# All Questions

677 views

### Is infrared light diffused on walls?

Lamps that produce visible light can light up a whole room because the ceiling and the walls diffuse light. This way, when there is a single lamp in a room, something that is in the shadows is not ...
1k views

### Do perfect spheres exist in nature?

Often in physics, Objects are approximated as spherical. However do any perfectly spherical objects actually exist in nature?
47 views

### Any new texts directly on second law of thermodynamics? [duplicate]

Possible Duplicate: Recommendations for Statistical Mechanics book Is anyone aware of any recent text that summarises the research and arguments on second law of thermodynamics and also on ...
437 views

### Can an induction coil heat two layers of metal?

Imagine we have an induction coil which is strong enough to heat a sheet of metal. We can put a sheet of ferromagnetic metal close to the coil at distance $h_1$, and it gets heated to temperature ...
156 views

### Effect on mass of Earth if it stops revolving around Sun

If the object in motion gains mass, will it affect the change in mass of Earth if it stops revolving around Sun, since mass is responsible for gravity how will be the gravitational force change?
342 views

### How does the evaporation of a black hole look for a distant observer?

Let's assume an observer looking at a distant black hole that is created by collapsing star. In observer frame of reference time near black hole horizon asymptotically slows down and he never see ...
11k views

### What exists in the Space between atoms

Apologies to all if this has been asked before, I searched but was unable to find one similar. This is a question that has been bugging me for a while that i haven't really been able to find a ...
455 views

### What is the “foamy space” hypothesis that has been debunked recently?

In "Space-Time Is Smooth, Not Foamy", a Space.com article, it is stated: In his general theory of relativity, Einstein described space-time as fundamentally smooth, warping only under the strain ...
419 views

### What physical quantity has SI unit $\mathrm{kg}/\mathrm{m}$?

What physical quantity has SI unit $\mathrm{kg}/\mathrm{m}$? For example, the physical quantity with SI unit $\mathrm{kg}\cdot\mathrm{m}/\mathrm{s}^2$ is force $F$ and the physical quantity with SI ...
1k views

### What amount of force is needed to topple a person?

Assuming no sliding and that the shoulder is 1.2m from the feet, what force is required to topple a person weighing 70 Kg standing with his feet spread 0.9 m? If possible, please include an ...
1k views

### How can the Moon have such a strong effect on the ocean?

The gravitational acceleration on Earth is approximately $10 \mathrm{m}/\mathrm{s}^2$. Compared to this, the tidal effect of the Moon's gravity gives a local variation in the acceleration of ...
163 views

### What is Resonance Width? Why we use it to distinguish different Regimes of the Anderson Model

The single inpurity Anderson Hamiltonian is ...
984 views

### Simple elastic collision

If a particle with mass $m$ collides with a wall at right angles, and the collision is perfectly elastic. The particle hits the wall at $v\ ms^{-1}$. There is no friction or gravity. So the particle ...
1k views

### What's the physical difference between a convective heater and an infrared heater?

Could someone please explain why there are 2 types of space heaters-- one that is convective and one that is infrared? Why does the first one not radiate and why does the second one not heat the air? ...
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### Experimental data for gas flow through pipelines

Maybe some of you could point a good source of experimental data of gas flow through pipelines? What I need is a very simple flow of some gases through some diameter simple linear pipeline. A graph of ...
6k views

### What causes an aircraft to roll when rudder is applied

When continuous rudder is applied in a typical light aircraft during straight and level flight at "normal" flying speeds and altitudes, the primary effect is that the aircraft will yaw to the left - ...
282 views

### If nothing is “objectively real” prior to “measurement”, what exactly is a “measurement”?

If nothing is "objectively real" prior to "measurement", what exactly is a "measurement"? Is there any "objective" criteria to demarcate a process as being a "measurement" or not? If "measurements" ...
477 views

What experiments have challenged or supported AdS/QCD, AdS/CMT, etc? What experiments should we look forward to do this?
907 views

### Do modern Formula One cars produce enough down-force to drive upside-down?

For example, if they were driving at top speed through a long tunnel, could they transition to and stay on the ceiling?
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### What are the resulting forms of energy after thermal equalization

When two bodies of matter are placed together, one hotter than the other, where does the energy go when the two temperatures equalize? Is there any energy in thermal difference at all? If not, how ...
262 views

### Number of gravitons launched by a proton

The wikipedia article http://en.wikipedia.org/wiki/Gauge_bosons describes how in QM exchanges of gauge bosons carry force, and describes how the graviton may also be a gauge boson. If the observable ...
257 views

### Was the singularity at Big Bang perfectly uniform and if so, why did the universe lose its uniformity?

Am I right in understanding that current theory states that Big Bang originated from a single point of singularity? If so, would this mean that this was a uniform point? If so, as the universe ...
273 views

### Why do Photons want to be together?

So I've heard that when a photon flies by a atom excited enough to release a photon there's a good chance it will. Because Photons want to be together and have the same direction etc? Is this true? ...
570 views

### Can you tell just from its gravity whether the Moon is above or below you?

If you are on a place of Earth where the Moon is currently directly above or directly below you, you experience a slightly reduced gravitational acceleration because of Moon's gravity. This is what ...
977 views

### Cyclic Coordinates in Hamiltonian Mechanics

I was reading up on Hamiltonian Mechanics and came across the following: If a generalized coordinate $q_j$ doesn't explicitly occur in the Hamiltonian, then $p_j$ is a constant of motion ...
920 views

### Quantum superposition of states: experimental verification

How can somebody demonstrate the quantum superposition of states directly by other means than the double slit experiment? And why can't macroscopic objects like a pen be in superpostion of states? ...
6k views

### How is the speed of light constant in all directions for all observers?

Please imagine the following thought-experiment: Order of Events: Pulse - A single pulse of light is emitted from the light towards the mirror Reflect - The pulse hits the mirror and is ...
300 views

### exercise concerning the inclined plane

I have an exercise to ask yourself. A ball of mass m = 5 g starts at rest and travels 50 cm along a ramp inclined at 45 ° to the horizontal. a) If we imagine frictionless motion of the ball along the ...
252 views

### Are probability-preserving variations of QT with respect to the Born rule mathematically possible?

Is it possible to create (m)any theoretically workable framework(s) - that do(es) produce probabilities - by taking QM and replacing the Born(-like) rule(s) with something that is not equivalent to it ...
280 views

### Approximating Rolling/Sliding in 2D Shape

I'm trying to find more information on how a 2D shape (could be defined by a function, such as ellipse, or by a polygon) will roll across a surface. The shape could be nearly circular or quite ...
222 views

### What is a good way to reason in physics? [closed]

I have recently made the decision to study Physics seriously. However, in the past, I've had some difficulty with the subject because of my primarily mathematical background. I find that sometimes ...
105 views

### Finding symmetry of a part of an equation, given the group transformation property of another part

I am reading this paper on Dyons and Duality in $\mathcal{N}=4$ super-symmetric gauge theory. The author finds the zero modes or a dirac equation obtained by considering first order perturbations to ...
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### Isometry group from information about the center of the group

I am reading this paper on Dyons and Duality in $\mathcal{N}=4$ super-symmetric gauge theory. The author finds the zero modes or a dirac equation obtained by considering first order perturbations to ...
394 views

### How does this chair move?

I have a chair like this(shown in figure). I was kneeling on the part where we sit. I just gave a sudden push to the part where we rest the back, without moving my legs. I found that the chair is ...
306 views

### Why is a critical system equal to a gapless system?

In condensed matter physics, people often say that a system without energy gap is a critical system. What does it mean? Any help is appreciated!
37 views

### How do stars look like from space? [duplicate]

Possible Duplicate: Optical explanation of images of stars? What are these rays that appear in photograph of sun? I thought stars should look round shaped (similar to a sphere, or oblate ...
4k views

### Is the normal force a conservative force?

Most of the time the normal force doesn't do any work because it's perpendicular to the direction of motion but if it does do work, would it be conservative or non-conservative? For example, consider ...
271 views

### Swords, impacts and elasticity for a noob

So I'm a game developer and I'm trying to understand some (extremely) basic facts of impact mechanics. I had read something entitled Dynamics of Hand-Held Impact Weapons, but it was a bit too ...
683 views

### D'Alembert operator and special relativity

We are currently covering special relativity in the theoretical physics lectures where we defined: $$\mathrm ds^2 := \mathrm dt^2 - \mathrm dx^2 - \mathrm dy^2 - \mathrm dz^2$$ In Road to Reality, ...
2k views

### Why do we weigh less when falling?

I don't want to go to science world to find out because it would be a long round-trip. I understand that acceleration/deceleration would effect the weight and I can also imagine that someone at ...
126 views

### What is the covariant expression for action of the Lorentz force density on charge-current density?

In a continuous medium the Lorentz force density is known to be written in the form: $f_\alpha = F_{\alpha \beta} J^\beta$, where $F_{\alpha \beta}$ is an electromagnetic field tensor, and $J^\beta$ ...
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### How does sound travel in space?

In relation to this question: How can a black hole produce sound? Which notes that the hole "produces" sound. The top answer states that: What you think of as the hard vacuum of outer space ...
67 views

### What is the first paper to report observations of polaritons?

I am seeking references to the first articles regarding the observation of polaritons.
308 views

### What information is stored on gramaphones/tape recorders/CDs/DVDs

I'm a Software Developer by profession and my physics knowledge is limited what I had learned at high school level. Please excuse me if the question is trivial. Question: From what I know, a sound ...
110 views

### Approach of the Andromeda Galaxy

So, it's theorized that Andromeda will collide with the Milky Way some time in the next 3 billion years... I don't get how galaxies can collide with each other. What is the cause of their movement? ...
595 views

### What is the apparent diameter of the sun as seen from earth?

I know this may not be physically accurate. For my purposes, basically 3D renders, I am assuming the sun rays are parallel rays from an infinitely far lamp. If the sky is clear, what size would that ...
2k views

### Can conservation of momentum related to Newton's first law?

I know this is a scrap of thought, but the first law states that (from Wikipedia): If an object experiences no net force, then its velocity is constant Is it describing the conservation of ...
533 views

### Why does weak equivalence principle say gravity is equivalent to acceleration?

I am told that the weak equivalent principle, that $m_i=m_g$ (inertial and gravitational masses are equivalent) is equivalent to the statement that in a small system you can't tell whether you are in ...
339 views

### Examples of systems with energy as an intensive variable

I need to consider a couple of examples of systems which have energies that are intensive variables - not extensive. I'be been thinking about this and I am not coming up with anything. My ...
132 views

### Dyson Series Second Order Term Importance vs. Time

Given a time ordered Dyson series expansion of $$H_I=e^{-\frac{i}{\hbar}\sigma_3t}V_0\sin(\omega t)\sigma_1e^{\frac{i}{\hbar}\sigma_3t}$$ {\cal ...

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