Newton's First Law of Motion, also known as the Law of Inertia, states 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. Galileo first proposed that a force is not needed to keep an object in motion and only friction or other forces can stop its motion. Isaac Newton later formalized this as his first law, establishing that an object's resistance to changes in its motion is defined as its inertia. The inertia of an object, or its mass, determines how much force is needed to change its motion.