1
vote
4answers
175 views

Why Light isn't like an Acoustic wave?

I just wanted to know why light isn't an Acoustic wave.Is it because light wave doesn't obey acoustic properties?
-1
votes
1answer
111 views

When you are standing in a running train if you jump means will you be in same place or will you be in front or back? [duplicate]

When you are standing in a running train if you jump means will you be in same place or will you be in front or back?
0
votes
1answer
79 views

Ideas for physics-related simulations to code up as weekend (or monthly) projects [closed]

What are some good ideas for simulations that I could try to implement/code in my spare time just for educational purposes? I'm thinking that it would be a good way for me to delve into the underlying ...
1
vote
1answer
52 views

What is difference between anisotropy and inhomogeinity of this type of composite material?

I am studying some types of composite materials having 2 phases - fibers and matrix. I have some questions and confusions. Any help is appreciated. The composite has fiber along length and I am ...
0
votes
1answer
95 views

Want to know about divergence [duplicate]

Can anyone please explain how to know whether a vector field has divergence or not by seeing its diagram? I have read that a vector field must change for having divergence but why is divergence zero ...
0
votes
1answer
216 views

Does varying centripetal force produce circular motion?

Centripetal force $\vec{F}$ is responsible for circular motion and is given by $$ \left|\vec{F}\right| = \dfrac{m\left|\vec{v}\right|^2}{r}$$ If the linear velocity $\vec{v}$ varies, the centripetal ...
0
votes
2answers
83 views

Length of Day and year on massive Earth

Say, a planet has the same orbital periods as Earth (365 days in a year, 24 hours in a day, etc. with respect to Earth-bound clocks), but the planet has double the mass. So, time must pass more slowly ...
1
vote
0answers
65 views

What is really interacting in weak interactions?

Only particles with chirality $-1$ do interact weakly. The corresponding eigenstate in the Dirac basis is $ \Psi_L = \begin{pmatrix}f \\ -f \end{pmatrix} = \begin{pmatrix}u_r {\mathrm{e}}^{-imt} \\ ...
0
votes
0answers
65 views

capacitors - Square circuit of two capacitors charging in a changing magnetic field

I've been studying capacitors and DC circuits, and I received this question as part of my homework: The above circuit is a square circuit with side a. E and F are the midpoints of AB and CD ...
0
votes
1answer
62 views

Why are lubicants like grease or oil slippery?

I know that grease or oil are slippery in nature but never thought of why? I want to know what causes this tendency.
2
votes
1answer
101 views

Does the $\bf{1+3}$ representation of $SU(2)$ also represent $SU(2)\times SU(2)$?

I'm a bit confused about this following issue concerning representations of $SU(2)$. Denote by 1 the 1-dimensional representation of the group $SU(2)$ (=the spin 0). Similarly, denote by 2 and 3 the ...
1
vote
0answers
45 views

Why are flight feathers asymmetric?

I've always heard that flight feathers were asymmetric because the shape generates lift, like an airplane wing airfoil shape. But, the feathers are not oriented with the shaft perpendicular to the ...
0
votes
0answers
56 views

Background field expansion in normal coordinates

Background field expansion following form $Y= X+\pi$ where $X$ is my background field and $\pi$ is the fluctuation. From the Normal coordinates we have the expansion of $\pi^{\mu} = ...
0
votes
0answers
10 views

Why molecules in this potential have a common average distance $r_m$ to their neighbors?

A commonly used potential describing intermolecular interactions in liquids and real gases is the so-called Lennard-Jones potential. In this model the interaction of atoms and molecules with each ...
0
votes
1answer
49 views

Why can the twin plane be uncoincide with the composition plane in reflection twin?

In the book Elements of X-Ray Diffraction, p67, I found the following sentence: In the case of a reflection twin, the composition plane may or may not coincide with the twin plane. But I think ...
1
vote
1answer
156 views

Calculate the mass of a Schwarzchild black hole with Komar integral [closed]

In Wald's GR, Komar integral is Eq. (11.2.9): $$M=-\frac{1}{8\pi}\int_S\epsilon_{abcd}\nabla^c\xi^d$$ $S$ can be chosen as a 2-sphere, the boundary of a spacelike hypersurface $\Sigma$ such that the ...
0
votes
1answer
25 views

Is it more difficult to create higher energy photons, given there is excess available energy?

Im not talking about pair production, I mean a single photon created from an energy transition between two electron shells. I'm studying K(alpha) and K(beta) fluorescence transitions in metals, and ...
0
votes
1answer
233 views

Calculating estimated HP from velocity, auto weight, and constant acceleration

I am working on a simulation program that runs theoretical performances of different cars and was wondering if there is a way to estimate the HP at any running RPM? the problem is (and this may be me ...
0
votes
0answers
42 views

Deriving an Expression for Helmoltz free energy

Given the equations of states for an isolated system: $$E=\frac32 pV$$ $$p=aVT^4$$ I was asked to find the Helmoltz free energy per particle, $F=E-TS$, as a function of $T$ and $V$. I began with the ...
0
votes
0answers
57 views

How $Z_{2}$ topological order is introduced to distinguish states?

In the $Z_{2}$ topological order and the quantum spin hall effect:Here is what the authors said" the states withe zero and on flux quantum threading the cylinder...However, the two states are ...
1
vote
2answers
86 views

A question over the reality of $\sin x$

Harmonic functions are in widespread use in physical descriptions of natural real phenomena. I am just wondering therefore how we can define $\sin(x)$ to be part of a real physical quantity (with ...
0
votes
0answers
82 views

Books on special and general relativity? [duplicate]

I have idea about all the theories related to space and time and stuff. But I wanna dig deep into it and understand all the theories like special and general relativity to begin with for now. Can ...
12
votes
6answers
577 views

What keeps electrons on a negatively-charged conductor from leaving?

Imagine a negatively charged conductor in a vacuum. The excess electrons will be spread out over the surface such that the net electric field inside the conductor is zero. What keeps these extra ...
1
vote
1answer
187 views

Work-Energy theorem vs conservation of mechanical energy? [closed]

Bodies A and B are moving in the same direction in a straight line with constant velocities on a frictionless surface. the mass and the velocity of A are: 2kg and 10m/s. The mass and the velocity ...
1
vote
0answers
40 views

Torricelli with constant vertical drain speed

Now I've been playing with this for some time but I can't seem to put things into perspective. I wan't to find out the curvature of the bowl so that the vertical drain speed of the liquid is ...
3
votes
3answers
229 views

Is torque independent of choice of the point of rotation?

Is torque or moment of couple independent of choice of point of rotation? Torque is given by $\tau =\vec r\times\vec F$. It depends on r, which is its distance from axis of rotation to the point ...
1
vote
3answers
116 views

What does it mean to say “a paramagnetic material is attracted to an external magnetic field?”

I'm just having a hard time wrapping my head around what actually goes on when a paramagnetic material is exposed to an external magnetic field. I understand that the individual dipoles line up so ...
7
votes
1answer
71 views

Why is the energy of particles in accelerators much higher than the energy of the particles they are trying to find?

I have been wondering. In the LHC, or other particle accelerators for that matter, they are colliding particles with energies above TeV. The LHC is going to be 14 TeV or something like that the next ...
1
vote
2answers
84 views

Wave packets and half-width at half-maximum

Suppose we have a Gaussian wave function and amplitude distribution function $$\psi(x) = (\frac{2}{\pi a^{2}})^{1/4}e^{-x^{2}/a^{2}}e^{ik_{0}x}, \qquad \phi(k) = (\frac{a^{2}}{2\pi})^{1/4}e^{-a^{2} ...
1
vote
0answers
36 views

Is there any advantage to using simple syrup over just dissolving sugar and waiting a minute?

When making coffee, I want to sweeten it with sugar. Many coffee shops offer simple syrup which is essentially just a solution of water saturated with sugar. Obviously, this has the advantage that ...
3
votes
1answer
258 views

What is fatal? Current or voltage? [duplicate]

I have been searching this answer for quite a while. I asked my physics teacher and he said it is current that causes death. A friend of mine, who is a EE student, and his answer was voltage. When i ...
35
votes
7answers
5k views

“Falling upward” - how far you have to be from Earth to start falling to the Moon?

Talking about gravity with my 9 y/o she asked when do we start "falling upward" to the Moon. What is the distance at which the Moon's gravitational attraction is higher than that of the Earth and thus ...
2
votes
3answers
168 views

Root of $i$, which one to take?

The propagator of a free particle in 1d is $$ K(x_b, t_b; x_a, t_a ) = \sqrt{\frac{m}{2\pi i \hbar (t_b-t_a)}} \exp \left [ \frac{i m (x_b-x_a)^2}{2 \hbar (t_b-t_a)} \quad \right ] .$$ It looks ...
1
vote
1answer
162 views

Feynman rule for propagator with derivatives

Suppose you have an interaction term of the form $$\mathcal{L}_{int} = \frac{h g}{3!}\phi^3\partial^2\phi$$ where $h $ and $g$ are both couplings. Now if I draw a diagram of the form given in the ...
0
votes
0answers
24 views

Expansion rate in matter dominated era [duplicate]

Do physicists know what the expansion rate of the Universe was at the beginning of the matter dominated era?
3
votes
1answer
81 views

Why is the value of the action integral in general relativity the same on all regions that are homologous?

In their famous paper Action integrals and partition functions in quantum gravity, Gibbons and Hawking argue that in order to avoid the singularity of a Schwarzschild black hole you can complexify ...
19
votes
2answers
3k views

Melting diamond and cool down as diamond

Is it possible to melt diamond? And if possible while let it cool will it became diamond again?
5
votes
4answers
1k views

What are Quarks made of and will they ever decay to this? [duplicate]

What is it that quarks are actually made of? Will they decay into this substance? As the up and down quarks are the lightest type of quark do they not decay? I was thinking that if this could happen, ...
1
vote
1answer
81 views

Why are (most) solids periodic?

Is there a rigorous proof that periodic arrangements minimize the energy of a group of particles?
2
votes
1answer
60 views

Integration of $e^{-it\sqrt{\mathbf{p}^2 + m^2}}$ for QM amplitude

My question might be more about maths than physics, but it originated in a Physics context. Take $\hbar$ = $c$ = 1. I was looking at the amplitude for a free particle to propagate from an initial ...
1
vote
1answer
105 views

Scalar field in a Schwarzschild metric

I have found this article recently published in Classical and Quantum Gravity giving the exact solution of a scalar field in the Kerr-Newman metric. These authors also derived Hawking radiation for ...
1
vote
1answer
60 views

Calculating speed of spheres in an asymmetric Newton's cradle?

If we assume there is an asymmetric Newton's cradle with 3 spheres of different masses and the first one is initialized with some certain speed can we calculate the outcome speed of the all the three ...
0
votes
1answer
126 views

Einsteins idea about solving the ether problem [duplicate]

I'm struggling a bit with my Special Relativity course. Most exercises are quite trivial and can be done without much effort, but when you really start to think and analyse why and how certain things ...
1
vote
3answers
769 views

Will an object rotate when we apply a force to it?

What would happen if the axis of rotation passes through the centre of mass of an object? Will the object rotate when we will apply a force to the object? Edit: The object is free, is not fixed to an ...
1
vote
3answers
99 views

Is the world we are living in discretized? [duplicate]

I do not know how to use professional words to ask my question, so I will try to use a layman language. Please bear with me for a moment. A ROUGH GUESS The world our eyes are seeing every moment is ...
0
votes
0answers
30 views

The physics behind capacitive touch sensing

Can someone explain the mechanism of capacitive touch sensing? According to http://www.planetanalog.com/document.asp?doc_id=527410&site=planetanalog&print=yes, it mentions "fringing fields", ...
1
vote
1answer
92 views

Abstract concept of wave propagating on a string

I'm a real beginner in physics with a really basic doubt about waves. Suppose i have a string ( perfect elastic material ) whose left-end i can manipulate ( i can change its heigth ) and whose ...
0
votes
0answers
19 views

Is there an applied force acting on the balloon? [duplicate]

A long string is running through a straw with a blown-up balloon taped onto the straw, and a penny taped onto the balloon. There should be a support force going up, gravity going down, friction force ...
3
votes
2answers
1k views

Why is less force required to open a door when we apply a force at a greater distance from the hinge?

I know that the magnitude of a torque increases with the distance from the axis of rotation. What is the reason of that : how can a force exert a greater torque when the force is applied at a ...
1
vote
3answers
581 views

New subatomic particles

In reference to the findings talked about here http://online.wsj.com/articles/two-new-subatomic-particles-found-using-large-hadron-collider-scientists-say-1416409980 and other similar articles ...

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