# All Questions

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### What did Feynman mean by “gravity itself is a pseudo force”? [duplicate]

Feynman writes: One very important feature of pseudo forces is that they are always proportional to the masses; the same is true of gravity. The possibility exists, therefore that gravity itself ...
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### Estimating the expansion of the universe using light from supernova

I understand that you can tell how far light has traveled by the measuring redshift but how accurate is this and could it be effected by anything like radiation from the sun it must be fairly accurate ...
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### Does the difference between contextuality, nonlocality and retrocausality depend on where we draw the boundaries? [closed]

Suppose we have a quantum system and a measuring apparatus in a superposition of detector settings. Different detector settings would measure "complementary" properties of the quantum system. The act ...
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### Measuring the pressure in a container without changing it [closed]

Let's say I have a closed, non-transparent metal container with pressurized gas and I'd like to measure the pressure inside without changing that pressure (or changing it as little as possible). The ...
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### Why does the superposition principle work? [closed]

I am reading Feynman volume 2. (I was reading Griffiths at first.) I don't really understand why the superposition principle of electrical forces (or fields) works. Is there anything deeper in that? ...
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### Inclined pulley. Finding the $\frac{M_1}{M_2}$ [closed]

Problem As you can see in the picture, $M_1$ and $M_2$ are attached by a massless rope over a pulley. the height difference of $M_1$ and $M_2$ is $h$. The masses start to move and after $τ$ seconds, ...
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### unknown stresses in double-layer glass window

I live in cold place where outside temperature drops to -20. Currently, we have -20 and on my window, which is doubled layer glass with trapped air in between, I found a "polarized stress spectrum" ...
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### Planetary nebulae, thermal pulses and mass loss

I'm reading about planetary nebulae and how they are formed, but as is sometimes the case, I've gotten a little confused. So, I have a star, let's say 5 times the mass of the sun. At some point, when ...
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### Near Infrared for distance determination

Would near-infrared be an efficient frequency to use in determining the distance from the source of the infrared signal to an object, maybe in the range of 5 to 20 meters away? I need a formula for ...
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### Is it possible to single out a proton, electron, neutron?

I've read that it's nearly impossible to take a proton from an element. But if it's "nearly" impossible then it is possible to some degree. If this has happened, what is exactly the process of taking ...
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### Sufficient criterion for su(2) invariant spin-1*spin_s bipartite density matrix

SU(2) invariant spin-1 and spin-S bipartite density matrix is given by $\rho ^{1,S}=\frac1{3*(2S+1)}[1+\alpha {S^A_i\times S^B_i}+\beta S^A_{ij}\times S^B_{ij}]$, i j varies from ...
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### Conditions of friction on constant acceleration [closed]

If a body is moving with constant acceleration, then what would the frictional force be? Zero In forward direction, In reverse direction all of above
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### What does the relation between mass and energy of a free particle mean?

What does the Hamiltonian for a free particle mean? Does it mean that the kinetic energy of the particle is in reverse relation with mass? $H$ or $E=\hbar^{2}k^{2}/2m$. Or better to ask: what's the ...
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### The wave function of transverse one is different from longitude one for convenience?

I use the book Fundamental of Physics Hallidays&Resnick 10th Edition Jearl Walker to study in my physics class while I got myself University Physics with Modern Physics Sears, Zemansky 13th ...
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### How can the electron be a wave? [duplicate]

I read about Rutherford's failed attempt to describe the atomic model. Then I read about Bohr's model and his postulates. Ok, that was fine. But then I read in my book a statement that shocked me: ...
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### Charging a spherical capacitance

How could one charge a spherical capacitor with a battery or any other emf source?
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### What's the connection between the spin of the photon and the polarisation of light?

In view of wave-particle duality, the spin of the photon must have a counterpart in the wave picture: is this polarisation?
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### In which direction due to a polarizing grid the photon's electric field is oriented?

After a photon passes the slit, is it's electric field oriented perpendicular or parallel to the slit and why this is so?
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### Why does heat lose its energy as we get further away?

Why does heat lose its energy dramatically as I move back? Say I have a fire around 0.5 meters in front of me, I can clearly feel the heat, however, as I move even very slightly back, say 1 meter ...
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### Heat diffusion: evanescent waves?

It has been recently pointed out to me that the solution of the heat equation in a semi-infinite material with an oscillating boundary condition at the surface is not an evanescent wave. The argument ...
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### Is placing a heater in the coldest part of a room the most efficient place to put it?

I've noticed that stores blow hot air in a "sheet" by doors and windows, which is "where cold air enters". I assume they do this because it saves money and/or keeps the store a uniform temperature. ...
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### Could it be possible that the universe doesn't exist [closed]

Could it be possible that the universe doesn't exist? That nothing exists, not even you or me? And by not existing, I mean totally not existing, as in not even existing as a computer simulation, or a ...
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### Why is the Great Dark Spot so shortliving unlike the Great Red Spot?

The Great Dark Spot is an anti-cyclone in Neptune. But unlike the Great Red Spot of Jupiter which lasts for more than hundred years, the Great Dark Spot exists for only one year or so. Why is it so??
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I am dealing with the following Lagrangian density $$\mathscr{L}_{em}= -\frac{1}{2}\rho\omega^2 u^2 +\frac{1}{2}\nabla u:\Sigma :\nabla ... 2answers 80 views ### Could we describe acids and bases using electromagnetism? Why don't we use Maxwell's equations in acids and base theory? Surely the interaction between two charged species is readily described by this. The theory I have so far come across is HASB theory. ... 1answer 98 views ### How fast am I moving? [duplicate] Given the speed of light is 299,792,458 meters per second and that it is constant through out the universe, (i.e a person who measures the speed of light while standing still will get the same result ... 1answer 99 views ### What topics do I need to study electromagnetism on the quantum scale? [closed] What topics do I need to study (in order) so that I can study electromagnetics on the quantum scale? What is the name of the discipline studying electromagnetism on the quantum scale? Do I need to ... 0answers 48 views ### Calculating buoyant force of air experimentally I'm planning a physics project to submit at school and since it has to be something where we experimentally determine some value, previously known theoretically, I had this following idea but I want ... 5answers 1k views ### What was the entropy of the universe at the time of the Big Bang? (I asked this question in Philosophy.SE; but I was advised to direct it here, despite it is, in my opinion, somewhat too speculative for physics.SE). High entropy generally means high disorder; and ... 2answers 815 views ### Energy Stored in a Capacitor with and without Dielectric I have seen the equation V = \frac {V_0}{K}, but also the equation V=\frac{1}{2}CV^2. The values of C and V increase in the same linear ration with K (because C=KC_0). However, as the energy ... 1answer 1k views ### Why does the tongue stick to a metal pole in the winter? since the Christmas season is here, I would like to ask a question about the movie, "A Christmas Story." In one of the subplots of the movie, Ralphie's friends were betting each other that their ... 1answer 87 views ### Question concerning the diagram for stress tensor I have a couple of questions related to the diagram in the answer by Kyle Kanos to this question, Why is stress a tensor quantity?. Q1. If the box shown is of finite size why are there not three ... 3answers 1k views ### How much heat from a fire actually warms your home? A fire in a hearth disperses heat to, I guess, three places: the bricks of the chimney out the hearth (where the person tending the fire is standing) out the chimney, above the house How would you ... 7answers 2k views ### Does the weight of an hourglass change when sands are falling inside? An hourglass H weighs h. When it's placed on a scale with all the sand rested in the lower portion, the scale reads weight x where x = h. Now, if you turn the hourglass upside down to let the sand ... 4answers 194 views ### Particles scattering on fluids: breakdown of the effective continuum description When does the macroscopic continuum description of a medium like a fluid break down? Say I'm interested in a scattering process of some particles with momentum p and energy E off a fluid of ... 0answers 82 views ### Why D^{\mu} D^{\nu} F_{\mu \nu}=0 ? (Noether Identity) [closed] I have to show that:$$D^{\mu} D^{\nu} F^A_{\mu \nu}=0$$vanish identically. This is the generalization to non Abelian groups of \partial^{\mu} \partial^{\nu} F_{\mu \nu}=0, apparently called ... 1answer 58 views ### Do magnets\ magnetic fields become weaker when exposed to para magnetic objects? Do magnets (permanent) become weaker as they are exposed to para-magnetic objects? I was thinking about this after seeing this. I am buying a magnet and wish to know if a magnet (permanent) loses its ... 1answer 268 views ### Is imaginary time a fifth dimension? [duplicate] I've read that by introducing the concept of imaginary time, the dimension of time can be treated like a spatial dimension mathematically. Assuming, without imaginary time, one considers the universe ... 2answers 163 views ### Why are galactic centers always brighter than the edges? As you can see the image below and other galaxy images, the center is generally much brighter. Why is that? Is there a very big star? A very big gravitational field? 1answer 75 views ### What are “two-centre integrals”? Reading through some condensed matter physics literature I came across the term "two-centre integrals". Could someone explain what is meant by this in general? CONTEXT: "the overlap matrix and the ... 0answers 57 views ### What is a nucleon field? A nucleon is either a proton or a neutron. A field is, as John Gribbin says, a physical quantity that has a value for each point in space and time. But what is meant by a nucleon field? Can anybody ... 4answers 640 views ### Exploring beyond event horizons In the expanding universe the velocity of separation between galaxies depends upon how far they are. If they are much far away will they have relative velocity of separation greater than speed of ... 0answers 79 views ### Where does light go when you switch it off? [duplicate] I have a lamp. When I switch it on, it immediately illuminates the surroundings. When I switch it off, the light goes away in an instant. Why is this? Where do the light particles go? Do they quickly ... 1answer 192 views ### Is Romer's letter on our search for the elementary proof of the spin-statics theorem out of date today? The following link provides a letter to the editor by Robert H. Romer who writes, In a 1994 "question" in this journal, Neuenschwander asked whether anyone had yet met Feynman’s challenge of ... 2answers 124 views ### Prove Biot-Savart law, assuming that \vec{A}=\frac I c\int \frac{d\vec{L}}{r} [closed] Prove Biot-Savart law, assuming that$$\vec{A}=\frac I c\int \frac{d\vec{L}}{r}\vec{B}=\nabla\times \vec{A} Any hint on what to do next?
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No Nobel prizes at stake, but just an idle thought and an idle question. How could one calculate the probability of a flipped coin landing stable on its edge, instead of heads or tails? I assume ...
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### What is the roaring in a roaring fire?

I was just starting a barbecue fire by blowing on the smouldering coals when I realised I had no idea what the sound was actually caused by. I can make the sound by blowing at almost any flame I can ...
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### Can a light be bent by a magnetic field?

I'm struck with two competing ideas on the question in the title. Listing #1: http://van.physics.illinois.edu/qa/listing.php?id=2009 Q: "How far can a magnetic field bend light?" A: "Unfortunately, ...
### Why is kinetic energy only “often $(1/2)mv^2$”?
I am reading the first few pages of Nakahara and refreshing my memory on physics I learned a while ago as a physics math undergrad. Nakahara defines a field $F$ to be conservative if it's the gradient ...