Question 42

[mcat_premed3832]

Actually, it's almost the opposite of what you are suggesting. First of all, we know that it is an elastic collision because one electron knocks out another but we are not told that they stick together (inelastic) and we are told that linear momentum is conserved.

Also, we cannot assume that e2 has an initial velocity of zero. In fact, because it is an electron in an atom, if we had to assume anything, we would have to assume that it is in orbit which means that it has a very important velocity.