## Top new questions this week:

### Why do the big nuts always remain at top? The Brazil-nut Effect

Most of the time I mixed different nuts in a bowl, I observed that the big Brazil nuts were always at top while the small peanuts settled near the base. No matter how you take it, if the big nuts are ...

everyday-life

### Is ice a type of glass?

It's kind of a tricky concept I assume, on one side you got those neat shared vertices of SiO2, on the other (water) you don't really have shared vertices, only kinda (but they still want to align ...

material-science material

### Why does a dish cloth absorb liquid better when it is damp?

Take a really dry dish cloth and try to wipe up some liquid you spilled on the kitchen counter. I will take up only so much of the liquid. Then try it with a damp cloth (or a wring out a wet one). It ...

everyday-life water

### Why don't free electrons fall from metals if shaken?

This is a question we were asked at a physics lecture.

condensed-matter electrons metals

### Underdetermined forces in a statics problem

I was trying to cook up a good exam problem on Newtonian mechanics, and I came up with one that I didn't fully understand myself. Always fun when you can outsmart yourself. In the figure a cylinder ...

newtonian-mechanics

### How Earth protected from the gamma rays generated by Sun?

Sun is generating energy by nuclear fusion. And nuclear fusion will emit energy in the form of gamma rays. Normally earth's ozone filtering the Ultraviolet radiation. And magnetic fields of earth ...

sun atmospheric-science gamma-rays

### Non-linear systems in classical mechanics

In general, what is meant by non-linear system in classical mechanics? Does it always concern the differential equations one ends up with (any examples would be greatly appreciated)? If so, is it ...

classical-mechanics non-linear-systems integrable-systems

## Greatest hits from previous weeks:

### If photons have no mass, how can they have momentum?

As an explanation of why a large gravitational field (such as a black hole) can bend light, I have heard that light has momentum. This is given as a solution to the problem of only massive objects ...

special-relativity electromagnetic-radiation photons mass momentum

### Why is filling a balloon from your mouth much harder initially?

Why is it that when you first fill up a balloon, it's hard to get air through, but after inflating it a bit, it becomes much easier to further inflate the balloon?

everyday-life air elasticity
 asked by seventeen years a bmw 69 votes

## Can you answer these?

### Non-integrability of the 2D double pendulum

Context: For an $n$-dimensional system, one has to deal with $2n$ independent coordinates ($2n$ dimensional phase space), of position $q$ and $\dot{q}$ in Lagrangian formulation, or independent ...

lagrangian-formalism hamiltonian-formalism chaos-theory integrable-systems integrals-of-motion

### Is there a null incomplete spacetime which is spacelike and timelike complete?

Geodesic completeness, the fact we can make the domain of the geodesic parametrized with respect an affine parameter the whole real line, is an important concept in GR. Especially, because the lack of ...

general-relativity mathematical-physics differential-geometry
We know that SUSY generators commute with translation $$[P_\mu,Q_\alpha]=0$$ I have some questions: What is this equation physical meaning? Is it possible to make "SUSY-like" generators that do ...