Auto Safety Features
How does a car make use of its safety features? Features
including the seat belt, air bags, the crumple zones and any other major
Auto safety devices -- seat belts, air bags, crumple zones, four wheel
drive, power steering and brakes, collapsible steering columns -- are
engineered to provide the following features: CONTAINMENT (keep the
occupant in place within the car. This is especially true in rollover
accidents where a major cause of death and injury is a result of being
crushed by the vehicle itself or being thrown against the ground at high
speeds.). It has been amply demonstrated that the occupant is much safer
remaining inside the auto. RESTRAINT (seat belts, air bags collapsible
steering columns) Keep the occupant from sudden impact with the frame of
the car. CONTROL (power steering and brakes, four wheel drive) Maximize the
ability of the driver to control the speed and direction of the auto before
(thus avoiding the collision), during (reducing the change in momentum at
impact), and control after impact to avoid secondary collisions if
possible. It should be pointed out that the kinetic energy of the vehicle
increases as the square of the speed, and all these devices above can be
overwhelmed by excessive speed.
All the safety features you mentioned function by reducing the acceleration
of the occupants of a car during an accident. If your head hits the
windshield or some rigid part of the car while continuing at, say, 50 mph
when the car has already stopped (perhaps it hit a brick wall), your head
will have to stop in a fraction of an inch if your head is not to be
fractured. However, the force needed to stop your head in 1 cm when
travelling at 50 mph (22 m/s) is about 120,000 Newtons or 25,000 lb. Such a
large force is guaranteed to break your head. I calculated this number
using F = ma and a = v^2/2x with m = 5 kg (11 lb), v = 22 m/s, x = 0.01 m (1
cm) giving a = 24,000 m/s^2.
With a crumple zone and an air bag, your head may travel as much as 1 m
before stopping which reduces the acceleration and therefore the force
needed to stop your head by a factor of 100. Your head can probably
withstand a force of 250 lb spread uniformly over the front of your head as
the air bag does.
Note that the crumple zone causes the car to stop in a foot or two when it
strikes a rigid body, which reduces the forces by a factor of 10 or so over
stopping the car in an inch or two.
The human body can withstand enormous forces if they are spread over a large
area of the body. Point forces, on the other hand are very destructive. I
remember the picture of a little girl with a very rigid cigarette lighter
handle sticking out of her head. She was killed although the accident was
quite minor because the cigarette light handle was rigid and the girl did
not have a seat belt of air bag to protect her. Thank goodness, engineers
have finally learned to worry about what sharp objects in the car can do to
occupants during an accident.
Best, Dick Plano,
Professor of Physics emeritus, Rutgers University
In their simplest forms, those are quite passive safety features. Seat belts
restrain passengers with minimum risk of injury inside the cabin of the
vehicle. A crumple zone is designed to not be as strong as the cabin, thus
allowing the cabin to remain intact (with it is passengers) while the front
or back of the vehicle is crushed instead. (like a pillow giving way below
a dropped book). Air bags are more active. When a sensor in the bumper
determines the car is in a collision, the airbag inflates like a balloon,
and immediately begins to deflate. (So it is more like a soft balloon than a
From what I see, the design of the final product is a result of design,
testing, and evolution.
My guess, initially , cars were based on wagons. In fact the station wagons,
if memory serves, had wood components, well into the 40's (but the war had
something to do with that). I do know Ford built the cars to last, as strong
as possible. Whether they were safe was another issue. I cannot pass this
comment up, Henry Ford paid his workers well, so they could afford the cars.
Now, we have Wal Mart, paying low wages, in effect, creating a market for
their low priced goods, the reverse.
Back to the cars, Ford did shift to profit, I know they went to junk yards to
see which parts were still good, so they could make them a little, less good
(cheaper). Why waste, but that was the start of obsolescence.
The car evolved, now the computer simulations and testing combine to produce
safety elements. The design evolving, based on past models, no different
Not to be cynical, but the crushing may be a safety failure to protect the
passengers, but it also destroys the product, so you buy new.
A little more on design, computer simulation can cover almost all situations,
so an element is designed, prototypes constructed. Then, they are
instrumented, the car and dummy passengers physically tested.
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Update: June 2012