Miscellaneous Car Science


Crash Science

Air Bags


  1. Why do you drive on a parkway and park in a driveway?
  2. Why are turnpikes straight?
  3. Why is there an interstate highway in Hawaii?
  4. Where does all the rubber go that wears off tires?
  5. When the chicken in a closed poultry truck take off is the truck lighter?


#4 The rubber wears off in a thin layer as the car drives and in a thicker layer in skid marks. As the car drives it leaves behind a layer of rubber averaging one atom thick.

If 1 cm of tread wears off in 100,000 km of driving then the wear is

10-2 m/108 m = 10-10 meters per meter and the circumference of a car tire is about 2 m.

Now 10-10 meter is the diameter of an atom. So the tire leaves an atom diameter thickness every turn or so of the wheel. Like a rubber snail-slime trail.

#5 No. To fly, the chickens push air down with a force equal to their weight, this air hits the bottom of the truck with the same force as the weight of the flying chickens.

In an open truck some of the air misses the bottom of the truck and it does get lighter.


A 60 mph crash is like a 140 foot fall. Math.
A 100 km/hr crash is like a 50 meter fall.

Without a seat belt: The car stops quickly, perhaps in 3 feet, 1m,
at 60 mph, 100 km/hr, this takes 1/15 of a second.
A blink of an eye.

Meanwhile, your body keeps going at 60 mph, 100 km/hr. What's to stop you?

You then collide with the dash board, or the windshield.

The impact is like jumping off a 12 story, (140 foot, or 50m, high) building, face first onto an oddly shaped piece of steel, the dashboard, and glass, the windshield. This is not good for your body. Math.

Some people think that they can brace" themselves against a car crash,
no one thinks they can brace themselves against a 140 foot, 50 m, fall.

The math

The acceleration needed to stop in s=1 meter from v = 100 km/hr, or 28 m/s is:

v2 = 2as

a = v2/2s = 282/2 = 392 m/s2 = 39 g

Wow! 39 g's

The time it takes to stop at 39 g is

v = at

t = v/a = 28/392 = 0.07 s = 1/15 s

How far and for how long do you have to fall to reach 100 km/hr? (60 mi/hr?)

v = at so t = v/a t = (28m/s) / (9.8 m/s2) = 3 s

d = 1/2 at2 = 0.5 * 9.8 *32 = 44 m = 143 ft.

Head-on crash

A crash into a cement barrier at 60 mph, 100 km/hr, is the same as a head-on crash into a similar car traveling toward you with the same speed. You move at 60 mph, it moves at 60 mph your relative speed is 120 mph, yet there are 2 cars to absorb the damage. Whereas the barrier is rather unyielding and doesn't absorb much damage. So all the energy goes into your car alone. In both cases you come to a stop in the distance your car crumples.


Air bags

If you fell off a 12 story building it be nice if you could hit a thick air-filled mattress? One that collapsed with a whoosh and brought you to a stop?

In fact people can survive stops from 200 km/hr, 120 mph if their stop is done in a 1 meter, 3 foot, distance and if the force is spread uniformly over their bodies. Terminal velocity for a human body in the parachutist's free fall position, face down,with body arched, is 200 km/hr or 120 mph. People have survived 10,000 foot, 3000 m, falls out of airplanes into powder snow. They are found, intact, at the bottom of a 1 meter deep crater.

This is the idea behind the automotive airbag.

In the blink of an eye 1/30 of a second after the car hits the wall you hit the inside of the car. math. The idea is to sense the collision and inflate an airbag in under 1/100 of a second. You only travel a few inches before the airbag is fully inflated. As you hit the airbag it spreads out the force over the whole surface of your body and collapses to bring you to a stop with less damage than hitting steel and glass. Airbags have saved many lives.

Here is the timing behind an airbag:

A millisecond, ms, is a thousandth of a second.

0 ms A collision happens.

20 ms sensors in the bumper detect a collision and process data.

25 ms airbag begins to inflate.

45 ms fully inflated (it expands at 200 mph!)

60 ms passenger collides with airbag and airbag begins to deflate through holes.

100 ms passenger continues to sink into airbag.

In the blink of an eye the airbag has deflated and the passenger has hit a cushion of air not a steel and glass car interior.

The blink of an eye is 1/3 of a second, 300 ms.

the math

How long does it take to travel 1 meter to the dashboard at 100 km/hr?(3 feet at 60 mph?) Note that 100 km/hr = 28 m/s.

d = vt or t = d/v = 1/28 = about 1/30 of a second.


(However, to inflate an airbag quickly enough it is inflated by a gas generator that is slightly slower than an explosive. The front of the bag moves toward you at 200 mph. (300 km/hr) If you are a kid sitting on the front edge of your seat or in a rear facing child seat, the airbag will hit you while it is still inflating. This could cause injury or death. The system is designed so that the passenger will hit the airbag after it has fully inflated and come to rest.

Seat Belts

The seat belt stops you. It is like jumping and being caught by a wide nylon strap after a 140 foot fall. Better than hitting steel and glass, but not as good as an airbag.

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Scientific Explorations with Paul Doherty

© 1999

19 Nov 99