A particle can acclerate in 2 ways, either
a change in direction or a change in speed. The
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. It
is broken into 4 parts: 1. A discussion
of the unit tangent and unit normal vectors 2. Arclength parameterization 3.
Curvature 4. Formulating the acceleration
in components a_{T} and a_{N}  the tangential anf normal components .
