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Elastic and Inelastic Collisions
-------------------------------

Elastic collisions are those in which no kinetic
energy is lost in the collision.  Thus,

KEinitial =  KEfinal

Inelastic collisions are those in which some
kinetic energy is lost in the collision at
the expense of heat, internal energy etc.
Thus,

KEinitial =  KEfinal + U

In both cases, however, the momentum IS conserved.
Thus

pinitial = pfinal

The extreme inelastic collision is one in
which the colliding objects stick together
after the collision.  The fraction of the
KE which is lost in the collision process is
determined by the combination of conservation
of energy and conservation of momentum.