Newton's Laws describe the relationship between an object's mass, its motion (including direction and speed), and the applied forces. Sir Isaac Newton published his work on the laws of motion and universal gravitation in his 1687 work Philosophiae Naturalis Principia Mathematica. The three laws are: An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force; Force equals mass times acceleration; and For every action, there is an equal and opposite reaction.