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Up to now, we have only analyzed the behaviour of dynamical systems
which consist of *single* point masses (*i.e.*, objects whose spatial extent is either
negligible or plays no role in their motion) or arrangements of point masses which are constrained to
move *together* because they are connected via inextensible cables. Let us now
broaden our approach somewhat in order to take into account systems of point masses which exert forces on
one another, but are not necessarily constrained to move together. The classic example of
such a multi-component point mass
system is one in which two (or more) freely moving masses *collide* with one another.
The physical concept which plays the central role in the dynamics
of multi-component point mass systems is the *conservation of momentum*.

Richard Fitzpatrick
2006-02-02