On a cold winter day, a penny (mass 2.50 g) and a nickel (mass 5.00 g) are lying on the smooth (frictionless) surface of a frozen lake. with your finger, you flick the penny toward the nickel with a speed of 2.25 m/s . the coins collide elastically; calculate both their final speeds. what are the directions of their final velocities, opposite or the same?

 cannot be done because they can deflect in various direction 
momentum is conserved. energy is changed to heat and sound "click" 
you could solve it it they were stuck in a straight line track 
no real collision is perfectly elastic 

did the penny go 2.25 hundred million miles and hour? pretty strong finger 
all problems need units

An  atom  (ours, not carroll's) consists of a positively charged  nucleus, surrounded by one or more negatively charged particles called  electrons. the positive charges equal the negative charges, so the atom has no overall charge; it is electrically neutral. most of an atom's mass is in its nucleus; the mass of an electron is only 1/1836 the mass of the lightest nucleus, that of hydrogen. although the nucleus is heavy, it is quite small compared with the overall size of an atom. the radius of a typical atom is around 0.1 to 0.25 nanometres (nm), whereas the radius of a nucleus is about 10-6  nm.  *  if an atom were enlarged to the size of the earth, its nucleus would be only 200 feet in diameter and could easily rest inside a small football stadium.read more on -