m)g/M-m but it is wrong the solution in the book says the tension is different but don't know why is it because ...