In the last section, we saw that any time a car comes to a sudden stop, a passenger comes to a sudden stop as well. A seatbelt's job is to spread the stopping force across sturdier parts of your body in order to minimize damage.
A typical seatbelt consists of a lap belt, which rests over your pelvis, and a shoulder belt, which extends across your chest. The two belt sections are tightly secured to the frame of the car in order to hold passengers in their seats.
When the belt is worn correctly, it will apply most of the stopping force to the rib cage and the pelvis, which are relatively sturdy parts of the body. Since the belts extend across a wide section of your body, the force isn't concentrated in a small area, so it can't do as much damage. Additionally, the seatbelt webbing is made of more flexible material than the dashboard or windshield. It stretches a little bit, which means the stop isn't quite so abrupt. The seatbelt shouldn't give more than a little, however, or you might bang into the steering wheel or side window. Safe seatbelts will only let you shift forward slightly.
A car's crumple zones do the real work of softening the blow. Crumple zones are areas in the front and rear of a car that collapse relatively easily. Instead of the entire car coming to an abrupt stop when it hits an obstacle, it absorbs some of the impact force by flattening, like an empty soda can. The car's cabin is much sturdier, so it does not crumple around the passengers. It continues moving briefly, crushing the front of the car against the obstacle. Of course, crumple zones will only protect you if you move with the cab of the car -- that is, if you are secured to the seat by your seatbelt.
The simplest sort of seatbelt, found in some roller coasters, consists of a length of webbing bolted to the body of the vehicle. These belts hold you tightly against the seat at all times, which is very safe but not particularly comfortable.
Car seatbelts 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. In the next section, we'll look at the machinery that makes all this possible.