  This paper addresses the problem of the separation of rotational and internal
motion. It introduces the concept of average angular velocity as the moment of
inertia weighted average of particle angular velocities. It extends and
elucidates the concept of Jellinek and Li (1989) of separation of the energy of
overall rotation in an arbitrary (non-linear) $N$-particle system. It
generalizes the so called Koenig's theorem on the two parts of the kinetic
energy (center of mass plus internal) to three parts: center of mass,
rotational, plus the remaining internal energy relative to an optimally
translating and rotating frame.
