ABSTRACT
Newton’s great contribution to the science of mechanics was to establish quantitative relationships which accounted for the changes in the motion of an object in terms of the forces acting on that object. To emphasize the limited validity of Newton’s first law, imagine a suitcase on the frictionless luggage rack in a coach which is travelling at constant velocity relative to a road. In stating Newton’s three laws of motion it has already been pointed out that both force and acceleration are vector quantities that are characterized by direction as well as magnitude. When external forces act on an extended body the motion of its centre of mass is identical to that of a particle with the same mass as the whole body subjected to the resultant of those external forces. The equation of motion of a system is obtained by combining Newton’s second law of motion with the force laws that are relevant to the system.
