The right hand, right forearm and the right upperarm that compose the right arm are internal to the system. Identifying the mechanical system is important in the sense that it will allow the analyst to distinguish the external forces , the forces acting on the system from outside of the system, from the internal forces , forces acting within the system among the elements composing the system. The linear motion of the center of mass of the system is affected only by the external forces. Therefore, the first task in motion analysis is to define the mechanical system. Among these, the simplest mechanical system that can move is a particle.
Particles are the building blocks of a rigid body.
The equations for a particle will be later used to develop those for a system of particles and a rigid body. Explanation requires the info about the object in motion. When one tries to explain someone else's behavior, one must know that person's personality. The personality of the object in motion is its inertial properties such as the mass and the moment of inertia. It is safe to say that any mechanical quantity that involves mass or moment of inertia in its equation is a kinetic quantity, such as force, momentum, energy, etc.