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.
-4
votes
1answer
70 views
Photonics: Slab As a Lens [closed]
The question can be found here: http://gyazo.com/fc4d26cd35e6ce368ad2a8ed504f1dcc
The refractive index it references can be found here: http://gyazo.com/94fd2f3b5ea7da9226c3acd56b0024c1
I'm not ...
1
vote
1answer
233 views
Kinematics - Find theta with Coefficient of Friction?
I recently found a problem that looked like this:
A box sits on a horizontal wooden ramp. The coefficient of static friction between the box and the ramp is ...
0
votes
3answers
183 views
Classical mechanics. One-dimensional motion
Here is one task below.
How to solve equation
$$
m\ddot {x} + ax = F(t), x(0) = \dot x (0) = 0
$$
in quadratures by using two methods?
I tried to create a system of equations
$$
\begin{matrix} \dot ...
0
votes
2answers
109 views
How do you calculate the time to emission of an electron from a metal given the incident radiation?
Here's the question:
A monochromatic point source of light radiates 25 W at a wavelength of 5000 angstroms. A plate of metal is placed 100 cm from the source. Atoms in the metal have a radius of 1 ...
-3
votes
2answers
745 views
How to add two perpendicular 2D vectors [closed]
vector D = 4 cm North
vector J = 4.5 cm West
what is D+J?
In a more general sense, how can two 2D vectors that are perpendicular to each other be added?
5
votes
1answer
262 views
Lagrangian density for a Piano String
So I'm trying to do this problem where I'm given the Lagrangian density for a piano string which can vibrate both transversely and longitudinally. $\eta(x,t)$ is the transverse displacement and ...
2
votes
0answers
159 views
include the stretch of the spring own weight in potential energy for spring pendulum?
we are given a problem with spring with its own mass $m$. I am confused how to set up the
PE term in the Lagrangian.
Assume the spring has length of $L_{0}$ when it is laying on a table
horizontally. ...
1
vote
1answer
203 views
Solving by using Gauss law
Task: find the vector $ \mathbf E $ in the center of the sphere with radius $R$, which has charge volume distribution $\rho , \rho = (\mathbf a \cdot \mathbf r ), \mathbf a = \operatorname{const}, ...
0
votes
2answers
215 views
Calculating two particle position and velocity with each other?
I've calculated the displacements and average velocities of both particles.
$A$ average velocity $= 1.1180m/s$ displacement is $11.1803m$.
$B$ average velocity $= 0m/s$ displacement is $0m$.
...
0
votes
0answers
477 views
Create general equation of racing cars with head start [closed]
Cars A and B are racing each other along the same straight road in the following manner: Car A has a head start and is a distance $d_A$ beyond the starting line at $t=0$. The starting line is at ...
1
vote
1answer
251 views
Modulus of Elasticity
Consider a cube of unit dimensions. Let $\alpha$ and $\beta$ be the lateral and longitudinal strains. The expressions for moduli of elasticity on applying unit tension -
1) At one edge: Young's ...
2
votes
1answer
156 views
Symbol for dashpot/damper (in a harmonic oscillator)
In diagrams that contain the dashpot symbol, sometimes the mass is attached to the "interior" end of the dashpot, other times the mass is attached to the "base" end.
For example, consider the ...
1
vote
2answers
276 views
Tensor product decomposition of SU(2)
I have a rather trivial question. I am looking for the decomposition of $1/2\otimes 1/2\otimes 1/2$. It should give, $0,1/2$ and $3/2$. I thought one must get as the overall dimension of this space 8, ...
3
votes
2answers
212 views
How to measure g using a metre stick and a ball
Can I measure the value of g using only a metre stick and a ball? I am not supposed to use a stopwatch and that has been the problem.
NOTE: I do not know if a solution exists or not.
1
vote
1answer
58 views
How to calculate the amout of time for oil to get out from a pneumatic cylinder?
I have a system. You can assume it is just a 1" diameter steel pneumatic cylinder (1 foot long). Assume we ignore the bore size issue at this point. One chamber is filled up by oil (6" of volume), ...
0
votes
0answers
81 views
Mass loss rate of planetary nebulae
The “interacting wind” model of planetary nebulae is based on the idea that the white dwarf phase of stellar evolution is preceded by a red giant phase. A fast wind from the hot white dwarf overtakes ...
3
votes
1answer
105 views
Help on unit conversion problem
This is a problem from school. I will show my attempt.
The question:
"The gas constant for dry air R is 287 $\frac{m^2}{s^2*K}$. Assuming the temperature is 330 K and the pressure is 1050 hPa, what ...
0
votes
1answer
180 views
Scattering problem: Finding the speed of the scatterer after collision
A particle of mass $M$ moving in a straight line with speed $v$ collides with a stationary particle of the same mass. In the center of mass coordinate system, the first particle is deflected by 90 ...
1
vote
1answer
87 views
What's the wavelength of an electron after hitting a potential barrier?
I have this question:
An electron with Energy $E = 40 eV$ hits a potential barrier with $E_0 = 30 eV$. What is the wavelength of the electron after hitting the potential barrier?
I worked from ...
0
votes
0answers
62 views
How to find the directional cosines?
l+m+n=0 where l,m,n are the directional cosines of a point P in the x,y,z plane. How do I find the angles the radius vector makes with the axes. I get two equations l+m+n=0 and l^2+m^2+n^2=1. But how ...
0
votes
0answers
813 views
Moment of Inertia problem [closed]
I'm revising for my exam and there's this one problem where I can't get the correct answer, so I'd like to know how to solve it. The problem is:
Two boys, each with a mass of 30kg, are sitting ...
1
vote
1answer
303 views
Calculating velocities using Reference frames
Suppose an object A is traveling at a velocity of 100 m/s, and another object B is traveling at 105 m/s. With both the objects traveling through the same direction, taking A as a reference frame, the ...
0
votes
1answer
281 views
Two Blocks of mass M1 and M2 are connected by a spring of force constant k [closed]
If Block 1 is elongated towards right to a distance $x_1$ and Block 2 is elongated towards left to a distance of $x_2$ simultaneously, what is the work done by the spring on each of these blocks ...
0
votes
1answer
72 views
Finding the number of particles scattered by a certain angle
I'm trying to do the problem below, but it seems like there is incomplete information.
PROBLEM STATEMENT:
In a scattering experiment, $10^6$ $\alpha$ particles are scattered at an angle of ...
0
votes
1answer
2k views
How to calculate work done by variable kinetic friction force?
A heavy chain with a mass per unit length $\rho$ is pulled by the constant force $F$ along a horizontal surface consisting of a smooth section and a rough section. The chain is initially at rest on ...
1
vote
0answers
104 views
How to show the oblique parameters S, T, and U are coefficients of d=6 operators
In Morii, Lim, Mukherjee, The Physics of the Standard Model and Beyond. 2004, ch. 8, they claim that the Peskin–Takeuchi oblique parameters S, T and U are in fact Wilson coefficients of certain ...
1
vote
1answer
255 views
What is physics of a normal jump?
What is the physics behind a normal human jump ?
when a normal human wants to jump.
First, some energy is stored in their thighs and The elastic tendons Just like a spring.
In mechanics and ...
2
votes
0answers
74 views
The force exerted by a ~10 Tesla magnet on a C13 isotope?
How much force would a ~10 Tesla magnet exert on a weakly magnetic C13 isotope? If I made a molecule of diamond with $N$ C13 atoms, how large would $N$ need to be for me to pull on it with something ...
0
votes
1answer
190 views
Time taken for object in space to fall to earth
The Problem
For a small mass a distance $R_i$ away from the center of the Earth, how long would it take for the object to fall to the surface of the Earth, assuming that the only force acting upon the ...
0
votes
1answer
141 views
Gauss's Law in action
Need someone to tell me if I got this done correctly
(a) Draw Gaussuian cylinder inside the black cylinder to find charge enclosed
$Q_{en} = Q(\frac{r}{a})^2$
Apply Gauss's Law
$E2\pi r \ell = ...
5
votes
2answers
284 views
relativistic acceleration equation
A Starship is going to accelerate from 0 to some final four-velocity, but it cannot accelerate faster than $g_M$, otherwise it will crush the astronauts.
what is the appropiate equation to constraint ...
0
votes
2answers
259 views
Maximum velocity allowed for motion profile with constant jerk
I´m trying to calculate what the maximum allowed velocity for a mechanical axis traveling towards a stop. The maximum velocity must be a secure speed so the axis have is able to stop in time.
I have:
...
0
votes
1answer
163 views
Mathematical derivation of $N(\lambda)d\lambda$
We all know that in Rayleigh-Jeans law,
$$N(f)df ~=~ 8\pi f^2 df/c^3.$$
How do you derive $N(\lambda)d\lambda$?
I am sort of confused...
3
votes
3answers
169 views
How to compute the speed necessary for an airplane to fly?
I give some physics lessons to a friend. She asked me a question that I am unable to answer. Could you help me ?
A plane has a weight of $2\times10^6$kg. The surface of the wing is $1200 \text{m}^2$. ...
3
votes
4answers
2k views
Calculating impact force for a falling object?
Good evening, I'm trying to calculate what kind of impact force a falling object would have once it hit something. This is my attempt so far:
Because $x= \frac{1}{2} at^2$, $t=\sqrt{2x/a}$
$v=at$, ...
2
votes
2answers
556 views
Electric potential of sphere
(a) I am a little confused about this part. The point at A to B isn't radial. The electric field is radially outward, but if I look at the integral
$$\int_{a}^{b}\mathbf{E}\cdot d\mathbf{s} = ...
0
votes
1answer
150 views
Condition for circular orbit
I am a little confused about the condition for circular orbit. Goldstein's Classical Mechanics has the condition for circular orbit as $$f^'=0\tag1$$ where $f^'$ is the effective force. I understand ...
0
votes
0answers
128 views
Reduction in velocity in pipe
I have two pipes divided from main pipe. If I add one more pipe, what would be the amount of velocity in each pipes. I know the velocity of main pipe, then diameter and radius of each pipe.
Edit:
1) ...
0
votes
3answers
453 views
When I connect two charged capacitors side by side, what will be the voltage across them?
Say, I have two charged capacitors, one 3mF and one 2mF. The voltage across them are 20V and 30V respectively. Now if I connect the two capacitors side by side as shown below, what will be the voltage ...
0
votes
1answer
169 views
Energy required to kick a planet orbiting the Sun from an elliptical to a parabolic path
I am trying to solve the following problem from Goldstein's Classical Mechanics:
A planet of mass $M$ is in orbit of eccentricity $e=1-\alpha$ where $\alpha<<1$, about the Sun. Assume that the ...
2
votes
3answers
954 views
How do I find work done by friction over a curve represented by a polynomial?
I am facing a problem in Physics.
Problem: What will be the work done by the frictional force over a polynomial curve if a body is sliding on this polynomial($a+bx+cx^2+dx^3+\ldots$) curve from rest ...
0
votes
1answer
88 views
Why are there two ways to solve for energy of a spring?
I can find the energy of a spring using $F = -kx$, or by using the formula $e = 1/2mv^2 + 1/2I\omega^2 + mgh + 1/2kx^2$. The first way, I get $mg/k = x$, but the second way, I get $2mg/k = x$. Which ...
0
votes
1answer
266 views
Calculating the magnetic flux density of a thin, flat conductor
I am currently preparing for a physics exam and I have troubles understanding how to solve the following exercise:
A current that has the constant current density $\vec{j}$ flows through a thin, ...
0
votes
0answers
65 views
Kinematics equation [closed]
The motors of an electric train can give it an acceleration of 1 m/ sec square and the brakes can give a negative acceleration 3 metre/ second square. Find shortest time interval in which train can ...
3
votes
3answers
1k views
0
votes
1answer
109 views
Scattering problem: Expression for angular momentum of particle
I'm reading Goldstein's Classical Mechanics, the part on "Scattering" in the "Central Force" chapter.
In relation to the figure below, he says that angular momentum, $l$, is given by $$l=mv_0s$$ ...
1
vote
1answer
616 views
Calculate mass of air in a tyre from pressure
How can one calculate the mass of air inside a tyre, given a particular tyre size; a pressure, in $kPa = \frac{1000kg}{m\cdot s^2}$; and assuming room temperature, and normal air composition? I can't ...
2
votes
0answers
69 views
Randomly sampling a “well-mixed” solution of Brownian particles
I place $N$ Brownian particles in $V$ liters of solution, shake until I assume that the particles are "well-mixed", and sample and randomly sample an $S$ liter volume. What is the probability ...
3
votes
0answers
140 views
How to build a lab at home/school? (general purpose, sophisticated, reliable and cheap) [closed]
The title includes the main objective and all the criteria; namely:
A. A lab is to be built.
B. It must adhere to the following criteria:
Reasonably cheap for a limited-fund ...
0
votes
0answers
539 views
Capstan equation [closed]
This problem is from An Introduction To Mechanics book, chapter 2, problem #24.
A device called a capstan is used aboard ships in order to control
a rope which is under great tension. The rope is ...
