(Original post by Stonebridge)
There can be some confusion in the terminology depending on which book you read.
When 2 objects collide there are an infinite number of possible outcomes.
In all cases momentum is conserved if there are no external forces.
The total kinetic energy of the objects usually decreases as some is converted into heat and/or sound with the possibility of some being used to permanently distort the bodies, leaving them with some potential energy.
Some books will say a collision is elastic
if no kinetic energy is lost. Others will call this perfectly elastic
. Actually, there is no need for the word "perfectly".
If some kinetic energy is lost in the collision, the collision is inelastic
. There is no need to call it "partially" elastic, although I have seen that word used.
There is also the special case where the two objects don't separate and stick together after colliding. This is called completely inelastic
. (Also totally
In an elastic collision (no loss in k.e.) you will also find that the objects separate from each other with the same relative speed that they approached each other.
In an inelastic collision the velocity of separation is less than that of approach.
In a completely inelastic collision the velocity of separation is zero.
The ratio of the relative velocity of separation to the relative velocity of approach is called the coefficient of restitution, and given the symbol e.
Mathematically, for an elastic collision e=1
For a completely inelastic collision e=0
Hope this helps.
If you want to find out more about "e" and (in)elastic collisions look up "Newton's Experimental Law of Impact".