21Seat 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 to 50 km/h, for example, the kinetic energy is
multiplied by four.
Because the passengers in our example are not restrained by seat belts, in
the case of a head-on collision all of their kinetic energy has to be absorbed
at the point of impact ⇒ fig. 7.
Even at speeds of 30 km/h to 50 km/h, the forces acting on bodies in a col-
lision can easily exceed one tonne (1000 kg). At greater speed these forces
are even higher.
Passengers not wearing seat belts are not “attached” to the vehicle. In a
head-on collision, they will move forward at the same speed their vehicle
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