law of motion

1 :  a statement in dynamics: a body at rest remains at rest and a body in motion remains in uniform motion in a straight line unless acted upon by an external force —called also Newton's first law of motion

2 :  a statement in dynamics: the acceleration of a body is directly proportional to the applied force and is in the direction of the straight line in which the force acts —called also Newton's second law of motion

3 :  a statement in dynamics: for every force there is an equal and opposite force or reaction —called also Newton's third law of motion