According to the National Highway Traffic and Safety Administration in the United States, a properly worn seat belt reduces the risk of death or serious injury for a front seat car occupant by about 50 per cent. Isn’t that an amazing feat for such a small strip of fabric?

What a seat belt does

A three-point seat belt consists of a lap belt, which rests over the pelvis, and a shoulder belt, which extends across the chest. The shoulder belt should be worn over the shoulder and across the chest, never under the arm. The lap belt should rest low across the hips.

The basic job of a seat belt is to stop you from flying through the windshield or hurtling toward the dashboard when your vehicle comes to an abrupt stop. In a frontal crash, the seat belt limits the forward movement of the body to avoid contact with the hard interior of the car and reduces the forces directed to the body.

The physics of a car crash

Before we can understand how a seat belt functions, let’s go over some physics principles that may be observed during a car crash.

In a moving car, the speeds at which the passengers and the vehicle are travelling forwards are almost the same. That’s why it feels like you and the car are moving as a single unit. But in fact, the car and the people inside it each have their own inertia.

Inertia is an object’s tendency to keep moving until something stops this motion. In other words, inertia is an object’s resistance to a change in direction and speed.

If your car were to crash into a stationary object, the force from the collision would bring the car to an abrupt stop, but your speed and that of the other vehicle occupants would remain the same as before the crash.

A properly fastened safety belt distributes the forces of rapid deceleration over larger and stronger parts of a person’s body, such as the chest, hips and shoulders.

Without a seat belt, the passengers remain in motion until the force from a sudden impact hits you to slow you down. That force could come from the dashboard, windshield or road. The seat belt stops you with the car so that your stopping distance is extended by four to five times. It greatly increases your chances of surviving a car crash.

Preventing the human collision

Car seat belts have the ability to extend and retract. You can lean forward easily while the belt stays fairly taut. But in a collision, the belt will suddenly tighten up and hold you in place.

Seat belts in newer vehicles are often equipped with pre-tensioners. During a crash, a pre-tensioner will tighten the belt almost instantaneously. This reduces the motion of the occupant in a violent crash.

Seat belts also prevent another form of human collision, that is, person-to-person. Passengers in the front are often hit by unbelted rear-seat passengers who become high-speed projectiles during a crash.