# All Questions

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### What's the true reason behind thermal expansion?

Thermal expansion is a normal concept everyday. There are 2 explanations: 1, thermal expansion result in stress, then result in deformation 2, thermal expansion result in deformation, then result in ...
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### How to calculate D0->K+K- decay with QCD factorization?

I am studying the decay of D0->K+K- and I calculated the result of Branching Ratio by Naive QCD and I wrote all the equations for amplitude and branching Ration and Hamiltonian and... for this decay ...
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### Could this planetary superalignment happen?

Here's the 'superalignment' I'm referring to: We've all heard the stories about 'mystical planetary alignments' that will increase/decrease the effective surface gravity experienced on Earth (one ...
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### When objects fall along geodesic paths of curved space-time, why is there no force acting on them?

On cseligman.com, it is written that So, we see things falling with an acceleration which we call the acceleration of gravity,and thinking that we live in a straight line , uniformly moving or ...
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### Is gravity a centripetal force?

In curved space-time, there are curved paths. Since curved paths in our experience require some centripetal force to create them, isn't then gravity a centripetal force?
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### If gravitation is due to space-time curvature, how can a body free-fall in a straight line?

According to general relativity, Gravity is due to space-time curvature. Then all paths must be curved. If so, how can there be any straight line motion? The body must follow a curved path. So, there ...
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### What are some most advanced theories explaining why same charge repel, opposite charge attract? [duplicate]

It is the holidays with a lot to think about. One of the thing that I couldn't get out of my mind was something I read in Stephen Hawking's "Brief History of Time". In it, he described the repelling ...
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### Problem with sound intensity equation in displacement amplitude form

The sound or wave intensity is defined by energy transfer rate with time (power) per unit of area: $$I = \frac{P}{A} \tag{1}$$ so this equation makes sense since the denominator is the area of the ...
19 views

### Physical meaning of RG transformation

When we do RG transformation in Statistical mechanics we eliminate unnecessary degrees of freedom and it leads us to the fixed point. How can I visualize it physically?
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### How can we derive the zero work done by the formula of potential energy [on hold]

**We throw a stone through a vertical direction and its fall done so give me explanation about zero work done by potential energy formula. **
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### Does the density of matter increase as we approach the big bang? [duplicate]

I am interested in knowing whether it is clear (undisputed) that the density of matter/energy increases as we approach the time of the big bang? Does this follow from the FLRW metric?
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### Loop quantum gravity and string theory [on hold]

I am wondering, with loop quantum gravity theory and string theory. They both are quite substantial in their own right, but What are the pros/cons of each in simple understandable terms?
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### Does it matter which blanket I take below and which one above when I cover myself with the two? [on hold]

If I take two different blankets to cover myself, does it matter which one I take below and which one above? Will both the arrangements give me the same comfort or one of them will give me better ...
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### Why are we still teaching Newtonian physics if it's “false”? [duplicate]

Why are we teaching an outdated theory when the math in general relativity isn't that hard? I really don't see a reason why we should teach a highly oversimplified theory in our schools. For me, it's ...
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### Do objects really move in the direction of the force applied?

According to the second law of motion: Direction of the force is in the direction of change in momentum. Now, I came before a problem. Though an easy one, I was perplexed at a point. A ...
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### How to simulate 2 AC electrodes putting into a pool in Maxwell3D (ANSOFT)? [on hold]

I am using Ansoft Maxwell3D to simulate the magnetic field produced by an AC current flowing through a water channel. Suppose i have 2 AC electrodes putting at two ends of a water channel or a pool ...
101 views

### Do electrons pop into and out of existence around the nucleus of an atom?

What surrounds the nucleus is the probability wave. But are the electrons constantly popping in and out of existence around the nucleus in the cloud?
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### Learning Physics [duplicate]

What's the best way to start learning Physics from home? I haven't touched it since my high school course. I know of Coursera and youtube videos? Are there any other sources that anyone can recommend? ...
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### Energy needed for exoergic nuclear reaction

To initiate endoergic nuclear reaction (negative Q Value)minimum energy is supplied which is slightly more than Q value. Then what will be the energy needed for intiating exoergic nuclear reaction ...
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### Gravitational… confinement?

This is a followup to Ergil's question "Weak isospin confinement?". According to the Wikipedia article on color confinement: The current theory is that confinement is due to the ...
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### Is friction acting for average speed if vehicle passing on a banked road?

Roads are usually banked for the average speed if vehicle passing over them. However, if the speed of a vehicle of a vehicle is somewhat less or more than this the self adjusting static friction ...
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### Is there a limit to what we zoom into if it is in fact infinite

I read that recently we were able to view the first atom (hydrogen) under a microscope. This got me thinking about the idea of measurements and zoom lenses. Say hypothetically we were able to build a ...
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### What is “special” and what is “general” in Relativity?

Initially I thought in special relativity the velocity was constant, whereas general relativity allowed treatment of accelerated frames as well. But now I have heard that SR is only valid locally?
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### What is the energy of a Gaussian wave packet?

Suppose we have a potential barrier situation, that is $V(x)$ is zero everywhere except on the interval $[-a,a]$, where it is equal to some $V_0 > 0$. Introduce some Gaussian shaped wave packet to ...
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### The subtle differences between angular momentum and centrifugal force?

I am a mathematician wanting to understand the differences between the concepts of angular momentum and centrifugal force. The following two ideas are clear to me from a physical point of view, but ...
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### What does vector space and bra/ket space mean? [on hold]

I wonder What are the similarities and dissimilarities between a vector space and bra/ket space?
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### Spivak's Physics for Mathematicians [on hold]

I would like to ask for some details on Spivak's Physics for Mathematicians (Mechanics): is this textbook suitable for a first exposure to physics for a math undergraduate? what knowledge is assumed ...
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### Collisions in two dimensions: using relative velocity

I am following this tutorial which reviews some physics on collision resolution. I understand relative velocity, the mathematics is not a problem for me. However, despite my head scratching and ...
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### Using infrared temperature sensor to measure water surface temperature

Can I use infrared temperature sensor (such as TS118-3) to measure water surface temperature? I'm afraid some effects such as reflection of infrared waves from water surface and blinking because of ...
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### Is wave superposition always equivalent to wave interference?

I'm confused when using these 2 words "wave superposition" and "wave interference" since their definition is very similar. So, are these 2 term the same?
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### Are all fermions massless at high temperatures?

According to the standard model, the electroweak symmetry is unbroken at high temperatures, and therefore all gauge bosons are massless then. But since fermions are said to acquire mass by a different ...
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### Biot-Savart for current carrying wire

A homework question has made me doubt my understanding of the application of the Biot-Savart law. Question: The magnetic field 43.0 cm away from a long, straight wire carrying current 5.00 A is ...
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### What are the relative limitations of the Schrödinger, Pauli, and Dirac Equations?

I know there are significant differences in the nature of the Schrödinger, Pauli, and Dirac equations. Although I know a bit about how each works, I don't understand the relative limitations of each ...
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### Modelling ions hitting a grid

I want to model the capture of ions by a negatively charged grid made of some proton conductor material like graphene oxide. Are ions captured as soon as they cross the surface, or do they have some ...
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### Does water maintain equal level in connected vessels?

Consider a system as illustrated below: There are two 55 gallon barrels used to store water coming down the gutter. These barrels are connected together on the bottom to maintain equal water level ...
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### Calculate the length of propellers needed on an electric motor to lift a mass [on hold]

I purchased an electric motor that is rated for 1.5-3VDC, it produces 11,600 RPM with no load and consumes .3A with no load. It is about an inch in diameter and height but I was wondering how I could ...
33 views

### Can we convert light to another spectrum?

Could we convert light from one spectrum to another just like how it does in a radium, using some device? If we could do that to a specific part of the emitted light, probably we could make 3-D ...
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### Do infrared lamps provide more heat than incandescent lamps?

Infrared lamps seem to be used in situations where you want to produce heat without producing (as much) light. But incandescent bulbs also produce heat. Why not just use incandescent bulbs? My ...
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### What does it mean for a Fresnel lens to have two finite conjugates?

I'm in a situation where I need to reverse engineer a product that my company already manufactures and sells, but it wasdesigned too long ago for anyone to remember. It uses a Fresnel lens with the ...
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### References or resource recommendation for the mathematics concerning fission

I am working on a statistical problem that appears similar (in some respects...) to nuclear fission. I am interested in the properties of a system undergoing fission around, or near, delayed ...
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### How does diffraction cause maxima and minima on a viewing screen

I have a problem in understanding the diffraction phenomenon. At first, it is described in simple way If a wave encounters a barrier that has an opening of dimensions similar to the wavelength, ...
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### Criterion of tangled state by using particular transposition of density matrix

Let's have (for simplicity) two-qubit density matrix $\rho$. If we make the particular transposition only by one qubit, we'll get some other matrix $\tilde \rho$. Then the criterion of the mixed ...