Transcript: Newton’s first law of motion says that an object will stay at rest or in uniform motion unless an unbalanced force acts on it. Newton’s first law is a major departure from Greek physics which stated that rest was the natural state of an object. Newton realized that when an object subject to friction, such as a light thing falling through the Earth’s atmosphere or an object rolling on the surface, when such an object slowed down it was because it was subject to a force that decelerated it. Newton realized that in an ideal situation, no air resistance or no friction, an object in uniform motion would continue its uniform motion. Conversely, when an object changed its uniform motion, either a change of speed or a change of direction, this is because a force is acting. So when an object is in orbit, even though the circular velocity is constant the direction of motion is constantly changing. An object in an orbit is subject to the force of gravity. Similarly, other forces, electric or magnetic, could cause changes to uniform motion. In this way Newton produced an idea that unified large areas of knowledge.