The second of Newton's three laws of motion tells us that applying a force on an object produces an acceleration proportional to the object's mass. When you're wearing your seat belt, it supplies the force to decelerate you in the event of a crash so that you don't hit the windshield.
Why Cars Have Seat Belts
When your car accelerates, the car seat supplies the force required to accelerate you along with it. The heavier you are and the faster the car accelerates, the stronger this force needs to be. When the car stops, you keep right on going until something supplies a force in the opposite direction to stop you. Your legs can supply this force if the cars slows down gradually, but if the car hits an obstacle, the deceleration and force are too much for your legs or arms to handle.
The Force of a Collision
The force required to stop a 68-kilogram (150-pound) person traveling at 26.8 meters per second (60 miles per hour) in 5 seconds is 364 newtons (1,800 pounds). If the car hits an obstacle and stops suddenly, that force goes up to 1,822 newtons (9,000 pounds). In the absence of seat belts, the force is supplied by the windshield or steering wheel, and the impact is more than enough to kill the person.
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Added Safety Precautions
A seat belt should include a shoulder harness to prevent the upper part of the body from continuing forward when the car stops. Injuries occur even in cars with this feature, however, because the the head can arch backwards when the body recoils from the force that stopped its forward motion. For this reason, contemporary autos have air bags to absorb the forward motion and disperse the stopping force over a wider area.