-3
votes
0answers
29 views

Size of upward force actin on the rocket! [on hold]

A 1500 kg rocket lifts off from the surface of the Moon by expelling exhaust from its lower end as shown. The force applied to the exhaust gas is 15 000 N downwards. Since the rocket is lifting off ...
0
votes
0answers
50 views

Why does the $\Lambda\mathrm{CDM}$ model end up a cold de Sitter space?

If space-time is expanding at $a(t)=e^{Ht}$, you plug acceleration into the Hawking-Unruh temperature equation, solve for time, and get the cosmic horizon at the Planck temperature within about $4.4 ...
2
votes
2answers
108 views

How to accurately mark with arrows an electric circuit

As I understood, we choose to mark the current with arrows going from the plus pole to the minus pole (even though we know that in reality it is the contrary). As I'm looking at electric circuits, I ...
11
votes
2answers
1k views

Why collide a moving particle with a particle at rest, rather than two moving particles?

I was just reading some lecture notes about relativistic and quantum mechanics, and in the later part of this page the author demonstrates that any relativistic particle collision in the "lab" ...
2
votes
1answer
62 views

Vacuum has zero spin in Dirac theory

I have troubles trying to prove a statement made by Peskin-Schroeder in page 61, section 3.5 where he says that the "spin" operator $J_z$ given by the non numbered equation $$ J_z= \int d^3 x ...
0
votes
2answers
51 views

How does light oscillate?

Why do we say that electromagnetic wave is oscillating? Or does light propagate really in a wavy form like this image? What is making the photons oscillate and how is it oscillating is it ...
0
votes
0answers
11 views

What criterion did Abbe use?

For a microscope (correct me if I am wrong) the Rayleigh Criterion gives us: $$ R=\frac{1.22 \lambda}{NA_{condenser}+NA_{objective}}$$ But with the Abbe diffraction limit: $$ ...
4
votes
1answer
56 views

What algebraic structure does the collection of all physical quantities form?

What algebraic structure -- by which I'm referring to abstract algebra theoretic ones such as ring, field, module, etc. -- does the collection of all physical quantities form? An related and/or ...
1
vote
1answer
117 views

Quantum simple harmonic oscillator interpretation

I am just wondering what does the SHO system from quantum mechanics actually physically represent? Is it just a SHO of a quantum particle, seems a little too obvious for quantum theory? I'm from a ...
1
vote
1answer
157 views

How to find the time evolution for two-component spinor?

Taking the wave function I found here as the initial condition for the Schrodinger equation. I would like to find how the wave function will depend on time, at least for small times, and calculate ...
3
votes
3answers
6k views

What happens if a wind mill rotates in opposite direction?

Suppose if a windmill is made torotate in clockwise direction and due to air flow if it rotates in anti clockwise direction then does it produce electricity?
0
votes
0answers
15 views

Does a negative and a positive charged body repel vs. attract a suspended negative charged ball with the same strength?

In this question I'm in trouble about my statement, that the measurement instrument does not influence the electron's electric field. It seems to me that only the repelling force between negatively ...
0
votes
1answer
17 views

Difference between a light wave in space and a wave in a pond

Apart from their nature and the medium utilized, what are the differences between an EM wave and a wave we see in a pond? When we throw a stone into a pond, can we imagine we are observing light ...
0
votes
0answers
9 views

Chi Square Minimization [migrated]

In my data analysis I've found that my $\chi^2$ is turning out to be very small $\sim10^{-3}$. But this can't be true, for the errors in the fitting variables themselves are of the order of ...
1
vote
2answers
136 views

$\nabla^{\mu}\nabla_{\mu}$ in general relativity [on hold]

I am trying to work out $\square=\nabla^{\mu}\nabla_{\mu}$ in the metric $ ds^{2}=-A(r)dt^{2}+B(r)^{-1}dr^{2}+r^{2}d\Omega^{2} $$ My work: when applying $\square$ to a scalar $\phi$, then $ ...
0
votes
0answers
16 views

Is the electron's magnetic dipole moment influenced by the measurement method?

The electric charge of an electron at rest is a constant value and is not influenced by the measurement instrument. The measurement instrument by itself can give more or less accurate result, but does ...
2
votes
3answers
147 views

Uniqueness or multiplicity of ADM masses for spacetime manifolds with more than one “end”?

The question is: What is the mathematical and/or physical basis for saying that a (static) spacetime manifold with more than one asymptotically flat region at infinity ("end") has a distinct ...
1
vote
1answer
113 views

Size of hydrogenic atoms

Positronium consists of an electron and a positron. By what factor is a positronium atom bigger than a hydrogen atom? The solution has been explained to me. The characteristic length when solving ...
6
votes
3answers
268 views

Recommended books for a “relativity for poets” class?

I teach physics at a community college and have developed a new course titled "Relativity for Poets," which I will be teaching for the first time in spring 2015. As implied by the title, it's a ...
1
vote
0answers
20 views

What are the reactions that take place inside battery at the quantum level?

I was just studying about how a battery works on the internet and found out that there are reactions of chemicals which make the electrons move. But what exactly happens inside a battery (lets take a ...
0
votes
0answers
19 views

What will be the final temperature of water when heating at a a rate 10J/s?

If I supply heat of rate 10W to water of mass 250g and initial temperature of water is 29 degC then what will be the final temperature of water when steady state is reached. **Water is heating in open ...
0
votes
2answers
44 views

Is it possible to make an electromagnetic shield?

Is it possible to make an electromagnetic shield to protect something (or ourselves) against things like bullets, human attacks, metals thrown on us or simple dust particles to protect a spaceship ...
1
vote
1answer
58 views

Is this the reason why acceleration is said absolute?

I've seem sometimes people saying that although uniform motion on a straight line cannot be detected and hence it is not absolute, acceleration is indeed absolute in Classical Mechanics (I don't know ...
1
vote
1answer
83 views

Luminance from multiple light sources

I have a LED diode: Given Luminance intensity [I] of $1.85$ cd. I want to have Luminance [B] from the light source of $75 \ {\rm cd/m^2}$. From $B=I/S$, where S is light source plane area, I get ...
0
votes
0answers
20 views

Do physicists theorize an origin to the big bang?

Do we have any theory that covers a possibility of the big bang's origin? This seems like the most difficult question of all. Logic tells us that every effect has a cause. But every cause must be the ...
5
votes
6answers
1k views

Is there a black hole in the centre of the Milky Way?

Is it true that the whole galaxy is actually revolving, and powered by a black hole? Has it been proven, and if it is true, how can our solar systems actually keep up the momentum to withstand the ...
0
votes
0answers
14 views

Is a big bang capable of producing infinite outcomes?

I hear commonly about the (perhaps silly)idea of there being an infinite number of universes, a new one for each possible difference. For example there would be a universe where I'm a millionaire. ...
5
votes
2answers
81 views

Once I calculate the Riemann curvature tensor, what do I do with it?

I am considering the Schwarzschild metric. I have calculated my Christoffel symbols and am able to calculate the Riemann tensor (I think). In short, I have done a bunch of work to find this thing ...
0
votes
2answers
79 views

How to connect these two formulations regarding the need for a density matrix in quantum mechanics?

I found these two formulations: The density matrix is: 1) "needed if we consider a system that is part of a larger closed system." 2) "needed for a system to be ...
3
votes
1answer
1k views

How do I derive the Lorentz contraction from the invariant interval?

Reviewing some basic special relativity, and I stumbled upon this problem: From the definition of the proper time: $$c^2d\tau^2=c^2dt^2-dx^2$$ I was able to derive the time dilation formula by using ...
9
votes
4answers
1k views

Intrinsic structure of electron

The electron contains finite negative charge.The same charges repel each other.What makes electron stable and why does it not burst? Is it a law of nature that the electron charge is the smallest ...
1
vote
1answer
63 views

Galilean Relativity is already included in Newton's Laws?

Usually I see an inertial reference frame being defined as a reference frame in which Newton's first and second laws holds. That means that if a particle is at rest, it stays at rest unless some ...
0
votes
1answer
21 views

First principle band structure calculation

In many literature, we see plots of the energy band structure from DFT simulation. How these eigen-energies are obtained as function of k within the DFT framework? Are they the physical quasiparticle ...
5
votes
4answers
155 views

Thermodynamics thought experiment

There is some ideal gas in a container moving with some velocity on a smooth surface and you suddenly stop it( say by using your hands) , will the temperature of the gas increase? It seems to me that ...
7
votes
3answers
159 views

Can the Unruh effect be confirmed by the LHC?

Two short questions regarding the Unruh effect. There are related answers on this forum and on wikipedia, but I am looking for confirmation of my own intuitive assumptions, so a straightfoward yes or ...
0
votes
1answer
39 views

Converting $F^2$ to $C^2$

I'm trying to convert Fahrenheit squared $F^2$ into Celsius squared $C^2$. I know how to convert a value $x$ in $F$ into $C$ with: $\frac{5}{9}(x - 32)$ I also know how to convert a value $x$ in ...
0
votes
1answer
35 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
1answer
127 views

nuclear physics- Energetics and Mechanics of Nuclear Reaction Homework

A sample of $^{24}_{12}\mathrm{Mg}$ is bombarded by a monoenergetic proton. If the resulting nucleus in a $^{24}_{12}\mathrm{Mg}(p,\gamma)$ reaction; $^{24}_{12}\mathrm{Mg}(p,\gamma)$ has its ...
0
votes
1answer
44 views

In circular motion, what's preventing me from calculating the circumference of the circle, dividing it by time to find the speed of the object?

There are equations to solve for the speed in circular motion. However, is there anything preventing me from calculating the circumference from the given radius, and then divide that over the time to ...
-4
votes
1answer
63 views

Could dark energy be what we think is spacetime?

I find it difficult to wrap my brain around how energy/matter, chemical reactions and resultant entropy or waste heat exist in abstract spacetime. First, how can a clock measure "time" without ...
-1
votes
0answers
36 views

Fourier Transform and Canonical Transform

According to a book I'm reading, Fourier transform is a canonical transform. Anyone can prove that?
-1
votes
0answers
20 views

What is the terminal velocity of a tungsten cylinder 20ft long and 2ft wide? (if it had a more aerodynamic end) [on hold]

I read somewhere that the drag coefficient of a sphere is 0.5. so I tried calculating with that and got 4150 MPH. I don't know if 1 would be a flat end or not. However I would assume if you were ...
19
votes
7answers
2k views

What exactly is heat?

Is it energy? Is it energy per unit volume? Is it energy per unit time i.e power? What is it?
0
votes
1answer
71 views

spinning a water bottle quickly

When we spin a water bottle so quickly, why don't the water inside the bottle come out ? It has to do with the normal force and the apparent weight , i think . but plz someone explain for me how does ...
0
votes
2answers
55 views

Using Maxwell's demon alongside quantum mechanics to contradict 2nd law of thermodynamics

Suppose we have a steady state universe with a gas chamber resembling that of Maxwell's demon that is used to power this hypothetical heat engine as molecules transfer to their respectable sides based ...
0
votes
1answer
107 views

I want to know why light even moves at all.

Never mind in a vacuum, why and how does light even move at all? What propels it?
1
vote
1answer
24 views

Are nuclear processes the only processes that release more energy than is input?

Are the nuclear processes of fission and fusion the only known processes or events that release more energy than is input to create those processes? I know I am being a little vague; I do not know the ...
3
votes
1answer
31 views

Tidal tails of galaxies after collision

When there is a collision of 2 disc shaped galaxies, there is a tail formation created from both the galaxies. I read here that this was due to tidal forces, but I couldn't figure out how this ...
1
vote
0answers
31 views

Translation of the Mechanique analytique [on hold]

Is there an English translation of the Mechanique analytique by Lagrange that is free? I have tried searching up online, however I only get French originals. The English translations seem all to be ...
2
votes
1answer
41 views

Electric field in a hollow object

I am currently visiting a course about electrodynamics. In my last lecture it was said that if a hollow sphere is inside of a bigger sphere, but only in the bigger sphere there are charges, the ...

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