Tagged Questions
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.
0
votes
1answer
17 views
Straw and Water
I drink from a glass of water with a vertical straw. What's the longest straw I can use and still drink water if the ambient pressure is 1 atm?
Details and assumptions
1 atm is 101,325 Pa.
The ...
-1
votes
0answers
11 views
How do I redraw this circuit?
To find the effective resistance between A and B, how do I redraw/simplify this circuit, considering each resistor has equal resistance R?
3
votes
0answers
42 views
Derivation of the quadratic form of the Dirac equation
I am asked to derive the quadratic form of the Dirac equation in an electromagnetic field,
$\left[\left(i\hbar \partial - \frac{e}{c}A\right)^2 - \frac{\hbar e}{2c} \sigma^{\mu\nu} F_{\mu\nu} - ...
2
votes
1answer
38 views
How to show that every Killing vector field is a matter collineation?
Various texts make this claim, but no proof is given.
Explicitly, let $L$ denote the Lie derivative. Suppose $L_X g_{ab} = 0$ for some vector field $X$, called a Killing vector field. Suppose that ...
0
votes
1answer
33 views
Force experienced on two particles in a rotating system?
I've a system of two particles of the same mass who rotate in a circle about the centre of mass of the two particles. Is the force experienced by the particles $F=MV^{2}/r$ or should I use ...
1
vote
1answer
29 views
How would you quantify and predict the internal friction and efficiency of a bungee cord?
I am in an introductory physics class and we have been working with Gravitational Potential Energy, Elastic Potential Energy, Kinetic Energy and the Law of Conservation of Energy in relation to bungee ...
-3
votes
0answers
37 views
Velocity of any point on the rim of a rolling body [on hold]
I recently read in a book that the velocity of any point on the rim of a Rolling body is:
$$v = v_{com} \sin \frac{\theta}{2} $$
where $\theta$ is angle made with vertical made by point with centre ...
-2
votes
0answers
39 views
Gravitation problem about field intensity [on hold]
Infinite number of bodies, each of mass 6 kg, are situated at distances 1 m, 2 m, 4 m, 8 m,...
from the origin on the y-axis. The resultant gravitational field intensity at the origin is
(A) 4G ...
0
votes
0answers
19 views
Rolling on inclined plane [on hold]
*A sphere is rolling on a horizontal plane and an inclined plane going downards comes on its way. Now the question is to find max value of velocity of COM so that the sphere rolls on the inclined ...
-1
votes
1answer
23 views
friction between 2 bodies [on hold]
There is a plank on which a block is kept. the block is given some velocity. Now if there is friction between block and plank it will act opposite to it's velocity for sure. but the plank is free to ...
2
votes
2answers
120 views
${\gamma^5}^\dagger = \gamma^5$
How do you show that ${\gamma^5}^\dagger = \gamma^5$, where:
$$ \gamma^5 := i\gamma^0 \gamma^1 \gamma^2 \gamma^3$$
and we know that:
$$ ...
-1
votes
1answer
26 views
Determining distance from object in concave lens [on hold]
so I was studying for my Yr11 Physics test tomorrow and I came across a question that I got the wrong answer on, all of my friends are getting the same answer.
Question: You wish to project the image ...
0
votes
0answers
31 views
Can somebody answer this question about “ Vector Integration ”, taken from Arfken's Essential Mathematical Methods for Physicist? [on hold]
I can't seem to figure out how to answer this vector integration problem in particular. Not to mention that I am not even a Physics Major, so I need a COMPLETE AS POSSIBLE solution to this one, please ...
3
votes
0answers
47 views
Adiabatic expansion in van der Waals gas
Given a Van der Waals gas with state equation:
$$\left( P+\frac{N^2 a}{V^2}\right)\left( V-Nb \right)=NkT,$$
show that the equation of an adiabatic process is:
$$\left( ...
1
vote
0answers
31 views
Buoyancy Correction for a Kater Pendulum
Question:
Consider a Kater pendulum: that is, a rod with two cylindrical masses at both ends, with a knife edge between the two masses and another between one ...
7
votes
3answers
64 views
Correct expression for experimental data
I am doing practices at the laboratory. I have some doubts about how to express correctly the errors; I read some pdfs in Google but I can't solve these questions:
Sometimes, it's correct to write ...
1
vote
2answers
54 views
Fluid Mechanics Piston Problem
The actual problem:
A hydraulic car lift has a reservoir of fluid connected to two cylindrical fluid filled pipes. The pipe directly below the car has a diameter of 1.8 m. the pipe on which the ...
0
votes
0answers
34 views
Determining the neutral axis of an I-shaped cross section with dissimilar materials
Determine the neutral axis of the I- shaped cross section with dissimilar materials.
The top rectangle of the cross section is made out of aluminum and the second and third part are titanium ( these ...
2
votes
1answer
63 views
Retarded (Advanced) GreenFunction [on hold]
I need to show that Green Function:
$\displaystyle G(\vec{r},t;\vec{r}^{\prime},t^{\prime}) = \frac{\delta(t-t^{\prime}\pm |\vec{r}-\vec{r}^{\prime}|\,/c)}{|\vec{r}-\vec{r}^{\prime}|}$
Obeys
...
0
votes
1answer
11 views
Want to achieve maximum magnetic force & find out the max point [on hold]
Hi Sir/ Madam
Want to achieve maximum magnetic force & find out the max point for this setting, want to know
is the strongest magnetic force must in the 'dot' area ( middle poin of setting )
...
1
vote
1answer
34 views
Commutator evolution operator and position operator
Let $H= \frac{p^2}{2m}$, then I am supposed to calculate $[x,e^{-iHt}]$.
My idea was to use $[x,p^n]=i \hbar n p^{n-1}$ and so I ended up by using the series for the exponential function with ...
-2
votes
1answer
47 views
Poisson's equation with a point charge source [on hold]
How do you derive the solution to Poisson's equation with a point charge source? Without using Coulomb's law or the electric field! To be more explicit, we have a point charge at $(0,0)$ of charge $q$ ...
-1
votes
1answer
24 views
Proving velocity of box independent on angle of incline [on hold]
a box slips with $a=0$ on incline with angle $\alpha$. when getting to the bottom of the incline, the box is pushed up in velocity $v=10\frac m s$. suppose the height of incline is 1.8 meters prove ...
-2
votes
0answers
25 views
Trolley with a pendulum [on hold]
The upper end of the thread is attached to the ceiling of a trolley of mass $M=4kg$.
Initially, the trolley is stationary and it is free to move along horizontal rails without friction. A ...
0
votes
0answers
11 views
Characteristic length of differential element of cylinder surface?
I am trying to find the Nusselt number for a small element of the outside of a cylinder that has a height of $Delta z$. I found the average Grashof number of a surface as
$$Gr_{L}=\frac{\beta \rho ...
-1
votes
0answers
20 views
Concentrations at equililibrium [migrated]
"At 25 $C^o$, K=0.090 for the reaction $H_2O_{(g)}+Cl_2O_{(g)}=2HOCL_{(g)}$
Calculate the concentrations of all species at equilibrium forthe case:
1.0 g $H_2O$ and 2.0 g $Cl_2O$are mixed in a 1.0 L ...
0
votes
0answers
37 views
Finding kinetic friction coefficient
A box is thrown up an incline with degree $\alpha$. the kinetic-friction coefficient is $\mu_k$. the body returns back to its start point.
a. prove ...
-1
votes
0answers
38 views
Driven oscillations [on hold]
A force of friction $\vec f =- k_1\vec v$
affects the block of a horizontal elastic pendulum where $\vec v$ is the velocity of the block at any time and $k_1$ is a positive constant. In order to have ...
0
votes
1answer
27 views
Electrodynamics: Polarisation [on hold]
A sphere of radius $R$ carries a polarisation give by $P(\vec{r}) = br^{2}\hat{r}$ where $r$ is the radial distance from the centre of the sphere and $b$ is some constant.
1). Calculate the bound ...
1
vote
0answers
59 views
Exponential map of 2x2 matrix [migrated]
I saw the following formula (in (7.25) of Lie Groups, Physics, and Geometry)
$$
\exp\left(
\begin{array}{c}
w & z\\
0 & 1\\
\end{array}
\right)
=
\left(
\begin{array}{ccc}
...
5
votes
1answer
51 views
How do I measure the spin along an arbitrary direction?
I need to measure the atoms spin along a direction perpendicular to the z-axis but at an angle
$$
\phi
$$
with the x-axis.
They then get the output of the spin measured is + with probability
$$
...
-2
votes
1answer
35 views
How to calculate how much temperature can a cpu fan reduce? [on hold]
I need help for a college project. I need to write an simple Java application that calculates the temperature of an CPU when over clocked.
My question is how can I calculate how much temperature the ...
-2
votes
1answer
43 views
What distance has the soldier travelled? [on hold]
A troop $5$ meters long starts marching. A soldier at the end of the file steps out and starts marching forward at a higher speed. On reaching the head of the column, he immediately turns around ...
-1
votes
1answer
25 views
Scalar Field of a long thin wire
Given the situation that $B=-\nabla A$ where B is magnetic field and A is some Scalar Field. How can I calculate the scalar field, A.
We are dealing with current free region, here.
I know we can ...
-1
votes
0answers
28 views
How to determine the electric potential energy for the array of three charges? [on hold]
The following is the question.
Determine the electric potential energy for the array of three charges
shown in the drawing, relative to its value when the charges are
infinitely far away.
...
0
votes
0answers
17 views
How to relate compression ratio to work input for a compressor in a heat pump
So I need to calculate the COP (Coefficient of Performance) for a 6-ton air conditioner system in terms of the compression ratio. I understand that:
$$COP=\frac{Q_L}{W_{net,in}}$$
I cannot find out ...
0
votes
0answers
16 views
Finite volume method to determine steady state temp in cylinder [migrated]
I am attempting to model the steady state behavior of a cylinder using the finite volume method (FVM) subjected to a variety of boundary conditions in Matlab. First off, I am treating the cylinder as ...
-2
votes
0answers
37 views
Performance of LRC circuit [on hold]
There is a LRC circuit connected with a resistor and capacitor in parallel, connected in series to the inductor. The circuit is connected to an AC source. I am unsure how to calculate the current $I$.
...
1
vote
2answers
59 views
Classical Mechanics - Allowed systems
I've edited my question to clear any possible confusing parts:
This an exercise from the book "The Theoretical minimum", I'm paraphrasing.
In classical mechanics, dynamical laws must be reversible ...
0
votes
1answer
60 views
Archimedes' principle for two liquid layers
Problem: I have a cylindrical vessel of height $H$ and radius $R$. There are two liquid layers in the vessel. The first has density $D_1$ and height $h_1$, the second has density $D_2$ and height ...
1
vote
2answers
31 views
Conservation of Angular Momentum and linear velocity
I have a problem where a cylinder is moving on a horizontal surface, starting with velocity $v_0$. It is given that its radius is $10\text{cm}$, its mass is $200\text g$ and the coefficient of ...
-4
votes
0answers
29 views
Projectiles and escape velocity [on hold]
A body is projected vertically upwards with a velocity equal to $1/3$ of the escape velocity. What is the height reached by it if the radius of the Earth is $6400\,\mathrm{ km}$?
-4
votes
2answers
48 views
How many years would it take the Pioneer space probe to travel to Proxima Centauri with its current speed? [on hold]
Pioneer space probe moving at a speed of $30km/s$.
Assuming its heading for Proxima Centauri, which is situated at $4.2ly$ from earth, calculate how long it would take to get there in years, to the ...
3
votes
0answers
102 views
The commutator of scalar field [on hold]
I have a real scalar field which is given by the propagator as:
$$[\phi(x),\phi(y) ] =\int \frac{d^3 p}{(2\pi)^3} \frac{1}{2E_0} (\exp(-ip\cdot (x-y)) -\exp (ip\cdot (x-y)))$$
And I am asked to show ...
0
votes
0answers
20 views
Two transverse waves with equal frequencies f and amplitudes A move towards each other at velocity v. What is the resulting wave? [on hold]
I came across this question in a SAT Physics Practice Test.
The possible answers are:
...
-1
votes
1answer
29 views
Four protons are held on the corners of a square. What is their speed when they are far apart? [on hold]
This is a homework question, but I honestly have no idea how to go about it. Even the textbook says it's difficult, based on their disclaimer here:
"This is a challenging multi-step problem. Solve it ...
1
vote
1answer
25 views
Molecular formula of a compund [on hold]
I have no idea on how to get information out of this one.
A compound contains only hydrogen and nitrogen. Nitrogen makes up for 87.4% of the mass. A sample of this have a density of 0.977 g/L at ...
1
vote
1answer
54 views
Spinors and Probabilities of Electron-Positron Pair
Question:
An electron and positron are moving in opposite directions, and are in the spin singlet state. Two Stern-Gerlach machines are orientated in some ...
2
votes
1answer
33 views
Smallest number of quantum gates to simulate other gates?
What is the smallest number of Fredkin gates needed to simulate a Toffoli gate? What is the smallest number of Toffoli gates needed to simulate a Fredkin gate?
Where the Toffoli's gate is the CCNOT ...
2
votes
2answers
54 views
Relative velocities of boats and a stone thrown by the boat [on hold]
Question:
A police boat is chasing a boat with criminals along a straight river
by moving against the stream. The speed of the river stream is 3 miles
per hour, the speed of the boat with ...
