Dynamics is defined as the branch of mechanics that deals with the effect of forces on the motion of object or something. A machine is a device that can convert the available forms of energy to useful workby using the energy to apply forces and control movement to perform an intended actionduring which each part of a machine, that undergoes relative motion with respect to some other part, is called kinematic link (or kinematic element). Kinematic links help in the transmission of motion, from one machine part to another.Theory of Machines is a 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. Kinematics of machine is a branch of theory of machines which deals with the study of relative motion of various elements of machine without taking into consideration the forces which act upon them.
Whereas Dynamics of machines deals with the study of relative motion of various elements of machine taking into consideration the forces which act upon them there by force motion relationship are developed and analysed for static and dynamic equilibrium. If not the cause and effect of unbalanced forces on the rotating and reciprocating masses has to be studied, analyse, control and rectify the undesirable effects so that the system will be performing it functions as per the needs which it is intend to perform.