All Questions

323 views

Will a wave packet undergo dispersion when traveling down a hanging rope?

Suppose I tie one end of a rope to my ceiling and the other end to a spot on my floor directly underneath it. Because the rope has some mass, the tension varies along the rope, from highest at the ...
397 views

What are the normal modes of a vertical rope?

Closely related to this question on traveling waves on a hanging rope, I would also like to know what the normal modes are on a rope that hangs vertically, fixed at both ends. Tension in the rope ...
961 views

Why does the cart move? [duplicate]

A while ago someone proposed the following thought experiment to me: A horse attached to a cart is resting on a horizontal road. If the horse attempts to move by pulling the cart, according to the ...
436 views

Rolling stone on a frictional surface

Consider a spherical rigid stone rotating with angular velocity $\omega$ being dropped vertically onto a horizontal rigid surface with the coefficient of friction $\mu$. Can the stone roll on the ...
6k views

Why do lightbulbs continue to glow after the light is turned off?

I've noticed that whenever I turn the lamp off in my room at night, the lightbulb seems to continue to glow for a minute or so after that. It's not bright though; the only way I even notice it is if ...
534 views

Why are “heavier” particles harder to detect than “lighter” ones?

Something I have read multiple times that I've never intuitively understood is that "heavier" particles are harder to detect than "lighter" ones... For example, I quote from Stephen Hawking's "The ...
94 views

Why is it that using cell phone can invite lighting strike? [duplicate]

Possible Duplicate: Is it safe to use any wireless device during a lightning storm? We often hear the tale that a person gets stuck by lighting when he is unfortunate enough to use a cell ...
475 views

How are classical optics phenomena explained in QED (color)?

How is the following classical optics phenomenon explained in quantum electrodynamics? Color According to Schroedinger's model of the atom, only particular colors are emitted depending on the type ...
1k views

Is it safe to use any wireless device during a lightning storm?

I need "educated" reasons whether it is safe to use any wireless device during a lightning storm. Most people said don't use it but they cannot explain why.
2k views

Ring theory in physics

Surely group theory is a very handy tool in the problems dealing with symmetry. But is there any application for ring theory in physics? If not, what's this that makes rings not applicable in physics ...
509 views

Is it possible to throw an object faster but over the same distance?

Is it possible to throw say a tennis ball at $1ms^{-1}$ for 20m and then be able to throw the same ball at $2ms^{-1}$ for 20m at the same angle? That is throwing the ball in a traditional curve withe ...
430 views

What is the pressure between two electric dipole sheets of finite extent?

I have recently become curious about modeling the repulsion of everyday objects in contact with one another. By repulsion I mean as you attempt to walk through a wall, the pain in your nose suddenly ...
614 views

Nonlinear optics as gauge theory

the widely used approach to nonlinear optics is a Taylor expansion of the dielectric displacement field $\mathbf{D} = \epsilon_0\cdot\mathbf{E} + \mathbf{P}$ in a Fourier representation of the ...
407 views

How much does it cost every time your car stops for a traffic light?

Every time we have to stop the car, it is costing us extra money, because we have to then accelerate to full speed again. I would like to know how much. In order to simplify the situation, we can ...
203 views

Material resistency to lasers beam

Keeping the average power constant, why some materials are more eager to be damaged by pulsed laser with respect of C.W. lasers, or viceversa? When i talk about pulsed lasers i think for examples of ...
1k views

The Concepts of Path Integral in Quantitative Finance

I realize that path integral techniques can be applied to quantitative finance such as option value calculation. But I don't quite understand how this is done. Is it possible to explain this to me ...
1k views

Electromagnetic Field as a Connection in a Vector Bundle

I would like to know more about Ehresmann connections in vector bundles and how they relate to the electromagnetic field and the electron in quantum mechanics. Background: The Schrödinger equation ...
564 views

Self Balancing Unicycle

Here is a link to a video of a self-balencing unicycle. I'm curious as to how this works. In particular: What provides the torque to keep the rider from falling over? I know it's somehow provided by ...
340 views

Would something like the uncertainty principle arise even if the universe was built on something like Newtonian mechanics?

I am thinking of a (greatly simplified) computer simulation of a universe that followed something like Newtonian rules. Inside the simulation are A.I.s that are made from those same rules, and can ...
364 views

Why is the ground state of the ferromagnetic tetrahedron threefold degenerate?

I'm preparing a presentation on Spin-Ice, but something's been bugging me for a while. On the Wikipedia page for Geometrical Frustration, it says the following about easy spins on a tetrahedron with ...
123 views

Computing delta temperature based on radiation and heat loss

Is there a formula that gives me the instantaneous change in temperature under ideal circumstances? Details: On a cloudless day, temperature is affected by two major things(?): While the Sun is ...
484 views

Navy's new Mach 7 Rail Gun Photo

There have been a few articles about the Navy's new Mach 7 33 Megajoule railgun. As a physics teacher, I have a couple of questions about this, and was hoping for some help. Is the kinetic energy ...
340 views

Notation for Sections of Vector Bundles

(Reformulation of part 1 of Electromagnetic Field as a Connection in a Vector Bundle) I am looking for a good notation for sections of vector bundles that is both invariant and references bundle ...
444 views

Phonons, rotons, and maxons

What are phonons, rotons, and maxons, and what does their dispersion curve have to do with superfluidity? I understand that they are quasiparticles, but I'm not entirely sure what that implies. Are ...
393 views

Physics of a fixed wheel moving on a flat surface held at distance from a center point

Ok, so my boss is trying to make a car turntable. In essence, he has a two boards that sit atop a rotating ring. He wants to put two wheels at the end of each board (8 wheels total). He thinks that ...
556 views

Does the cube stay on the loop-de-loop?

Suppose a cube of mass $m$ enters a loop-de-loop of radius $r$, travelling halfway around and reaching the top. At the top, it has velocity $v$, and the second half of the loop is frictionless. How ...
184 views

Buckyballs in vacuum

I've read about the idea that buckyballs and other nanostructures could be used to hold drugs and things until they reach certain places in the body and then get released. So I was wondering, if you ...
1k views

Where is the Atiyah-Singer index theorem used in physics?

I'm trying to get motivated in learning the Atiyah-Singer index theorem. In most places I read about it, e.g. wikipedia, it is mentioned that the theorem is important in theoretical physics. So my ...
428 views

Orbital mechanics of Dragon's Egg

In the novel Dragon's Egg, the human crew use one asteroid to swing other asteroids in place to counter the gravity of the neutron star. I understood that it was similar to a gravity sling shot, but I ...
470 views

which areas in physics overlap with those of social network theory for the analysis of the graphs?

I am studying social networks in terms of graph theory and linear algebra. I know that physicists have published and worked alot in this field. This causes me to assume that there are sub-fields in ...
848 views

Distance travelled in free-fall

When an object is in free fall, we have: $a(t) = g - \frac{c}{m}v(t)^2$ where $g$ is acceleration due to gravity, $m$ is the mass of the object, and $c$ is the coefficient of air resistance. How ...
2k views

Why are physicists interested in graph theory?

Can you tell me how graph theory comes into physics, and the concept of small world graphs? (inspired to ask from comment from sean tilson in): which areas in physics overlap with those of social ...
331 views

Soliton mechanics

How much of classical mechanics can be modelled with solitons? What I am aware of is that single solitons behave in a way like free particles: they move along as stable entities with constant ...
1k views

How to calculate a ballistic trajectory for a suborbital flight?

I'm trying to calculate the initial launch angles and velocity of a projectile (atmosphere's effects can be neglected), assuming that I know the lat/lon coordinates of both the launch and the ...
890 views

How many Onsager's solutions are there?

Update: I provided an answer of my own (reflecting the things I discovered since I asked the question). But there is still lot to be added. I'd love to hear about other people's opinions on the ...
153 views

Does a towel that's spread out cool faster than one that isn't?

I was thinking about how they say those sails on top of some dinosaurs helped regulate their body temperature. If a dinosaur didn't have that sail, would it really make any difference? If you heated ...
867 views

Why some nuclei with “magic” numbers of neutrons have a half-life less than their neighbor isotopes?

It's easy to find the "magic" numbers of neutrons on the diagrams of alpha-decay energy: 82, 126, 152, 162. Such "magic" nuclei should be more stable than their neighbors. But why some nuclei ...
929 views

What does an atom radiate: a wave packet or a single photon?

What does an atom radiate: a wave packet or a single photon?
1k views

Electron transitions in an infinite square well

Consider an electron in an infinite square well. The expectation values of momentum and angular momentum are all zero for energy eigenstates. An electron transition is accompanied by the emission or ...
204 views

Experimental samples with rare earth metal

Many experiments, such as optical, superconductivity, etc, use the samples that involve rare earth metals and transition metals. Why are they used that often. Is the main reasons: They have the ...
3k views

Is crystal momentum really momentum?

Almost every solid state physics textbook says crystal momentum is not really physical momentum. For example, phonons always carry crystal momentum but they do not cause a translation of the sample at ...
954 views

Current from induced emf

If the induced emf in a circuit is negative, and current from this emf is the emf over the resistance, what happens to the negative sign in the induced emf when solving for the current? Surely there's ...
482 views

Are all objects in motion?

It seems like everything in the universe is in motion, at least relative to some other object. That leads me to believe that all objects are in motion. But how do we measure motion when we are ...
236 views

Isotope properties plotting tool?

I'm looking for something that will generate scatter plots comparing different properties of isotopes. Ideally I'd like some web page that lets me select axis and click go but a CSV file with lost of ...
128 views

How much can a laser's position be fine-tuned?

Suppose you wanted to do a time-resolved experiment with a molecular beam traveling at, say, 300 m/s involving a mobile excitation (pump) laser that scans across the length of the molecular beam and a ...
121 views

In photo-electric experiment, if the light is exposed to a metal plate for a long enough interval, does the plate become lighter?

I know from the book that electrons will be kicked out from the metal plate if the light of appropriate wavelength is exposed to the metal plate. My mental model says if we let the light expose the ...
291 views

Magnetism-Related Terminology

A few questions about a magnet and a paperclip: What do you call a material that attracts another material via magnetism? (i.e. the magnet) What do you call the material that is attracted in #1? ...
121 views

How long does it take a object captured by a star falling to the center?

If the captured object do not have tangential velocity, it's just the free-fall time. But when it has, it may take longer time to fall in, right ? The function should be \$\ddot{r} = -GM/r^2 + ...