In Mechanics, the change in position of one body with respect to another. The
study of the motion of bodies is called dynamics. The Time rate of linear motion in a given direction by a body is its velocity; this
rate is called the speed if the direction is unspecified. If during a time t a
body travels over a distance s, then the average speed of that body is s/t. The
change in velocity (in magnitude and/or direction) of a body with respect to
time is its acceleration. The relationship between Force and motion was expressed by Newton, Sir Isaac in his three laws of motion: (1) a body at rest tends to remain at rest, or a
body in motion tends to remain in motion at a constant speed in a straight
line, unless acted on by an outside force; (2) the acceleration a of a mass m by a
force F is directly proportional to the force and inversely proportional to
the mass, or a = F/m; (3) for every action there is an equal and opposite
reaction. The third law implies that the total Momentum of a system of bodies not acted on by an external force remains constant (see Conservation Laws). Motion at speeds approaching that of Light must be described by the theory of Relativity, and the motions of extremely small objects (Atoms and Elementary Particles) are described by quantum mechanics (see Quantum Theory).