19Seat belts
Why wear seat belts?
Frontal collisions and the laws of physics
Fig. 5 Vehicle about to
hit a wall: the vehicle oc-
cupants are not wearing
seat belts
Fig. 6 The vehicle hits
the wall: the vehicle oc-
cupants 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. 5, this is a certain amount of
energy known as kinetic energy both in the vehicle and in the vehicle occu-
pants.
The higher the speed and the greater the weight of the vehicle, the more
energy there is to be absorbed 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 (15 to 30 mph), for example, the
kinetic energy is multiplied by four.
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 of the vehicle and the vehicle occupants, the more ener-
gy there is to be absorbed in an accident.
Vehicle occupants not wearing seat belts are not “attached” to the vehicle.
As a result, in a frontal collision they will continue to move forward at the
speed their vehicle was travelling just before the impact until something
stops them! Because the vehicle occupants in our example are not restrain-
ed by seat belts, all of the vehicle occupants' kinetic energy has to be ab-
sorbed at the point of impact ⇒ Fig. 6.
At speeds of 30 km/h to 50 km/h (20 to 30 mph), the forces acting on bod-
ies in a collision can easily exceed one tonne (1000 kg). At greater speed
these forces are even higher.
This example applies not only to head-on collisions, but to all accidents and
collisions.
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