Because Newton’s first law of motion expresses that an object in motion will remain in motion at a constant speed in a straight line until acted upon by an outside force, when a car crashes, inertia will keep everything inside the car in motion at the speed that the car was travelling when a car crashes. While the momentum of the car stops immediately, the momentum of everything in the car (people included), carries the contents of the car forward even after the impact of the crash. Because of the close proximity of the driver and passenger to the dashboard/steering wheel and the great impulse (change in momentum) after the impact of the car crash, the job of the airbag is to prevent injury in a short amount of time. This means the airbag must counter the momentum of the people in the car to slow them before they are hurt by the impact. This is why the airbag inflates so quickly. It must absorb the momentum and slow the person down rather than allowing the person to hit the solid dashboard and risk likely injury.
