Isaac Newton, a 17th century scientist, put forth a variety of laws that explain why objects move, or don't move, as they do. These three laws have become known as Newton's three laws of motion. The first law, commonly referred to as the law of inertia, explains that 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. The state of motion of an object is maintained as long as the object is not acted upon by an unbalanced force, and all objects resist changes in their state of motion.