Seat belts
Head-on collisions and the laws of
physics Fig. 8
A driver not wearing a seat belt is thrown
f orw
ard violently. Fig. 9
The unbelted passenger in the rear seat
is thr o
wn forward violently, hitting the driver
who is wearing a seat belt. The effects of the laws of physics in the case
of a head-on collision ar
e easy t
o explain: the
moment a vehicle starts moving, a type of en- ergy called “kinetic energy” starts acting on
both the vehicl
e and its passengers.
The amount of “kinetic energy” depends on
the speed of the vehicle and on the weight of
the vehicle and of its passengers. The higher
they are, the more energy there is to be “ab-
sorbed” in the event of 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 corresponding kinetic ener-
gy is multiplied by four.
Given that the passengers of the vehicle in
our example do not have their seat belts fas-
tened, in the event of a collision the entire
amount of the passengers' kinetic energy will
be only absorbed by the mentioned impact.
Even at speeds of 30 km/h (19 mph) to
50 km/h (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.
Vehicle occupants 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 was travelling just before
the impact. This example applies not only to
head-on collisions, but to all accidents and
collisions.
Even at low speeds the forces acting on the
body in a collision are so great that it is not possible to brace oneself with one's hands. In
a front
al collision, unbelted passengers are
thrown forward and will make violent contact
with the steering wheel, dash panel, wind-
screen or whatever else is in the way
››› Fig. 8.
It is also important for rear passengers to
wear seat belts properly, as they could other-
wise be thrown forward violently through the
vehicle interior in an accident. Passengers in
the rear seats who do not use seat belts en-
danger not only themselves but also the front
occupants ››› Fig. 9
.
19
