  An introductory exposition of Chandrasekhar's gravitational dynamical
friction, appropriate for an undergraduate class in mec hanics, is presented.
This friction results when a massive particle moving through a ``sea'' of much
lighter star particles experiences a retarding force du to an exchange of
energy and momentum. General features of dynamical friction are presented, both
in an elementary and in a more elaborate way using hyperbolic two-body
interactions. The orbital decay of a massive particle in an homogeneous
gravitational system is solved analytically, that leads to an underdamped
harmonic oscillator type of motion. A numerical integration of the equation of
motion in a more realistic c ase is done. These results are compared to those
of an N-body computer simulation. Several problems and projects are suggested
to students for further st udy.
