A mechanism is a kinematic chain with one of the links in a fixed position. In a mechanism (which is a kinematic chain) every particle of every link must move along a definite constrained (pre-estimated or pre-specified) path which may be a straight path or curvilinear path. Generally the curvilinear path may be an arc with a fixed radius or an arc with a different radius from instant to instant.
Associated with motion are concepts of velocity, acceleration, angular displacement, angular acceleration and harmonic motion.
Motion occurs due to force. Momentum is defined as mass multiplied by velocity, and forced is defined as change in momentum and it is thus equal to mass and acceleration.