The document discusses equivalent dynamical systems used to model the motion of rigid bodies acted on by external forces. It defines an equivalent dynamical system as one where: 1) the sum of the masses equals the total body mass, 2) the center of gravity of the masses matches the body's, and 3) the sum of the mass moments of inertia about the center of gravity equals the body's. It then gives an example of calculating the equivalent dynamical system for a connecting rod suspended and oscillating, determining the radius of gyration and placing one mass at the small end center and the other a distance l2 from the center of gravity.