law of motion

any of three laws of classical mechanics, either the law that a body remains at rest or in motion with a constant velocity unless an external force acts on the body (first law of motion ), the law that the sum of the forces acting on a body is equal to the product of the mass of the body and the acceleration produced by the forces, with motion in the direction of the resultant of the forces (second law of motion ), or the law that for every force acting on a body, the body exerts a force having equal magnitude and the opposite direction along the same line of action as the original force (third law of motion, or law of action and reaction ).