Video narration
Why does your body lurch forward when a car suddenly stops? The car stops, but your body keeps moving in the same direction. You were moving with the car; stopping it does not instantly stop you. Newton's first law says an object at rest stays at rest, while one moving steadily keeps the same speed and direction, unless a net external force changes it. Stopping is not automatic. Motion changes only when a net external force acts. That resistance to change is called inertia; it is not a force. When the car brakes, it stops first, while your body continues forward. The seat belt supplies the external force that changes your motion and stops you during that sudden braking. So why do you lurch forward? Because of Newton's first law: your body keeps moving until the seat belt provides a stopping force.