# All Questions

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### What are these rays that appear in photograph of sun?

In many images of light emitting objects we see such rays. Why do they appear ? What is the math behind their number and direction?
582 views

### Rotating/Translating Disk

I was trying to understand an aspect of rotational dynamics and thought of a problem to help me learn. I'm sure this problem has been considered by countless people in the past, but I'm having some ...
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### A Different Lasing Medium [closed]

Powerful lasers are highly intense, diverge negligibly and are also coherent. These radiations are emitted through partially reflecting mirrors after simultaneous reflections within the lasing medium. ...
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### Lab observation correct? As distance decreases, velocity increases, stderr decreases

The experiment goes like this: Allow a moving cart to move from the top of an incline plane ($x_0$) downwards. The time taken will be recorded by the picket fence (those things you see wired up). ...
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### How can an action be dependent on both its past and future?

Is it true that whenever an action takes place it is dependent on both its past and future? I mean if we already know that whatever we are doing is dependent on future as much as it is dependent on ...
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### Could we make a trebuchet that could launch objects to a stable orbit?

Inspired by this xkcd, which calculated the energy requirements for accelerating individual humans to escape velocity (regardless of consideration for what that would do to your organs), I am ...
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### Maximum Possible Information in the universe?

I remember hearing about this in one of the programs in discovery science. The physicist claimed that the maximum possible information in the universe is (10)^(10^123) whereas the maximum possible ...
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### Is thermal noise “quantum random”?

Is the randomness that can be extracted from thermal noise "as random" (that is, even theoretically inaccessible to measurement according to our knowledge of quantum mechanics, and not just random for ...
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### the difference between the operators between $\delta$ and $d$

In classical mechanics, when talking about the principle of virtual work, what is difference between $\delta r$ and $dr$? e.g. $W=\int \overrightarrow{F} \cdot \delta \overrightarrow{r}$ and ...
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### 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 ...
326 views

### Do I have the meaning of the property temperature correct?

OKay my book just starts out talking about the vague definition we have for temperature and we ended up with the Zeroth law of Thermodynamics which states: Two systems are in thermal equilibrium ...
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### Fictitious Forces $\overset{?}{⇔}$ Constraint Forces (re: D'Alembert's Principle)

Are fictitious forces and constraint forces the same thing?
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### thermodynamics of a dual-face surface in space

This question is a continuation from this one. A material disk have two sides, one that is reflective and another absorptive of electromagnetic radiation in the range where the background cosmic ...
2k views

### Why is there an escape velocity?

I've been trying for days, but I just can't understand why escape velocities exist. I've searched the web and even this site, and although I've read many explanations, I haven't been able to truly ...
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### 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 ...
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### Why does earth look blue from outer space?

I know it's more than 70% water. But what has it got to do with earth's colour ?
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### easy or formal proof of Hawking effect … that anyone can understand?

where could i find an explanation (appart from Wikipedia) of how the Hawking's effect is obtained from quantum field theory GR and thermondynamic :D
1k views

### What exactly is a Fluorescent lamp?

A fluorescent tube (home-based) works on the principle of discharge of electricity through gases, as far as I can tell (I don't know much about cathode rays or gas discharge) What happens inside the ...
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### Is acceleration due to gravity constant?

I was taught in school that acceleration due to gravity is constant. But recently, when I checked Physics textbook, I noted that $$F = \dfrac{G \cdot m_1 \cdot m_2}{r^2}$$ So, as the body falls ...
833 views

### Calculating diffraction patterns using FFT

I'm trying to write a piece of code that calculates a diffraction pattern similar to an X-ray experiment using a FFT. From my knowledge, the diffraction pattern for point particles can be calculated ...
971 views

### Ewings Molecular Theory of Magnetism & Induction Cookers

This question regards Ewings molecular theory of magnetism I.) Ewing's molecular theory of magnetism describes every magnetic substance as being a collection of dipoles that are initially in a state ...
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### Validity of Cutkosky cutting rules for fermions

It is rather obvious for me that the generalized optical theorem (see e.g. Peskin&Schroeder) must hold for S-matrix elements for fermions as it is directly related to the unitarity of the ...
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### What does adding a scalar field component to the Einstein field equations mean for black holes and string theory?

If a scalar field component has to be added to the Einstein field equations (see below) to solve dark matter/energy, then how would string theory need to be modified and do black holes still exist? ...
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### Why a black hole sucks nearly everything, but emits gravitons?

What is so inherently different to the idea of gravitons from other particles, that a black hole draws everything possible including massless photons, but emits gravitons the more the stronger it ...
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### false vacuum original paper

I'm trying to track down the origin of the False Vacuum paradigm. Is it Coleman and de Luccia's "Gravitational Effects on and of Vacuum Decay" ...
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### Explanation for classic mechanics puzzle

I'm trying to figure out a nice way to describe to a kid the physics behind these experiments: Assuming ideal conditions, we have a small boat with a sale, close to a lake's shore and a fan fixed on ...
243 views

### Why isn't heavy water much heavier, and what use is it for fission reactions?

With confusion I experienced recently, I'd like to clarify some stuff for myself about nuclear energy, that I find are usually not properly explained. What exactly is heavy water, just water where ...
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### Is it possible to describe the entire universe with the behavior of an $\mathbb{R}^n$ field?

Suppose every phenomena in this universe (of course most are reducible to some particular general ideal ones - basically I'm talking about those!) could be described as ...
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### A problem of missing energy when charging a second capacitor

A capacitor is charged. It is then connected to an identical uncharged capacitor using superconducting wires. Each capacitor has 1/2 the charge as the original, so 1/4 the energy - so we only have 1/2 ...
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### Intensity of the diffraction pattern of the double slit

I am trying another approach for my last unanswered question. (Bounty still on for 3 days. Anyone? Please?) Note that this is not the same question but a greatly simplified version concerning a much ...
380 views

### difference between battery and capacitor in urdu languege [closed]

Heading List item List itementer code here Blockquote*emphasized text*
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### Single photon and double-slit experiment [duplicate]

Laser fires single particles of light, called photons, through the slits. Even though only single photons of light are being fired through the slits and They create three pattern again. How single ...
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### Derivation of the Lorentz transformation

I have been reading Young's book called: University physics with modern physics and on page 1284 author states that we can derive Lorenz time transformation by eliminating two equations for Lorentz ...
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### Does a particle lose its (location) wavefunction if its location is measured exactly?

As the title says, does a particle lose its location wavefunction if its location is measured exactly (I know this would be impossible in reality)? Also, in reality, if one measures a particle, does ...
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### How does one make a particle with particular wavefunction?

I was always curious how scientists achieve a particle with particular wavefunction (of location and spins etc.) So how do they achieve it? Or is this impossible?
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### Counterpart of the Klein Gordon Equation on the “Coordinate Shell”

The relation $$\psi=Ce^{i/\hbar(Et-\mathbf{p}\cdot\mathbf{x})}\tag{1}$$ satisfies the Klein Gordon equation on the mass shell, i.e. for $E^2=p^2+m^2$. Now let's think in the reverse direction. ...
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### Is there a deep reason why springs combine like capacitors?

I was solving a practice Physics GRE and there was a question about springs connected in series and parallel. I was too lazy to derive the way the spring constants add in each case. But I knew how ...
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### Relating theta_QCD to neutron EDM

How do I relate the topological $\theta_\text{QCD}$ parameter to the electric dipole moment (EDM) of the neutron? I am very familiar with chiral perturbation theory. I just need to know how to take ...
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### Was Jupiter's mass “guessed at” by Kepler or Galileo?

Following Kepler's publication of his 3rd law of planetary motion1, $$p^2 / r^3 = 1$$ in 1619, it would have been possible to use telescopic observations to arrive at an estimate of the orbital ...
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### Conservation Laws in Photon Parametric Down-Conversion

As Wikipedia explains, one photon passing through a crystal sometimes down-converts to two photons. Wikipedia says total energy and momentum are conserved by just considering the three photon states; ...
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### Why does it take until the middle of summer before lakes have warm water, but desert sand heats up in hours?

My sister asked me this question and I keep thinking that water would conduct heat much faster than sand. Hence the energy transfer of heat across the lake does not allow it to heat up soon. Sand on ...
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What's a physical meaning of, for example, complex part of the solution for coordinate change of the anharmonic oscillator? Why after substitute (for diff. equation solve) for real x we can earn $x = ... 3answers 264 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 613 views ### Is the letter delta generally only used to express change in variable or quantity? I was speaking with a friend of mine earlier and he said "Oh look, delta, the sign of uncertainty" (he doesn't study physics often so had only seen in in Heisenberg's Uncertainty Principle equations). ... 2answers 726 views ### Quantum Mechanics and nuclear fusion I've been told that, according to QM, when Hydrogen atoms are left together there is a non-zero probability that they spontaneously fuse (I accept this bit). I've been told further that, because of ... 1answer 326 views ### Why do hydrogen atoms attract? That is, why is the potential energy with the orbitals overlapping less than with the Hydrogen atoms 'independent'. Similarly, why is a noble gas configuration stabler than if an electron were to be ... 5answers 332 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 283 views ### Why are atoms particles? The Oxford English Dictionary definition of particle is as follows: "A component of the physical world smaller than the atom." I read an article in NewScientist and it said "...all particles from ... 1answer 151 views ### Why is the$\langle v_{x}^{2} \rangle=\frac{1}{3} \langle v^2 \rangle\$?

For a randomly moving particle. Or, I suppose that 1/3 could generalise to 1/n, where n is the non rotational degrees of freedom for that particle. Related reference Kinetic Theory of Gasses.