5
votes
3answers
319 views

Why aren't there more natural satellites orbiting the Earth than just the Moon?

Considering that there's a lot of debris in space and that impacts fling out rocks into space all the time, why do we only have one large natural satellite - the Moon? Shouldn't there be all kinds of ...
8
votes
2answers
2k views

Does the equivalence between inertial and gravitational mass imply anything about the Higgs mechanism?

For example: the role it might play in a theory of quantum gravity (ie causing space-time curvature)? I realize that inertial mass can result from binding energy alone. Has the equivalence principle ...
4
votes
3answers
181 views

Does frequency f = $\frac1T$ if there is non-periodic signal

Through what I learned from school, frequency $f=\frac1T$ while $T$ is the period. However, things get harder in college where there are non-periodic signal. That means they have no period or their ...
0
votes
2answers
110 views

The speed of light, objects moving against each other and the effects of a clash

Physics is a hobby interest of mine, so I may be asking a senseless question, but there is something I cannot get my head around when it comes to relative frames, the constant speed of light (as a ...
5
votes
0answers
106 views

Is this constraint non-holonomic?

I am working on a variational problem involving elastic stability of a beam. The deformation of the beam is given by six functions of the material coordinate along the beams longitudinal axis. The ...
0
votes
1answer
70 views

Calculating electric potential from a changing electric field

Assuming that I calculated the electric field in a single point between a uniform charged positive sphere and an infinite long wire charged positive uniformly. Now, I want to calculate the velocity of ...
1
vote
1answer
761 views

Coriolis Acceleration and Latitude?

We had a lecture on the Coriolis effect today and I am very very confused, this is the first time I've heard of it as such. Anyway, I know that Coriolis acceleration is calculated by $\vec{a} = -2 ...
2
votes
1answer
2k views

Kolmogorov-Smirnov test vs Chi-squared test

What is the difference between the Kolmogorov-Smirnov test and the Chi-squared test? When should we use one instead of the other? I was reading this article, and I got confused a lot. It is hard to ...
2
votes
2answers
161 views

Stress-energy-momentum tensor

In Wald's General Relativity, he writes on pg 61 For an observer with 4-velocity $v^a$, the component $T_{ab}v^a v^b$ is interpreted as the energy density, i.e. the mass-energy per unit volume, as ...
33
votes
3answers
3k views

Why do rocket engines have a throat?

Diagrams of rocket engines like this one, (source) always seem to show a combustion chamber with a throat, followed by a nozzle. Why is there a throat? Wouldn't the thrust be the same if the ...
0
votes
1answer
54 views

Concerning Electrogravitics in an inertial frame

Listening to Feynman, He pointed out the fact that if a magnet is in an inertial frame with respect to a coil, there can be no electromotive force and hence no electricity produced. It is only when ...
2
votes
1answer
110 views

Sign convention for EMF

When we define the field generate by EMF, why there is not negative sign in $\mathcal{E} = \oint \vec{E} \cdot d\vec{l}$? Usually we talk about potential, there should be a negative sign, right?
0
votes
1answer
159 views

Symmetric, antisymmetric and mixed symmetry particles

Can someone explain to me the concept of symmetric, antisymmetric, and mixed symmetry when talking about the states of identical particles?
0
votes
0answers
41 views

Is spectrum of Discrete-Time Fourier Transform (DTFT) periodic or not

I can't think of any better title. Here is the content that I got question http://cnx.org/content/m10247/2.31/ As it state the nature of DTFT's spectrum is periodic as it show in figure 1 However, ...
1
vote
2answers
172 views

On the Existence of Interstellar Meteoroids

A meteoroid in orbit around the Sun has a heliocentric velocity in the vicinity of Earth of about 42.1 km/s. So if a meteoroid has a heliocentric velocity exceeding that, it will have an open ...
0
votes
2answers
105 views

Zero photon energy in moving frame

Suppose I have a photon with energy $E$ (in its rest frame) moving along the $z$ direction with momentum $\mathbf{p} = p_z\mathbf{\hat{z}}$: The Lorentz transform in the laboratory frame should be: ...
0
votes
1answer
174 views

Electron Decay, Why are there P and higher orbitals?

Related: Decay from excited state to ground state My confusion arose initially from the definition of binding energy being the lowest energy state (n=1) in the hydrogen atom. This, I assume, is ...
2
votes
1answer
49 views

Evolution of minimization of surface tension

What are governing equations (or/and variational principles) for evolution of a simply connected body of water in vacuum? Initial state - for time $t=0$ we have a bounded simply connected set ...
1
vote
0answers
26 views

Time scale when measuring total activity from radioactive sample

I have measured the total activity from a radioactive sample during an experiment by counting the number of decays happening in 3 seconds intervals. I got data as: Time bin: 0 1 2 3 4 5 6 7 ... ...
3
votes
5answers
4k views

How to find the poles on a spherical magnet?

To any nonmagnet, the whole sphere is a magnet. To another spherical magnet though, there is a rough area on the surface where it is strongly repelled. Given a spherical magnet, how should the poles ...
0
votes
2answers
60 views

Heating a black body

By definition, Black body has absotivity=emmisstivity=1. This means the black body radiates all energy it accepts. Does this mean the black body cant be heated?
0
votes
2answers
138 views

Molecular mean free path probability

Let's let $p(\xi)$ be the probability a molecule travels at least $\xi$ between collisions, lets say $\xi=0.01$. When I think of this statement, I think it it is the probability the molecule is able ...
3
votes
1answer
99 views

Stress tensor in product of 2D CFTs

I was struggling with a question, hoping someone could point me in the right direction. I'm interested in 2D CFTs on a cylinder. I want to take the tensor product of two CFTs. My questions are these: ...
3
votes
2answers
85 views

force on a moving charge in magnetic field

Need help in understanding the direction of magnetic force in the magnetic field!Totally confused by directions. Why is it that magnetic force is perpendicular to the direction of magnetic field and ...
0
votes
1answer
78 views

Resonance and the driving frequency?

Why does resonance occur in a mass-spring system when the natural frequency = the driving frequency. I think it is because the driving force is always contributing to the kinetic energy of the system ...
7
votes
5answers
2k views

Can one create mass from energy?

Due to $ E =m c^2 $, one can convert mass to energy. A classic example would be matter/anti-matter annihilation to produce energy (photons, etc.). Can one do the reverse? So could one do something to ...
1
vote
1answer
61 views

Normalization of $\langle p_1 p_2 \vert p\rangle$ in RelQM and NonRelQM

Suppose a particle p of three momentum $\vec p$ decays into two particles of 3-momentum $\vec p_1$ and $\vec p_2$. I know the question might sound stupid but right now my brain is full stop: Is the ...
0
votes
1answer
145 views

Relativistic addition of velocities of spaceships [duplicate]

If Spaceship 1 is traveling at speed $.5c$ relative to Earth, and Spaceship 2 is traveling at speed $.3c$ relative to Earth in the same direction, what does Spaceship 2 see Spaceship 1's speed as? I ...
1
vote
1answer
55 views

Do gases have a general upper limit of density?

Is there some limit for the density of gases, at which no change in condition could make it more dense without making it fluid, or solid - or something 'in between'?
1
vote
1answer
76 views

Multiparticle generalization of $\langle \vec k \vert E,l,m \rangle$ spherical harmonics.

From Sakurai eq. 6.4.21a we have that $$\langle {\bf k} \vert E,l,m \rangle=\frac{\hbar}{\sqrt{M k}}\delta\left(E-\frac{\hbar^2 k^2 }{2M}\right) Y_l^m({\bf\hat k}),$$ where $M$ is the mass of the ...
0
votes
0answers
24 views

When a pole of magnet comes near another pole possessing the same charge does it generate electricity?

That's all there is to it. Well... I was daydreaming about making gloves that had the ability to shock people when you punched them. I thought how it would be better if they didn't need a battery and ...
2
votes
1answer
40 views

Why are the true variance of these orbits out by ~pi?

I have an object that is orbit around a point mass in a 2D environment with a known speed, radius and zenith. I calculate the following orbital parameters as such and can confirm that these values are ...
0
votes
1answer
206 views

Single-channel vs multi-channel scattering

I am studying quantum scattering and stumbled upon the "scattering channel" and "single- and multi-channel scattering" terms. However, I didn't manage to find any sufficiently formal definitions of ...
1
vote
4answers
139 views

Gravity and acceleration

I've imagined this little scenario to help me conceptualize things. Let's say we have a doughnut-shaped object with a hole whose diameter is greater than that of a sphere. Let's say that the sphere ...
2
votes
0answers
115 views

How to understand the transmission coefficient from the following question?

I want to understand the transmission coefficient and construct a time-independent Schrodinger equation where $$ V(x)=\left\{ \begin{array}{c c} \delta(x), & |x| < 1 \\ + \infty, & ...
3
votes
1answer
612 views

Transmission and reflection

What is the transmission amplitude of a wavefunction $\phi(x)=e^{ikx}(\tanh x -ik)$? I would have thought that it is $\tanh x -ik$ since this is the factor associated with the forward travelling ...
0
votes
1answer
108 views

Total momentum in linear monoatomic chain

Context: Solid state physics. Monoatomic linear chain. Question: To prove that the total momentum of the chain is zero. Attempted solution: I consider the sum: \begin{align*} p = \sum_{n=1}^{N} m ...
0
votes
1answer
73 views

Is $\langle k \vert k_1k_2\rangle=0$

Using that $$ \vert k_1k_2\rangle = a^\dagger({\bf k_1})a^\dagger({\bf k_2})\vert 0 \rangle$$ and the commutation relations $$[a({\bf k}),a^\dagger({\bf k'})]=(2\pi)^32\omega\delta^3(\bf {k}- \bf ...
0
votes
3answers
167 views

Time dilation in special relativity

Suppose a star ship is moving with some velocity. Two light pulses one in direction similar to star ship another opposite to it is shot towards the space ship. Then how time inside space ship adjust ...
18
votes
4answers
2k views

Why doesn't dark matter affect planetary motion?

If the universe is made up of ~95% dark matter, and it interacts only gravitationally then why didn't Newton and Kepler discover it before ? Why does it show itself only in the radial velocity profile ...
0
votes
0answers
94 views

Does non-matter energy curve spacetime? [duplicate]

I know that matter (mass) curves spacetime, but do other forms of energy do the same? I.e. is matter the only form of energy that curves spacetime?
9
votes
5answers
2k views

What happens when we bring an electron and a proton together?

I have a couple of conceptual questions that I have always been asking myself. Suppose we have an electron and a proton at very large distance apart, with nothing in their way. They would feel each ...
0
votes
1answer
80 views

speed of gravitational waves [duplicate]

Do gravitational waves have a certain speed? Is it the speed of light or infinite, or am I misunderstanding what a gravitational wave is? I think it is a ripple in spacetime caused by interactions ...
0
votes
1answer
349 views

Calculating a radioactive sample's mass from its activity [closed]

The question reads as such: "What is the mass of a 1.05 µCi carbon-14 source?" First I convert to decays/s: $R = 1.05 µCi=3.885 \times 10^4 decays/s$. The half-life for carbon-14 that we've been ...
1
vote
2answers
63 views

If the net force on a current loop in a magnetic field is zero, why is torque independent of choice of origin?

Im trying to show that the integral over a closed loop of a crossproduct stays the same if I choose a different origin with $\overrightarrow{r}=\overrightarrow{r}\prime+\overrightarrow{r_0}$ and ...
3
votes
1answer
195 views

Are gravitons bound by the event horizon?

I understand that photons, even when traveling at the speed of light, cannot escape the event horizon of a black hole. Are gravitons and other virtual particles traveling at the speed of light also ...
1
vote
2answers
1k views

If energy can neither be created nor be destroyed,what is the ultimate source of energy?

We usually say that the ultimate source of energy for earth is the Sun. So this means that sun creates energy but according to the law of the conservation of energy energy can neither be created nor ...
1
vote
0answers
116 views

Finding Time & Required Angle for Projectile Motion with Quadratic Air Resistance

This is a physics/calculus/computer science problem, but I think I'll get better results in the Physics SE. I have a fun little project I've been working on (hobby, not homework/production), that ...
1
vote
1answer
112 views

Normalizing continuous eigenstates

As far as I understand, to normalize the eigenfunctions, corresponding to the continuous spectrum, we use Dirac delta function: $\langle \psi_\lambda \mid \psi_{\lambda'} \rangle = \delta(\lambda - ...
2
votes
4answers
102 views

Acceleration and Circular Motion

Lets assume that there is a force that makes our body moves in circular motion. We know that the acceleration of a body that moves in circular motion is Velocity ^ 2 / Radius . How is it ...

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