A seat belt is a device used in motor vehicles, boats and aeroplanes designed to reduce the chances of injury and death in the event of a crash. A seat belt usually consists of one or more flexible but sturdy cloth straps that bind a passenger to his seat. The most common type of seat belt in cars are three-point belts which consist of a lap belt around the waist, and a shoulder strap which extends up from one side of the lap belt around the shoulder. When the car crashes, a locking mechanism in the seat belt is triggered and prevents it from extending, while holding the passenger firmly in place so that they do not slam into the steering wheel or wind shield. One common type of locking mechanism is a weighted pendulum which swings forward and obstructs a spool that allows the seat belt to extend whenever the car stops abruptly. Another is a locking mechanism built into the belt spool itself, which uses the centrifugal force of the spool when spun quickly, as in a crash, to move a lever that causes the spool to become obstructed and lock.
Seat belt Physics
The physical components of a seat belt are designed both to keep a passenger from flying out of their seat and to absorb a person's inertia during a car crash while extending the time of impact. When a person drives a car, each passenger's inertia--or tendency to stay at the same speed-- is separate from the car itself. When the car is involved in an accident, it suddenly slows down, but the person diving maintains their inertia. So if a car that was travelling 60 miles per hour suddenly stops, the driver's inertia will cause them continue travelling into the steering wheel or windshield at 60 miles per hour. A seat belt holds the driver back and allows them to expel the kinetic energy of their movement into the soft fibres of the belt rather than the rigid objects in front of the drive. Since seat belts are made of cloth and are somewhat flexible, they increase the time the body takes to slow down. By slowing down more gradually the force put on the body is many times less then it would be if it stopped almost instantly is it would if slamming into a rigid object. Also, the lap belt and shoulder strap design balances pressure across the more sturdy parts of the body. Without a seat belt, the head is likely to strike part of the car focusing a huge amount of impact force into a small vital area.
Seat Belts and Airbags
While seat belts are an extremely important safety feature of many vehicles, their effectiveness can be increases when used in concert with airbags. A three point seat belt is great at stopping a person from flying into the wind shield, but it has a tendency to cause the person's head to jerk forward suddenly around the tension of the shoulder strap, which often results in neck and spine injury. When an airbag deploys, it creates a cushion which helps stop the inertia of the head while the seat belt restrains the body. Used together the neck and spine are much less likely to be seriously injured, and the shoulder strap will cause less stress and tearing on the collar bone and shoulder.