where m is the mass of the rigid body; N v ¯ ¯ is the velocity of the center of mass of the rigid body, as viewed by an observer fixed in an inertial frame N; N H ¯ ¯ is the angular momentum of the rigid body about the center of mass, also taken in the inertial frame N; and N ω ω R is the angular velocity of the rigid body R relative to the inertial frame N.