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Why use safety belts?
Frontal collisions and the law of physics
Frontal crashes create very strong forces for peo-
ple riding in vehicles.
Fig. 141 Unbelted occupants in a vehicle heading for a wall
Fig. 14 2 The vehicle crashes into the wall
The physical principles are simple. Both the vehi-
cle and the passengers possess energy which var-
ies with vehicle speed and body weight. Engi-
neers call this energy "kinetic energy."
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 significant facto r.
If
the speed doubles from 15 to 30 mph (25 to 50
km/h), the energy increases 4 times!
Because the passengers of t his veh icle are not us-
ing safety belts
r::!.> fig. 141, they will keep moving
at the same speed the vehicle was moving j ust
before the crash, unt il something stops them -
here, the wall
r::!.> fig. 142.
The same pr inciples apply to people sitting in a
vehicle that is involved in a frontal collision. Even
at city speeds of 20 to 30 mph (30 to SO km/h),
the forces acting on the body can reach one ton
Safet y bel t s
(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 a lso not
attached to the ir vehicle. In a fron t al coll ision
they will also keep moving forward at the speed
their vehicle was travelling just before the crash.
Of course, the laws of physics don't just apply to
frontal collisions, they determine what happens
in all kinds of accidents and collisions.
What happens to occupants not wearing
safety belts?
In crashes unbelted occupants cannot stop
themselves from flying forward and being in-
jured or killed. Always wear your safety belts!
Fig. 14 3 A driver not wearing a safety belt is violently
thrown forward
Fig. 144 A rear passenger not wearing a safety belt will f ly
forwa rd and strike the driver
Unbelted occupants are not able to resist the tre-
mendous forces of impact by holding tight or
bracing themselves. W ithout the benefit of safe-
ty restraint systems, the unrestrained occupant
will slam vio lently into t he stee ring wheel, in-
strument pane l, windshield, or whatever e lse is
in the way
r::!.>fig. 143. This impact with the vehi-
cle inter ior has a ll the energy they had just be-
fore the crash.
IJI,,-
1 3 5
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