# All Questions

709 views

### Friction Between Liquid and Solid Surface

How does friction between liquid surface and solid surface differ from viscosity, the friction between liquid and liquid surfaces? Also has a word describing the process been invented or is ...
79 views

### How does the force between two charges becomes zero?

In case of Coulombs law, $$F=k\frac{q_1q_2}{d^2}$$ So, if $F=0$, then $q_1q_2$ must be 0. Is this true?
74 views

### Rotational Mechanics: Conservation of Angular Momentum

Consider a case where a person stands on top of a rotating disk. The disc is given to rotate at a constant rate. There are two possible movements of the man: He moves away from the center: In this ...
92 views

### How to get result $3 \otimes 3 = 6 \oplus \bar{3}$ for $SU(3)$ irreducible representations?

Let's have $SU(3)$ irreducible representations $3, \bar{3}$. How to get result that $$3\otimes 3 =6 \oplus \bar{3}~?$$ I'm interested in $\bar{3}$ part. It's clear that for $3 \otimes 3$ we can use ...
34 views

### Change velocity formula from a function of displacement to a function of time [closed]

I have this problem from my dynamics book and I can't figure out how to solve it. I've played around with all the formulas and asked a few friends who did the course in the previous years but I can't ...
5k views

### If your hair was on fire and you entered a vacuum, could you outlast the fire?

Let's say a person's hair was on fire and suddenly we (instantaneously) pumped all the air out of the (sealed) room, then after time period X we put all the air back. Is there an X such that we will ...
42 views

### Making An Energy Momentum Plot For A Rashba Model (Using Discretization)

I want to make a plot of the Energy versus the Momentum of the Rashba model, using discrete matrices. First Ill show how I did this for the free particle. Subsequently I will show what goes wrong for ...
139 views

### How, in practice, could instantaneous signalling violate causality?

I know that instantaneous signalling can result in different observers not agreeing on the order of events, but how can that result in causality violation in practice? In other words, if one had two ...
30 views

Please look at the following link for the sketch of the problem: Adiabatic Expansion with expanding control volume I have a certain amount of gas at a certain pressure and temperature at the start. ...
21 views

### Photon absorption by a hydrogen atom : [duplicate]

How does the photon absorption takes place in a hydrogen? The classical mechanics shows the absorption of photonic energy resulting in the excitation of atom. Intuitively, a photon with frequency ...
29 views

### Zeta regularization for a series and an integral

Given the UV divergence $$\int_{0}^{\infty}dx$$ in order to regularize it, I introduce a regulator $|x|^{-s}$, so I have $$\int_{0}^{\infty}x^{-s}dx$$ My problem and doubt is how can I relate ...
614 views

### If we traveled at almost light speed with a mirror in hand [duplicate]

What would we see if we hold the mirror in front of us and behind us.
3k views

### Heat and work are not the state functions of the system. Why?

Heat and work, unlike temperature, pressure, and volume, are not intrinsic properties of a system. They have meaning only as they describe the transfer of energy into or out of a system. This is ...
61 views

### Generalization of Feynman's derivation of the formula for gravitational potential energy in FLP volume I

In the Feynman Lectures on Physics, Feynman derives the formula for gravitational potential energy by applying a line of reasoning he borrowed from Carnot on reversible weight lifting machines (FLP I, ...
969 views

### Is the fifth dimension of Interstellar the same as Block Universe theory? [closed]

Is the movie Interstellar (2014) referring to Block Universe theory of space-time with its concept of a fifth dimension?
26 views

### Is simultaneous pole-to-pole stacked orbit of planets around a star possible? [closed]

I read recently that mercury, mars and earth once had concentric simultaneous orbits pole-to-pole under each other; all of which orbited under Saturns south pole around the sun. In sort of a stacked ...
105 views

### Luminance from multiple light sources

I have a LED diode: Given Luminance intensity [I] of $1.85$ cd. I want to have Luminance [B] from the light source of $75 \ {\rm cd/m^2}$. From $B=I/S$, where S is light source plane area, I get ...
89 views

### How does aerogel attain its enormous heat isolation properties?

In a text about properties of aerogel I read some interesting facts - clothes with aerogel proved to be impractical, being far too warm to wear. If you isolated a house with aerogel and lit a candle ...
52 views

### How many grams of anthracite coal is needed to convert to steam? [closed]

So I have searched the internet but it would not give me the answer that will truly satisfy the question. I hope someone could answer this
177 views

### Testing Many-Worlds Interpretation (MWI) with a causality-violating configuration of “superluminal cables”

Suppose we managed to arrange a causality-violating transmission of data with hypothetical “superluminal cables” (SLC; see both links for respective descriptions) and expect, similarly to ideas ...
298 views

### About the definition of the Møller operator

The Møller operator is defined as $$\Omega_+ = \lim_{t\rightarrow -\infty} U^\dagger (t) U_0(t).$$ Does the operator $$\lim_{t\rightarrow -\infty} U_0^\dagger (t) U(t)$$ also make sense? Is it ...
647 views

### Velocity of car down an Inclined Plane

I was learning to make a car game and came across this situation where my car is on an inclined plane. It's initial velocity is 0. Now, the problem is that my memory serves me this formula to ...
245 views

### Is it possible for photon to run in circle by its own gravity?

I have heard that gravity came from energy and momentum so photon has gravity too. Then there are theory state that photon has energy tied to frequency. So if a photon has very very high frequency ...
25 views

### What does it mean to charge a length of wire up to potential v?

I have to study the text "On the Theory of the Electric Telegraph," By William Thomson, 1855. I am having trouble gaining a conceptual understanding of the opening: "Let c be the electro-statical ...
42 views

### Kinetic energy of the spring

Suppose we have spring of mass $m$ initially at rest , now instantaneous velocity of $v$ is given at both ends in opposite direction (nothing is attached to spring) so what will be kinetic energy of ...
109 views

### Laplace equation vs Gauss theorem

Two concentric conducting spherical shells of radii $a_1$ and $a_2$ ($a_2>a_1$), are charged to potentials $φ_1$ and $φ_2$, respectively. Determine the electric potential and field in the ...
20 views

### Can the granular quantum nature of light be used to engineer a maroscopic optical phenomena?

Today we have optical metamaterials and metasurfaces: materials and surfaces that are made of unit cells with an approximate size of tens of nanometers, that can that interact with light and can have ...
35 views

### Where is the direction of kinetic friction in this question?

Suppose there is on object of 20.0kg sliding to the right. A person exert a force of 30N left on the object. The object has an acceleration of 2.5m/s^2 left. What is the magnitude and direction of ...
111 views

### How is strong time dilation consistent with weak tidal forces?

Nolan's latest film, Interstellar, takes pains to explain to lay audience members that the passage of time slows in the presence of strong gravitational fields (as per Einstein's theory of General ...
40 views

### How is spectrum controlled? [closed]

I understand that spectrum is considered a national resource for many governments. It is akin to land, water and other resources. It is also a scarce resource. I wonder how is it that government ...
38 views

The generating functional for a free Dirac field is $$Z_0[\eta,\bar{\eta}]=\int D\bar{\psi}D\psi \mathrm{exp}\{i\int [\bar{\psi}(x)S^{-1}\psi(x)+\bar{\eta}(x)\psi(x)+\bar{\psi}(x)\eta(x)]dx\}$$ where ...
89 views

### Two layers of glass reflection related to wave interference [closed]

A highly reflective mirror can be made for a particular wavelength at normal incidence by using two thin layers of transparent materials of indices of refraction $n1$ and $n2 (1 < n1<n2 )$on ...
104 views

### Time Dilation Experiment [duplicate]

I am currently thinking of doing an experiment for science fair involving time dilation which will use the equations for both time dilation based off of speed and for gravitational dilation to ...
114 views

### Bound electrons don't move, right?

I've never been satisfied with the explanation that electrons have a “speed” and move close to the speed of light, thus incurring relativistic effects that make gold pretty and mercury wet. I learned ...
69 views

### What are Maximally Helicity Violating (MHV) Amplitudes?

Definition of MHV amplitudes on Wikipedia: In theoretical particle physics, maximally helicity violating amplitudes are amplitudes with n external gauge bosons, where n-2 gauge bosons have a ...
21 views

### Relating Angles in Elliptical Orbit

I've been trying to model the orbit of a comet of arbitrary mass around our sun from having a known initial condition with $r_0, v_0$ with angle $\alpha$ at $t_0$ around our sun. I've been able to ...
125 views

### Twin paradox where the twins start at different locations

Suppose we have this scenario with twins A and B: 1) Instead of the twins starting at the same location, let's say the twins start out some distance apart, in the same reference frame. 2) The ...
74 views

### Rolling without slipping [closed]

A uniform, solid cylinder of mass $m$ and radius $r$ rolls without slipping down a plane, inclined at an angle $θ$ to the horizontal. You may assume, without proof, that the moment of inertia of ...
48 views

### Entropy maximum postulate and reversibility

I'm studying Thermodynamics on Callen's book and he introduces entropy through the entropy maximum postulate, namely: There exists a function (called the entropy $S$) of the extensive parameters ...