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In analyzing an object's motion, we need to focus our attention to certain portion (a part or the whole) of the object which we conceptually separate it from the environment external to it. This specific portion subject to analysis is called as the mechanical system. For example, if one is interested in analyzing motion of the right arm of a subject, the subject's right arm is the mechanical system. The rest of the body (the head, left arm, trunk and the lower extremities) are all external to the mechanical system. 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.

There are several different types of mechanical systems: a particle, a system of particles, a rigid body, and a system of [linked] rigid bodies. Among these, the simplest mechanical system that can move is a particle. A particle has mass but no volume: mass point. 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. Some of the important mechanical quantities of a particle are classified and summarized on the following pages:

 Kinematics Linear Kinetics Angular Kinetics

Kinematics is basically 'the study of description of motion'. while kinetics is 'the study of explanation of motion'. Description focuses on the effects (motion) while explanation focuses on the cause (force & torque). 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. Since the inertial properties are known, one can relate the cause (force & torque) and effects (motion) in the kinetics. On the other hand, kinematic quantities have no reference to the inertial properties of the object at all, such as in position, velocity and acceleration.