In general, forces are developed from within the cell via the cytoskeleton (endogenous forces) or come from outside the cell (applied forces). Forces exerted on the cell are often dynamic in nature, requiring the cell to constantly re-evaluate its status and adjust its internal and external morphology accordingly. The best prominent example of a motor protein is the muscle protein myosin which "motors" the contraction of muscle fibers in animals. Motor proteins are the driving force behind most active transport of proteins and vesicles in the cytoplasm.
