0
votes
1answer
42 views

Why isn't momentum conserved in this pulley problem?

I have some conceptual doubt about method of solving this problem. 24. A block of mass $m$ and a pan of equal mass are connected by a string going over a smooth light pulley as shown in figure ...
0
votes
3answers
43 views

Regarding synchronization of clocks in special relativity

I am trying to read of synchronization of two clocks in same inertial frame in special relativity. Suppose we have two synchronized clocks in an inertial frame placed at positions $x_1$ and $x_2$ in ...
0
votes
0answers
7 views

Why do people wear black in the Middle East?

I have read various dubious explanations as to why people often wear black in the heat, from cultural to somehow encouraging the evaporation of sweat (unconvincing). So, does anyone know what, if any ...
0
votes
1answer
11 views

All possible photons wavelengths

As far as I know all photons come from electrons loosing their energy. I remember from physics and chemistry classes, that electron can loose or get only certain determined amount of energy. Also I ...
1
vote
2answers
89 views

Phonon spectrum

I had a question regarding phonon spectrum in condensed matter. Consider a cubic lattice with '$p$' atoms per primitive cell. Consider the lattice plane used for derivation of the phonon spectrum to ...
0
votes
0answers
23 views

Addition of spins and projection

This is a calculational question but is not a homework question (rest assured I'm past that stage). Still, I don't quite know how to show the following. Say we have two $SU(2)$ spins $\vec{S}_1, ...
1
vote
1answer
13 views

Why does moving the source-slit closer to the double-slit plane decrease the sharpness of the interference-pattern?

Thomas Young used a single-slit plane before the two-slits plane in order to make the light source coherent. My book (NCERT Physics II) presents the above query & writes: Let $s$ be the size ...
0
votes
0answers
29 views

Did Einstein miscalculate something when deriving LT in his 1905 paper?

In his 1905 paper, Einstein derives the Lorentz transformation using the two postulates of SR;constancy of $c$ for all inertial frames and the Invariance of the laws of physics for all inertial ...
2
votes
0answers
48 views

Polarization Sums in QCD

In the case of a Spin 1 massive particle it is possible to demonstrate that $$\sum_{\lambda=0,\pm1}\epsilon_{\lambda}^{* \ \mu}\epsilon_{\lambda}^{\nu}=-g_{\mu\nu}+\frac{q^\mu q^\nu}{q^2}$$ for a ...
1
vote
0answers
49 views
+100

Can pure-bosonic string theories exist in curved spacetime?

Question: Can there be a consistent non-supersymmetric pure-bosonic string theory in some curved spacetimes? Reason: Since fields with certain amount of negative mass can exist in curved spacetime ...
6
votes
0answers
54 views
+100

Highest symmetric non-maximally symmetric spacetime

What is the highest number of symmetries (Killing vectors) that a (4-dimensional) spacetime can have without being maximally symmetric? From what I can see, it seems to be 7 (which includes the ...
0
votes
0answers
13 views

Why do high altitudes have larger diurnal temperature variation than lower altitudes?

It seems like the lack of atmosphere should not be playing a role in the diurnal temperature variation because that's what makes it colder. Mountains are not that dry, usually.
0
votes
2answers
122 views

Question about a car on a banked turn with no friction

A popular physics page explaining how a car can make a banked turn even with no friction : gives the following free body diagram and then says In the vertical direction there is no ...
0
votes
0answers
10 views

Why are WZW models interesting?

I realise this is a very broad question, but when I was studying for my thesis I randomly came across WZW models for a few times and I never quit understood them. So I understand that these models ...
0
votes
2answers
87 views

Coriolis force in free fall: Directions?

Inspired by this Phys.SE question, Coriolis force in free fall, I have to ask: Why does the Coriolis force act in the east-direction? I would say, if I jump from a high distance and look at the ...
0
votes
0answers
43 views

$\vec{E}$-and $\vec{B}$-fields in RCL circuit from Maxwell equations

Consider a basic RCL-Circuit consisting of an ideal resistor, an ideal capacitor and an ideal inductor in series. Assume that at time $t = 0$ the current is zero and the capacitor is charged with ...
-1
votes
0answers
30 views

Why do electrical charges given to a conductor come to the surface? [duplicate]

It is said that any charge given to a conductor comes to its surface. Should all the protons come to the surface? Should all the electrons come to the surface, or should all the free electrons come ...
1
vote
2answers
33 views

Why do so many galaxies in clusters have a near zero velocity?

I'm looking at a velocity chart of the Coma Cluster: And the question occurred to me: why are there so many galaxies that have a zero velocity (relative to the core of the Coma Cluster which is ...
0
votes
0answers
45 views

Solving Laplace's Equation for a point charge at orgin with Fourier transform? [on hold]

Ok So I have Laplaces equation: $\nabla^{2} \phi(\vec{r}) = \frac{q}{\epsilon_{o}} \delta(\vec{r})$ And I want to solve by using Fourier transforms. So I take the Fourier transform of the right ...
2
votes
1answer
33 views

Why do electric shocks send you flying?

One question which has recently been interesting me is: Why exactly do people get thrown back when they are shocked by electricity. I have done some research to find the answer to this, and i am yet ...
-2
votes
0answers
9 views

How do I calculate the energy produced in a beta decay reaction?

How do I calculate the energy produced in this reaction: Masses of subatomic particles: proton=1.007276u neutron=1.008665u electron=0.000549u Don't know if these are also relevant: Rest mass ...
0
votes
0answers
10 views

How does a Pulsed Plasma Thuster work?

My friend was describing a Pulsed Plasma Thruster (PPT) to me, and we couldn't figure out how it actually worked. The setup involves two conducting plates with a potential difference and a solid, ...
-1
votes
0answers
11 views

Quantum mechanics Hermitian problems [on hold]

how to determine its non zero Eigenvalues and eigenfunctions how to prove A is hermitian in problem 2
0
votes
0answers
19 views

The action of a relativistic point particle, the meaning of its parametrization invariance and possible potential terms

i am currently toying around with the behaviour of a classical relativistic point particle a bit. For a free one we get the action \begin{align} S =\int_\tau - m\sqrt{- \dot X_\mu \dot X^\mu}. ...
1
vote
1answer
200 views

Does Lyapunov exponent equate to exponential inflation?

Physics can be modeled by dynamical systems $f^t(x)$ as well as by PDEs. The most common dynamical system has hyperbolic fixed point and can be an attractor or a repellor. The dynamics at repellors ...
0
votes
0answers
24 views

Is there an intuitive explanation for the fact that the Lorentz algebra contains SU(2) twice?

The Lie algebra of the Lorentz group contains "two copies" of $SU2)$. I mean it's kind of obvious that one instance of $SU(2)$ is there, because $SU(2)$ is the Lie algebra of $SO(3)$ and $SO(3)$ is a ...
0
votes
0answers
19 views

Is it possible to “leech” a usable amount of current from a 220V line?

Inspired by wireless charging technology and "current clamp" type meters - I've been wondering if one could "leech" a small amount of power (say ~2.5W) from a 220V line carrying ~2200W using a wire ...
0
votes
0answers
21 views

4-vectors in special relativity and spacetime interval

In one of my vector mechanics lectures, the lecturer said the the space time interval was the dot product of the four position vector. But then he proceeded to show it was this: $s^2$ = $\Delta r^2$ - ...
2
votes
3answers
164 views

What is supposition of equilibrium? How do Rayleigh, Jean know the electromagnetic wave in equilibrium behave?

In a cavity of dimension L, the wave must give zero amplitude at the wall, means wave equation has zero amplitude. Why? Answer from hyperphysics "since a non-zero value would dissipate energy and ...
-1
votes
0answers
6 views

Viewing one object at different positions

Persistence of vision is 1/24 of a second.If I move an article at a higher speed from position A to position B and from B to A and repeat the process will I have the sensation that there are two ...
6
votes
2answers
150 views

What canonical momenta are the “right” ones?

I'm doing some classical field theory exercises with the Lagrangian $$\mathscr{L} = -\frac{1}{4}F_{\mu \nu}F^{\mu \nu}$$ where $F_{\mu \nu} = \partial_\mu A_\nu - \partial_\nu A_\mu$. To find the ...
2
votes
1answer
33 views

Complex / real representations of the Lorentz group

In Michele Maggiore's book "A Modern Introduction to QFT" he describes the spinorial representations of the Lorentz group as The representations are in general complex. I always thought the ...
1
vote
0answers
19 views

Are there any concrete data on the period of large amplitudes? To what extent are correction formulas useful?

Is there a site/pdf where I can find results of real and accurate experiments concerning the period of pendulums from 0 to 90°? (actual periods and not formulas). I once found one, but in spite of ...
0
votes
1answer
25 views

Does schwarzschild metric have cosmological horizon?

Since the space is not expanding in the schwarzschild metric does that mean that there is no cosmological horizon? Also, what if the shwartzchild metric was not asymptotically flat and we replace the ...
5
votes
2answers
91 views

Why do we not require higher derivatives to match at boundary when solving the Schrödinger equation in a given potential?

When solving the time independent Schrödinger equation for a given potential in 1D, the main part of the solving involves matching boundary conditions. Usually, we require the value and the first ...
0
votes
0answers
9 views

redistribution of charge between plates

How is charge redistributed when a charged conducting plate is placed between two identical wire-connected uncharged plates? For eg,
0
votes
1answer
31 views

How do inert helium cores in sub giant stars create a pressure force?

I'm reading about the sub giant branch (SGB) and the evolution to the red giant branch (RGB). On the SGB stars have burned all hydrogen into helium, as a result, they have an inert helium core. The ...
0
votes
0answers
21 views

Why are racing catamarans' hulls not teardrop shaped?

Looking at the America's Cup 72 class catamarans, I was wondering about their hull shape, which has a sharp wave piercing bow. My question is: since an airfoil shape experiences less drag than a sharp ...
4
votes
2answers
54 views

Do individual rays of light lose energy via the inverse square law?

We've all heard of the inverse square law, but apparently that refers to the flux or intensity or number of photons hitting an imaginary surface area. This is not exactly what I want to ask about. ...
4
votes
1answer
472 views

Does string theory pose a photon mass problem?

A few weeks ago, I started reading books on string theory. One thing that really seemed confusing or contradictory was that string theory explains that the energy of a superstring gives mass to the ...
0
votes
0answers
15 views

how much heat will be produced when air is compressed to liquid air [on hold]

I understand about 700 litres of ambient air becomes 1 litre of liquid air. I want to know how much heat will be dissipated when this happens. I appreciate any help.
1
vote
1answer
185 views

Are the authors saying that the observer effect plays no role in Bohr's thought experiment of the Heisenberg uncertainty principle?

Here is an excerpt from Eisberg & Resnick's Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles. Here is introducing Bohr's though experiment to establish a physical origin for the ...
0
votes
1answer
43 views

How to prove that sum converges to integral using density of states?

Essentially, I would like to prove $$ \sum_k f(k) \to \int f(k) \rho dE \tag{1}$$ where $$ \rho = \frac{dk}{dE} \tag{2}$$ is the density of states and $k \to \infty$. The model is that there is a ...
4
votes
1answer
52 views

Would neutronium emit heat?

Since accelerating a charge creates EM radiation, and there are no charges in neutrons to be accelerated, would that mean neutronium emits no radiation? Consequently, to preserve the second law of ...
2
votes
0answers
16 views

Why would an alpha-beryllium neutron source generating a radioisotope not produce controllable net power?

I have a question. I was looking through the table of nuclides and noticed that the energy needed to produce certain radioisotopes was less than the energy given off by the radioactive decay of that ...
1
vote
1answer
30 views

Is it theoretically possible to prevent or deter a star from becoming a black hole?

Suppose NASA decides too many black holes are being produced in a year, so it decides to do something about that. Given our current scientific knowledge, is there theoretically any way NASA could ...
0
votes
1answer
37 views

Allowed Virtual Displacements

I am having trouble understanding the kinds of virtual displacements which are permitted for a given constrained system. I have a specific example in mind: A block of wood resting on a table parallel ...
30
votes
5answers
7k views

Why can we see through rain?

I am gazing through my office window into a heavy rain. I am thinking that raindrops are like small lenses that bend the light. Thus I am surprised, that I can clearly see other buildings through the ...
0
votes
1answer
52 views

Best twin paradox trajectory for constant acceleration rocket?

Let's say I have a rocket that can provide constant (proper) acceleration (let's say of 1 gee) for an infinte amount of time. Let's say I want to use this rocket as a means of "suspended animation," ...
0
votes
1answer
126 views

Explanation of force amplification inside a solenoid

For a system being actuated by a motor, the force can be amplified by gearing. The energy is being used for force instead of distance, so it produces more torque but moves slower. For a system being ...

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