The document discusses the concept of inertia. It explains that an object at rest stays at rest and an object in motion stays in motion with the same speed and direction unless acted on by an unbalanced force. It provides an example of how inertia causes an unbelted crash dummy to continue moving forward at the same speed as the car during a collision and be thrown through the windshield, while the car is stopped by the force of impact. The best place to observe inertia is in outer space, where there is no friction or air resistance to slow moving objects.