# All Questions

0answers
2 views

### Explanation of quote from Steven Weinberg

When skimming through a book, I found this quote: The universe is an enormous direct product of representations of symmetry groups. —Steven Weinberg I am a mathematician (so I know only basic ...
0answers
2 views

### With what frequency triangle swings if it shuns from equilibrium?

Guide V-shaped is with no friction in the swivel bearings A and B (see Fig.) Of the magnet with a vertical magnetic field of 0.8 T. The weight of the guide is 50 g, the length AB 4 cm, and the length ...
2answers
14 views

### How do vinyl records work?

I know that vinyl records save the sound as ups and downs on the record plate surface. Let's say two people with different voice timbers say "I like vinyl records" with the same level of voice. It ...
1answer
35 views

### Speed of light paradox

If I send two rockets from the Earth in opposite directions, at, say, 60% of the speed of light relative to the Earth, then relative to each other they are travelling at 120% of the speed of light. ...
0answers
8 views

### Calculating curvature of spacetime when energy is present

I am only about half-way of studying SR and GR and I am not yet familiar with a formula to calculate the curvature of spacetime when energy is present. To be more specific, I want to calculate ...
0answers
4 views

### What is the value (order of magnitude) of electric charge density of the atom and the nucleus?

The mass density of the nucleus is constant. But what is the electric charge density of the nucleus and the atom (any order of magnitude)?
0answers
12 views

### How to count the symmetric wavefunctions made by 6 (u, d, s, $\bar{u},\bar{d},\bar{s}$) quarks to form Baryons (i.e., 3 particle state)?

Peskin said 6 (particles + Anti-Particles) guys make (6($\Rightarrow$particles) x7x8)/(3x2)= 56 total symmetric state. How?
1answer
6 views

### does the internal energy and enthalpy of the GAS changes in a free expansion process?

when a balloon kept in an evacuated room is punctured and the ideal gas in the balloon goes on to fill the entire room,does the gas perform any work in this expansion, and is there in any change in ...
1answer
12 views

### 'Doppler redshift' Vs 'Cosmological redshift'

It is not difficult to understand the differences between 'Doppler redshift' and 'Cosmological redshift' conceptually. But how do astronomers distinguish them when doing observations?
2answers
25 views

### How much is if my internet provider says “we have increased your WiFi connection by 2 decibels”?

I understand the definition of decibel as a logarithmic unit for intensity of anything. I just cannot imagine its "actual size" when comes to the wireless connection. Can you provide something ...
0answers
12 views

### Why wouldn't any Emission Theory work?

http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/origins_pathway/#Emission Here, at the Emission theories of light, I loved the discussed theory. There seems to be a contradiction right ...
0answers
7 views

### Reverse polarity vs falling in Millikans experiment

There are multiple methods of hot to perform Millikans oil drop experiment. One of them lets the drop rise through an electric field (and measures the velocity) then lets if fall (without an electric ...
1answer
20 views

### Acceleration of uncharged particles

Following on from this comment - do uncharged particles radiate under acceleration? (no offense to the commenter, just clarifying as I've not heard of this before) Why is this? I vaguely recall a ...
1answer
21 views

### Absoluteness of Simultaneity

I think this is a very important question because if simultaneity is absolute, then it shakes the foundation of relativity. But it was asked here a month ago, and no one answered it. Neither does ...
1answer
29 views

### Question about scalar product of 2 four-vectors

I'm looking for help to prove that $\mathbf{a\cdot b}=-ab\ cosh \ \theta$, where: $\mathbf{a}=\sqrt{\mathbf{-a\cdot a}}$, $\mathbf{b}=\sqrt{\mathbf{-b\cdot b}}$, and $cosh \ \theta$ is $\gamma$, ...
0answers
24 views

### Photograph of Light as Wave and Particle

what is this? actually its the first photo of light as wave and a particle. The bottom "slice" of the image shows the particles, while the top image shows light as a wave. i have questions 1.how ...
0answers
12 views

### Coulomb barrier in nuclear fusion

Suppose there exists an alpha particle in the nucleus. Within a radius of 2 femtometer, the dominating force is the nuclear force but beyond this radius, the Coulomb force becomes the effective ...
2answers
28 views

### Are spherical coordinates distances or angles?

I've become confused about spherical coordinates when dealing with electric fields. The way I always understood spherical coordinates is something like the below picture. To define a vector, you give ...
0answers
9 views

### Derivation of the kinetic energy of decay product

By conservation of momentum: $$M_{\alpha }V_{\alpha }+M_{Y}V_{Y}$$ By the conservation of energy: $$(\frac{1}{2})M_{\alpha }V_{\alpha }^{2} + (\frac{1}{2})M_{Y }V_{Y }^{2}$$ Here I begin the ...
0answers
13 views

### Klein paradox in graphene

When considering the Klein paradox in graphene Katsnelson, Novoselov and Geim introduce a potential barrier (see http://www.nature.com/nphys/journal/v2/n9/full/nphys384.html). But I cannot understand ...
1answer
24 views

### Maxwell, speed of light and the electric permittivity and magnetic constants

When Maxwell found that the already-measured speed of light could also be derived from the already-measured electric and magnetic constants, what were the electric and magnetic constants measured to ...
2answers
43 views

### Why can velocity and acceleration be negative?

Why is speed and acceleration negative when $V_1$ of an object is say 150m/s, $V_2$ is 0 m/s and $\Delta d=0.50\,\rm m$? I found the time it takes which is 0.0033s and the acceleration to be 90909.09 ...
0answers
27 views

### Is it more effective to skim through a topic then go back for details or deeply understand each concept/equation before moving on?

For example, you are going through a textbook chapter on rotational motion. You come across the concept of moment of inertia and the relevant equations for the first time. You could skim over moment ...
1answer
46 views

### Conceptual question regarding work

Here is a problem I am having some trouble with: The solution is below, my question is, why is conservation of energy valid in this situation. From my understanding, just before the box hits the ...
5answers
88 views

### Why does moving electron emits photons?

I have read that moving or oscillating electron emits photons. But why and how does it so? And why only photons? There are other bosons like gluons, W and Z bosons, so why does electron emit only ...
2answers
20 views

### Collsion in 2D, find angle and final momentum

Suppose that the collision is inelastic. Disregard friction as the surface is smooth. Suppose that one is in the direction of $(1,0)$ and the other $(0,1)$. Is the final momentum the vector addition ...
1answer
49 views

### How to prove that any rotation can be represented by 3 Euler angles

How can one prove that any rotation of a rigid object in 3-dimensional (3D) space can be represented by a sequence of three rotations around pre-fixed axes by 3 Euler angles? I see this statement in ...
2answers
53 views

### Solution of simple problems using only Maxwell equations in differential form

Solve simple electrostatic or magnetostatic problems using only Maxwell equations. For example: In every book there is an excercise to find a magnetic field outside a thin wire of radius $a$ with ...
0answers
19 views

### finding velocity of solid particle right after collision

If a particle collides with a droplet, depending to magnitude of their velocities, different scenarios might happen: rebound, penetration, disintegration, etc. If particle starts penetrating into ...
1answer
55 views

### Why did the matter in the early universe not stick together due to gravity?

(Feel free to correct any mistaken assumptions I have) The overall question I have is: given that the early universe started as an incredibly dense ball of matter and energy, why didn't that mass ...
0answers
14 views

### Seeking Reference on Transport of Momentum by Diffusion of Mass

I'm looking at continuum mechanics from the perspective of De Groot and Mazur's "Non-Equilibrium Thermodynamics" - the first reference that I've come across that seems to do a good job of bringing ...
1answer
19 views

### Non-linear relationship between power supplied and perceived brightness in a filament lamp?

I was expecting the relationship between these two to be linear, but after conducting an experiment my data fits almost exactly on the curve, $y = 2800.2 x^{2.0173}$, where $y$ is the brightness in ...
1answer
29 views

### Athletics on the moon

The moon is a lot smaller than the earth ($\frac{1}{81}$ of the mass and $\frac{3}{11}$ of the diameter). a)How high would a professional vaulter jump on the moon? Assume that he would jump ...
0answers
25 views

### Quantum dimensions in SU(N) level k Kac-Moody current algebra

The CFT of the SU(N) level k Kac-Moody current algebra has many Kac-Moody primary fields. I wonder if any one has calculated the quantum dimensions of those Kac-Moody primary fields. I know that, for ...
0answers
46 views

### Halley's comet. (Questions from a visiting astronomer)

Okay, so we had an apparently famous astronomer in class today. He is giving lectures at our university this week and my class is one of them. So he just comes in on his first day and ends his ...
0answers
12 views

### Electron confinement energy

I was introduced the electron confinement energy. At room temperature thermal energy of a particle is about $k_B T$, where $T=298K$, giving about $25meV$. I was told that $E_{confinement}=50emV$, ...
2answers
21 views

### Does the Photoelectric Effect cause any kind of decay?

From my understanding, the Photoelectric Effect knocks electrons off of some metal using photons. Since electrons are being thrown out of the metal, does this cause some kind of decay?
0answers
11 views

### Nuclear powered flux compression generator?

A flux compression generator is a device for converting the chemical energy of explosives into magnetic field energy, with reported efficiencies of up to 20%. Is there a limit on the conversion ...
0answers
21 views

### Central Force - Trajectory

There is a central force $\vec{F}(\vec{r})=f(r)\vec{e_r}$. 1.1.: The trajectory/flight path is defined by $r=r(\phi)$, compute $f(r)$. 1.2.: The path is defined by $u=u(\phi)$ with ...
1answer
31 views

### Explanation of Interference of Electromagnetic Waves

There is a question on a test which goes like this: "Given two electromagnetic waves, one of wavelength 6.0 X 10-7 m and the other of wavelength 7.0 X 10-7 m, travelling in space. When the two ...
2answers
35 views

### 'schrodinger' picture in measurement based topological quantum computation

I am looking at the measurement processes in topological quantum computation (TQC) as mentioned here http://arxiv.org/abs/1210.7929 and in other measurement based TQC papers. Let's say I start with ...
1answer
22 views

### Coriolis Effect vs airplane

So, if I build a highway from north pole towards equator and sit in a car, speed pretty fast towards it, I should feel force with the Earths rotation due to the different rotation speeds of this ...
1answer
23 views

### Overloading stresses wooden planks beyond their elastic range, causing them to fail?

With safe use there is the fact that a car (a not so light load) can board a ship successfully going over two wooden planks. On youtube one can see this by a car boarding a ship going over two wooden ...
0answers
28 views

### My own thoughts about singularities and spacetime [on hold]

I believe the assumptions about black holes are mostly wrong due to the idea that time dilates relative to an outside observer as you get closer to the singularity at the center of the black hole. ...
0answers
10 views

### Bethe lattice nn Ising model - phase diagram

How does phase diagram for nearest neighbors ising model looks like?
3answers
91 views

### Physical reason why the derivative of a wavefunction has to be continuous?

Question What is the physical reason (i.e. without any maths) that the derivative of a wavefunction (except with infinite potentials) has to be continuous? Other info I know that in the classical ...
1answer
45 views

### If time-like paths are geodesics, what physical principle applies to space-like intervals?

If I have a number of particles interacting with one another locally, then the center of mass of the system moves along a geodesic. Taking this further with the particles interacting via an EM field, ...
0answers
20 views

### How to make wormhole? [duplicate]

Today in my Physics Class, My teacher gave us an introduction of relativity and I found this very fascinating. He Said that its very hard to make a wormhole because it requires a lot of energy. I am ...
1answer
11 views

### Effect of small changes in atmospheric pressure on open cylinder piston

OK, I'm not a physics student, but I played one for 2 semesters 35 years ago... now I'm a competitive shooter working a theory about why small changes in atmospheric conditions cause our hyper-tuned ...
1answer
22 views

### Torque in a non-inertial frame

How can we calculate torque in a non-inertial frame? Take for instance a bar in free fall with two masses, one on either end, $M_1$ and $M_2$. Taking the point of rotation to not be the center of ...

15 30 50 per page