20 Seat belts
Why wear seat belts?
Physical principles of frontal collisions
In the event of a frontal collision, a large amount of kinetic
energy must be absorbed.
Fig. 6 Vehicle about to
hit a wall: the occupants
are not wearing seat
belts
Fig. 7 The vehicle hits
the wall: the occupants
are not wearing seat
belts
It is easy to explain how the laws of physics work in the case of a head-on
collision: When a vehicle starts moving ⇒ Fig. 6, a certain amount of energy
known as kinetic energy is produced in the vehicle and its occupants.
The amount of kinetic energy depends on the speed of the vehicle and the
weight of the vehicle and its passengers. The higher the speed and the
greater the weight, the more energy there is to be released in an accident.
The most significant factor, however, is the speed of the vehicle. If the
speed doubles from 25 km/h (15 mph) to 50 km/h (30 mph), for example,
the kinetic energy is multiplied by four.
Because the vehicle occupants in our example are not restrained by seat
belts, all of the occupants' kinetic energy has to be absorbed at the point of
impact ⇒ Fig. 7.
Even at speeds of 30 km/h (20 mph) to 50 km/h (30 mph), the forces acting
on bodies in a collision can easily exceed one tonne (1000 kg). At greater
speed these forces are even higher.
