# 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.

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 ...
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 ...
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 ...
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 ...
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?
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 ...
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. ...
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}, ...
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 $0m$. ...
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 ...
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 ...
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 ...
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, ...
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.
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), ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 = ...
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 ...
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: ...
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...
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 $1200 \text{m}^2$. ...
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 $x= \frac{1}{2} at^2$, $t=\sqrt{2x/a}$ $v=at$, ...
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} = ...
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 ...
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) ...
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 ...
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 ...
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 ...
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 ...
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, ...
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 ...
3answers
1k views

...
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 $$l=mv_0s$$ ...
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 ...
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 ...
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 ...
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 ...
{"extraction_info": {"found_math": true, "script_math_tex": 0, "script_math_asciimath": 0, "math_annotations": 0, "math_alttext": 0, "mathml": 0, "mathjax_tag": 0, "mathjax_inline_tex": 58, "mathjax_display_tex": 4, "mathjax_asciimath": 0, "img_math": 0, "codecogs_latex": 0, "wp_latex": 0, "mimetex.cgi": 0, "/images/math/codecogs": 0, "mathtex.cgi": 0}, "config": {"markdown_headings": true, "markdown_code": true, "boilerplate_config": {"ratio_threshold": 0.18, "absolute_threshold": 10, "end_threshold": 5, "enable": true}, "remove_buttons": true, "remove_image_figures": true, "remove_link_clusters": true, "table_config": {"min_rows": 2, "min_cols": 3, "format": "github"}, "remove_chinese": true, "remove_edit_buttons": true, "extract_latex": true}, "english_confidence": 0.9140781760215759, "perplexity_flag": "middle"}