While solving the problems in dynamics, one must take account of the constraints that limit the motion of the system. Examples (i) In rigid bodies, constraints keep distance between the particles unchanged. (ii) The beads of an abacus are constrained to one dimensional motion by the supporting wires. (iii) Gas molecules in a container are constrained by the walls of the vessel to move only inside the container. (iv) A particle placed on the surface of a solid body is subject to the constraint that it can move only on the surface or in the region exterior to the body.