A mechanism is a device that transforms motion to some desirable pattern and typically develops very low forces and transmits little power. A machine typically contains mechanisms that are designed to provide significant forces and transmit significant power. Some examples of typical mechanisms are a stapler, a door lock, car window wiper, etc. Some examples of machines that possess motions similar to the mechanisms above are an automobile engine, a crane, and a robot. There is no clear line of difference between mechanisms and machines. They differ in degree rather than definition.

If a mechanism involves light forces and is run at slow speeds, it can sometimes be strictly treated as a kinematic device; that is, it can be analyzed kinematically without regard to forces. Machines (and mechanisms running at higher speeds), on the other hand, must be first treated as mechanisms. A kinematic analysis of their velocities and accelerations must be done and then they must be treated as dynamic systems in which their static and dynamic forces due to accelerations are analyzed using the principles of kinetics. Most of the applications in robotics involve motions at lower speeds and low or moderate forces are involved. So we will restrict our discussion only to the kinematics of mechanisms in this chapter. However, there are certain instances where the study of the dynamics becomes very essential in robotics. A discussion of those instances is beyond the scope of this book.