Physics Weekly Newsletter - Monday, March 30, 2015

Top new questions this week: How small does sand have to be to get wet? I think of sand as a lot of very small rocks. Suppose I have a pile of rocks, each about 1cm in size, and the pile is a meter tall. If I pour a bucket of water on the rocks, most of the water will ... scaling wetting   asked by Mark Eichenlaub 43 votes answered by Johannes 26 votes Could I break the sound barrier using circular motion? (And potentially create a sonic boom?) Ok, Lets say I get out my household vaccum cleaner, the typical RPM for a dyson vaccum cleaner reachers 104K RPM, Or 1.733K RPS. In theory, this disc would be travelling with a time period of ... newtonian-mechanics acoustics   asked by FastestManAlive 24 votes answered by tpg2114 32 votes If the earth is a good conductor of electricity, why don't people get electrocuted every time they touch the earth? Since the earth is a good conductor of electricity, is it safe to assume that any charge that flows down to the earth must be redistributed into the earth in and along all directions? Does this also ... current earth   asked by Swami 22 votes answered by zeldredge 33 votes In an electron-positron annihilation, in what direction are the photons released? I read that, in an electron-positron annihilation, at least 2 photons are produced, because of the law of conservation of momentum. my question is: in what direction are those photons released? and ... kinematics photons electrons quantum-electrodynamics collision   asked by Moozda 13 votes answered by Ali Moh 19 votes Why do lines in atomic spectra have thickness? (Bohr's Model) Consider the atomic spectrum (absorption) of hydrogen. The Bohr's model postulates that there are only certain fixed orbits allowed in the atom. An atom will only be excited to a higher orbit, if ... quantum-mechanics atomic-physics spectroscopy doppler-effect absorption   asked by Gerard 12 votes answered by m0nhawk 6 votes Why collide a moving particle with a particle at rest, rather than two moving particles? I was just reading some lecture notes about relativistic and quantum mechanics, and in the later part of this page the author demonstrates that any relativistic particle collision in the "lab" ... special-relativity kinematics reference-frames collision particle-accelerators   asked by Ixrec 11 votes answered by PhotonicBoom 13 votes Is the lay explanation of the equivalence principle wrong? The common explanation/trope for the equivalence principle always has something to do with you being inside an elevator or spaceship, and your supposed inability to differentiate, say, gravity from a ... equivalence-principle   asked by Nickolas 8 votes answered by Alfred Centauri 3 votes Greatest hits from previous weeks: Relation between water flow and pressure Is there any equation that states the relation between pressure and water flow. I.e. Let's say that in 1 hour with 8mca (water collum meters) pressure I obtain 50m3. What if (giving the same ... fluid-dynamics pressure water flow bernoulli-equation   asked by Paulo Bueno 1 vote answered by Ron Maimon 9 votes Gravity on the International Space Station We created a table in my physics class which contained the strength of gravity on different planet and objects in space. At altitude 0(earth), the gravitational strength is 100%. On the moon at ... newtonian-gravity orbital-motion reference-frames centrifugal-force celestial-mechanics   asked by SuperStack 10 votes answered by Keith Thompson 22 votes Can you answer these? How to calculate the ground states' Berry phases with doubly degeneracy, such as that due to the particle-hole symmetry or time reversal symmetry? Suppose the ground states of a system are doubly degenerate due to an anti-unitary symmetry $K$, which are $|\psi>$ and $|K\psi>$. If the system is an one-dimensional Fermion system and ... quantum-mechanics majorana-fermions berry-pancharatnam-phase   asked by Dong 2 votes Why does band gap form for Silicon? The image on the right shows that as the interatomic distance becomes closer together, the 3P and 3S band that joined together begins to split up forming a gap in allowed energies. Why does this ... electronic-band-theory   asked by Illegal Immigrant 1 vote Calculating imaginary part of a loop diagram using cutting rules for phi^4 theory I'm trying to calculate the imaginary part of this diagram in $\phi^4$ theory, using the optical theorem, and I'm having trouble. The cutting rules seem to suggest that this diagram is equal to ... quantum-field-theory feynman-diagram   asked by calavicci 2 votes

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