A set of three postulates first set forth by Sir Isaac Newton in the middle of the seventeenth century. According to the first law (the law of inertia), a body (point mass) remains at rest or in a state of uniform motion unless acted on by a force, where the position of the body is specified relative to an inertial reference frame. The second law states that the time rate of change of (linear) momentum of a body is equal to the force on the body; Newton took momentum to be the product of mass and velocity, which is only an approximation valid at speeds much less than that of light. Finally, the third law states that if two bodies exert forces on each other, they are equal in magnitude and opposite in direction. Although apparently simple and unambiguous, Newton's laws have engendered considerable discussion over the extent to which some of them are mere definitions (e. G. , of inertial reference frame, mass, and force) or are true laws in the sense of being subject to experimental verification. Despite the ambiguity of Newton's laws, they have proven to be an efficient way of describing the physical world (within limits) and form the basis for the equations of motion of fluids.