Newton's three laws of motion are:
- Every body continues in a state of uniform motion in a straight line unless acted upon by some external force.
- The time rate of change of momentum (mass x velocity) is proportional to the impressed force. In the usual case where the mass does not change, this law can be expressed in the familiar form: force = mass x acceleration or F = ma.
- To every force or action, there is always an equal and opposite reaction.
Kepler's three laws of planetary motion, which had been derived empirically by Johannes Kepler, were obtained with mathematical rigor as a consequence of Newton's law of universal gravitation in conjunction with his three laws of motion.