Why safety belts?
Frontal collisions and the law of physics
Frontal crashes create very strong forces for
people riding in vehicles .
Fig. 16 5 Unbelted occupa nts in a vehicle heading for a
wall
Fig. 166 The vehicle crashes into the wall
The physical principles are simp le. Both the
vehicle and the passengers possess energy
which varies with vehicle speed and body
weight . Enginee rs call this energy "kinetic en -
ergy."
The higher the speed of the vehicle and the
greater the vehicle 's weight, the more energy
that has to be "absorbed" in the crash .
Vehicle speed is the most sign ificant factor . If
the speed doubles from 15 to 30 mph (25 to
50 km/h) , the energy increases 4 t imes !
Because t he passengers of t his ve hicle are not
using safety belts
r::;, fig . 165, they will keep
moving at the same speed the vehicle was
moving just before the crash, unt il something
stops them - here, the wall
c:> fig . 166 .
The same pr inciples apply to peop le sitting in
a vehicle that is involved in a frontal collision .
Even at city speeds of 20 to 30 mph (30 to 50
km/h), the forces act ing on the body can
Safety belts 153
reach one ton (2,000 lbs . or 1,000 kg) or
more . At greater speeds, these forces are
even
higher .
People who do not use safety belts are also
not attached to their vehicle. In a frontal colli-
sio n they will also keep moving forward at the
speed their veh icle was t ravell ing ju st before
the crash. Of course, the laws of physics don't
just app ly to frontal collisions, they determine
what happens in all kinds of accidents and col-
lisions .
What happens to occupants not wearing
safety belts?
In crashes unbelt ed occupants cannot stop
themselves from flying forward and being in-
jured or killed. Always wear your safety belts!
Fig. 1 67 A driver not wear ing a safety belt is violentl y
thrown forwa rd
Fig. 1 68 A rear passeng er not wear ing a safety be lt
will fly forwar d and st rike the d river
Unbelted occupants are not able to resist the
tremendous forces of impact by hold ing tight
or bracing themselves . Without the benefit of
safe ty restra int systems, the unrestrained oc-
cupan t will slam violen t ly into the st eer ing
whee l, instr ument pane l, windshield, or what-
ever else is in the way ¢ fig. 167 . This impact Ill-
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