Nib asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
how do airbags inflate so fast
An airbag is not blown up from a tank of air. In most, a solid chemical makes the gas on the spot, so the bag is full in about thirty milliseconds.
Thirty milliseconds is three hundredths of a second. You are thrown forward and reach the bag about fifty milliseconds after the crash, so it must be ready.
Here is the chain. Crash sensors feel a sudden slowdown and send an electric signal. That fires a tiny igniter, whose heat sets off the solid chemical.
In older bags that chemical is sodium azide, which falls apart into sodium and nitrogen gas. About one hundred thirty grams makes sixty-seven litres, enough for a bag.
The bag has to be full before you arrive. But a hard, full balloon would hit you almost like the wheel, so the bag is built to give.
Small vent holes let the gas out as you press in. You sink into the bag and slow down gently, and it is already going flat as you do.
So the whole thing: a sensor feels the slowdown, an igniter lights a solid chemical, nitrogen fills the bag in thirty milliseconds, and vents let you sink in.
Next time you buckle up, remember: a little chemistry is built to get there before you do.