Newton's First Law of Motion, or the Law of Inertia, states that objects at rest will stay at rest, or object in motion will stay in motion, unless acted upon by an outside force. Therefor if a person is riding in a car going 80mph, and the car crashes head-on, the car will stop, but the person will keep moving at 80mph until it too is acted upon by an outside force, in this case the air bag. If there was no air bag, the person would collide with the car's dashboard or windshield.
