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Why safety
belts?
Frontal collisions and the law of physics
Frontal crashes create very strong forces for
people riding in vehicles .
Fig. 131
Unbe lted occupa nts in a vehicle hea ding for a
wall
Fig. 1 32
The vehicle cras hes into the wall
The physical principles are simp le. Both the
ve hicle and the passengers possess energy
which varies with vehicle speed and body
weight . Enginee rs call this energy "kinetic en -
e rgy."
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 . 131, they will keep
moving at the same speed the vehicle was
moving just before t he crash, unt il something
stops them - here, the wall
c:>
fig . 132 .
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
135
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 coll i-
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 33
A driver not wear ing a s afety belt is violentl y
thrown fo rwa rd
Fig. 1 34 A rear passeng er not wear ing a safety be lt
will fly forwar d a nd st rike the d river
Unbelted occupants a re not able to resist the
tremendous forces of impact by holdi ng 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. 133 . This impact
Ill-
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