Theory  of  Machines  may  be  defined  as  that  branch  of   Engineering-science, which deals with the study of relative motion between the various parts of  a machine, and forces which act on them. The knowledge of this subject is very essential for an engineer in designing the various parts of a machine.

 Classification of Theory of Machines as following four branches:

 1. Kinematics: It deals with the relative motion between the various parts of the machines.

 2. Dynamics: It deals with the forces and their effects, while acting upon the machine parts in motion.

                3. Kinetics: It deals with the inertia forces which arise from the combined effect of the mass and motion of the machine parts.

                4. Statics: It deals with the forces and their effects while the machine parts are at rest. The mass of the parts is assumed to be negligible.