A particle can acclerate in 2 ways, either
a change in direction or a change in speed.
purpose of this discussion is to formulate velocity and acceleration of a particle at a point in terms of vectors tangent
and perpendicular to the trajectory at this point.
is broken into 4 parts:
1. A discussion
of the unit tangent and unit normal vectors
2. Arclength parameterization
4. Formulating the acceleration
in components aT and aN - the tangential anf normal components .