# All Questions

317 views

### Angular Momentum Operator

I'm looking at a review question I was given and it quite frankly has me stumped. "Using matrix representations find $L^{3}_{x},L^{3}_{y},L^{3}_{z}$ and from these show that $L_{x}, L_{y},L_{z}$ ...
8k views

### Book against a wall and forces

If you take a book with mass of 1kg and push it against the wall. With how much force do you have to push the book so it does not fall? The problem is I know how to calculate this problem, you say ...
4k views

### What are the differences between Comet versus Asteroid versus Meteoroid versus Meteor versus Meteorite?

What are the differences between comet versus asteroid versus meteoroid versus meteor versus meteorite in practical terms?
119 views

### Is the Apophis asteroid a concern?

I was watching the TED lecture How to defend Earth from asteroids , and the speaker Phil Plait spoke of the possibility of this asteroid hitting Earth. He also said something about a Mayan legend. He ...
201 views

### Drum head coupled with an RLC circuit

I'm thinking about the possibility of an electroacoustic drum that would be tuned with some kind of RLC circuit. The drum head would have an electromagnet attached to its center, which would be ...
710 views

### Do the properties of two streams flowing into an engine add together when they mix?

Let's say I have two stream supplys that have separate mass flow rates, enthalpies, and velocities that flow into an engine. Upstream of the engine's inlet, the two streams mix to form one stream that ...
217 views

### Is there any know weakness with GNUplot fits? (especially with gauss) [closed]

my faculty push pretty hard on us using GNUplot but it gives me somewhat unconvincing results while attempting to fit a gauss-like function. Is there any known weakness with GNUplot fits or should I ...
797 views

### The Sun as a gravitational lens

Since the Sun is a gravitational lens with as focal length of 550 AU for visible light, with an immense amplification factor, shouldn't it light up objects hanging out there? We should get solar ...
1k views

### What is the force between two magnetic dipoles?

What is the force between two magnetic dipoles? If I have two current loops parallel to each other with currents $I_1$ and $I_2$ and radii $R_1$ and $R_2$ a distance $z$ from each other, what is the ...
147 views

### How does one geometrically quantize the Bloch equations?

I've just now rated David Bar Moshe's post (below) as an "answer", for which appreciation and thanks are given. Nonetheless there's more to be said, and in hopes of stimulating further posts, I've ...
135 views

### More data points or more averages?

Perhaps an elementary questions. Given a time limited measurement situation, would it be better for one to measure more averages or more data points? More averages will increase the SNR by ...
78 views

### Examples of heterotic CFTs

I'm trying to get a global idea of the world of conformal field theories. Many authors restrict attention to CFTs where the algebras of left and right movers agree. I'd like to increase my intuition ...
287 views

### Projectile motion equations @ very (!) high starting velocity

I've already searched Physics StackExchange for some similar question but I didn't find anything about this. Assumptions: Earth is a perfect sphere with it's core (X,Y,Z) -> (0,0,0) as a ...
1k views

### How to determine donor or acceptor impurity

I am working on a practice exam for semiconductor physics. Can anyone help me figure out how to determine whether we have donor or acceptor impurity in Gallium Arsenide? ...
96 views

### Could a neutron star be active, fusioning neutrons into something more dense and releasing constant power?

Could a neutron star be a bit like our Sun or any other star? Just to have a different scale: A neutron star fusioning not hydrogen and other elements but neutrons and quarks in its core at a ...
156 views

### How would a large a mass be stable at the Earth Sun L4 or L5 point?

I've heard about the Trojan asteroids and there is the famous idea of putting a space colony at one of these points, but the explanations I see for how something is stable at those points it they are ...
336 views

### Proof of the Hydrostatic weighing equation?

I've been trying to derive this equation for about two hours: But I can't seem to get it. In my class, we are using it to accurately measure the volume of irregulary-shaped objects, but I ...
4k views

### split gravitational force into x, y, and z componenets

I am writing a program for a computer science class in which I am doing an n-body simulation in 3-dimensional space. Currently, I have figured out the gravitational force along the hypotenuse between ...
63 views

### What would the universe look like if it started out without any irregularities?

(Warning! Newbie question coming up!) Background As seen on this picture of the cosmic microwave background (take from the Wikipedia entry on the very same topic) there exists irregularities in the ...
423 views

### How are possible microstates discerned in Gibb's entropy formula?

On the entry of Gibb's entropy formula on Wikipedia, the following definition is given: "The macroscopic state of the system is defined by a distribution on the microstates that are accessible to a ...
938 views

### How would you improve braking capability on a hovercraft? [closed]

Pretty much letting my mind free-wheel. Assume a fleet of air-supported hover-craft were to replace cars/etc on the streets. Assume also that the present traffic-signals/pedestrian rules remain ...
1k views

### Calculating the gravitational acceleration inside of a planet

Plant X has a radius of 5000 km and is composed of two layers. The first inner layer ranges from the centre to 2000 km from centre, it's density is 8 kg / dm^3. The second layer ranges from 2000 ...
622 views

### Taylor approximation in physics

In a textbook about semicoductor physics, I came across a passage about deriving the carrier concentration at thermal equilibrium in semiconductors I didn't quite grasp: The recombination ...
485 views

### Why does the Higgs field only couple to opposite-chiralities fermions?

According to the Wikipedia page on the Standard Model, the Higgs field interact with fermions through a Yukawa interaction coupling only left to right chiralities. What is the reason for that? Is that ...
1k views

### How fast does force propagate through matter? [duplicate]

Possible Duplicate: Is it possible for information to be transmitted faster than light? Consider the following thought experiment. You have a long perfectly rigid beam (for the sake of ...
340 views

### Expected number of quanta in harmonic oscillator states

I'm working my way through A Squeezed State Primer, filling in details along the way. Let $a$ and $a^\dagger$ be the usual annihilation and creation operators with $[a,a^\dagger]=1$ and ...
9k views

### Finding induced current in a loop at an instant [closed]

I'm working on this problem; however, I cannot seem to get anywhere. Given information: The rectangular loop in the figure has 2.1x10^-2 ohm resistance. What is the induced current in the loop at ...
31 views

### Liquid Water in “mid-ice” on Europa. Mechanism?

Recently I read this entry by Dr. Phil Plait (a.k.a. The Bad Astronomer). He is talking about a lake of water contained within the ice layer of Europa. the lake is completely embedded in the ice ...
1k views

### Is PHD in Theoretical Physics(Quantum Mechanics) on my own possible? [closed]

Do you think it is possible to earn a PHD degree in Theoretical Physics on your own? I have a M. Sc. in Applied Computer Science but I really want to go in the direction of Physics. Is there a way one ...
220 views

### What sets the resolution on analog film?

When taking a picture with old fashioned film what sets the resolution of the picture? Is it the wavelength, or the chemical makeup of the film?
857 views

### Problems that Lagranges equations of the 1st kind can solve whereas the 2nd kind can't?

Can anyone give examples of mechanics problems which can be solved by Lagrange equations of the first kind, but not the second kind?
488 views

### Finding work done by rotational force?

A disk with a rotational inertia of 5.0 kg·m2 and a radius of 0.25 m rotates on a fixed axis perpendicular to the disk and through its center. A force of 2.0 N is applied tangentially to the ...
437 views

### Finding stopping time when only given initial angular velocity and an expression for angular acceleration?

Question: A wheel starts is spinning at $27\text{ rad/s}$ but is slowing with an angular acceleration that has a magnitude given by $\alpha(t) = (3.0\;\mathrm{rad/s^4})t^2$. It stops in a time ...
424 views

### Differential Equation in Spherical Harmonics Derivation

I've been reviewing derivations of the spherical harmonics in quantum mechanics; mostly as review but also to make sure I understand where the concepts arise from. However, every derivation I've ...
851 views

### Rigorous proof of Bohr-Sommerfeld quantization

Bohr-Sommerfeld quantization provides an approximate recipe for recovering the spectrum of a quantum integrable system. Is there a mathematically rigorous explanation why this recipe works? In ...
108 views

### what compactifications of the Poincare group have been studied?

as we know the Poincare group is non-compact. Poincare invariance have been observed in velocities and energies up to $10^{20}$ eV in cosmic rays. The other day i was thinking in how $SU(2)$ ...
789 views

### Is quantum mechanics intrinsically dualistic?

In just about every interpretation of quantum mechanics, there appears to be some form of dualism. Is this inevitable or not? In the orthodox Copenhagen interpretation by Bohr and Heisenberg, the ...
3k views

### How do we know Dark Matter isn't simply Neutrinos?

What evidence is there that dark matter isn't one of the known types of neutrinos? If it were, how would this be measurable?
120 views

### Is stopping something work?

If somebody pushes against a mass moving with $3 \frac{m}{s}$ to slow it down to $2 \frac{m}{s}$, he will drain the moving system of kinetic energy. Does he do work then or does he consume work? My ...
568 views

### Why is geometry mathematics and not physics? [closed]

This has always bothered me: it would seem that the concept of Euclidean space and real numbers themselves arose out of necessity for describing the physical universe that we live in. Mathematics, on ...
447 views

### Why the pion does not get mass under Spontaneus breaking of chiral symmetry, but the quarks do?

Some sources state that when the mass of a quark goes to zero, it allows for Spontaneous Breaking of Chiral Symmetry and gets a constituent mass of about $200\, \mathrm{MeV}$. Other sources state ...
125 views

### Expectation values of interacting fields

I was motivated to ask this question by the equality claimed in equation 10.3.3 of Weinberg's volume 1 of QFT books. My interpretation of that, If $O_s$ is a quantum field of spin $s$, $\psi_s$ is ...
215 views

### Bloch oscillations - Scattering to other bands

In the free electron approximation, a Bloch state $|k\rangle$ is the linear superposition of free plane wave states $\sum_G C_G(k) |k+G\rangle$, where $G$ are the conjugate lattice. Since the ...
6k views

### What happens to a fully charged battery, when you continue to charge it?

If I leave my cell-phone charging the whole night, it will be fully charged after a while. What happens with the battery and the excess energy I add? Also, I noticed my charger emitting a different ...
52 views

### Which arguments for $m_u \approx 0$ are still in the market?

The RPP note on quarks masses has traditionally carried, and it is still there, the comment that It is particularly important to determine the quark mass ratio mu/md, since there is no strong CP ...
18k views

### Can magnetic fields be redirected and focused at one point?

I know that magnetic fields can be redirected, but... given a situation where you have static magnetic field over a large area, and you want to quickly change the magnetic field strength. Is it ...
173 views

### What are the implications of the speed of light broken? [duplicate]

Possible Duplicate: What would be the effects on theoretical physics if neutrinos go faster than light? I don't know if it's been asked before, but I couldn't find a thread about it. I ...
58 views

### Derivation of Eq. 7.12 in the review paper of Kraus

I'm reading "Lectures on black holes and the $AdS_3/CFT_2$ correspondence" by Kraus. http://arxiv.org/abs/hep-th/0609074 I don't know how one can obtain Eq.7.12. My stupid question is how to obtain ...