Normally, something blowing up in your face is bad.

But right about now, this chemical reaction could very well save your life. It takes 50 milliseconds for you to hit the steering wheel in a crash. Everything we’re about to tell you goes down even faster. Your car must be able to detect when a potentially deadly crash is happening. You don’t want your airbag deploying every time you hit a pothole or get in a fender-bender. Your car has an accelerometer — several, actually, in case one fails or goes off by mistake — that detects sudden, rapid changes in velocity.

In one model, a ball bearing is normally held in place by a magnet, but in a crash, it comes flying free and it flies right into an electrical circuit that tells the car’s computer to deploy the airbags. Airbags don’t just fill with gas like a balloon blowing up. That’d be way too slow. The gas that fills airbags is produced by a chemical reaction. The classic example is sodium azide, which is stable until it’s heated. In a crash, a small explosive or heating element called a squib goes off and ignites the sodium azide which decomposes quickly to nitrogen gas and sodium metal That sodium metal could react with water in the air to produce corrosive sodium hydroxide, so airbag manufacturers include some other compounds to react it into sodium oxide and alkaline silicate instead.

By the way, this technology we described is a little outdated. Nowadays, most frontal airbags are inflated with guanidine nitrate, but it works just the same as sodium azide: you’re creating a lot of Nitrogen gas. And accelerometers have changed with the times, too. They’re a lot more sophisticated than the ball bearing thing. Meanwhile, the nitrogen gas is totally harmless, being the main ingredient in, y’know, air. Just a small handful of sodium azide can produce 67 liters of nitrogen. And that volume of gas creates enough pressure to fill the airbag in less than 40 milliseconds.

Quick 1st Law refresher: an object in motion stays in motion. So: When your car hits something, it stops, but your body keeps moving forward. And if it keeps moving forward right into the steering wheel or dashboard, that’s bad news for you. So, the airbag is there to slow you down more gradually than a steering wheel would.

But this rapidly inflating cushion isn’t pillow soft. The front face of the airbag moves toward you at between 150 and 250 mph. And if you were to hit it while it was still inflating, that might be almost as bad as hitting the steering wheel. To properly slow you down, an airbag must have blown up already when you hit it. So that means the accelerometer detects the crash, the squib ignites the sodium azide, the nitrogen gas is produced — and in 50 milliseconds, it’s already blown up in your face and started to wind down.’ Between 1987 and 2015, frontal airbags saved 44,869 lives.

That’s a summer evening at Dodger Stadium’s worth of people, and a lot of people around to cheer thanks to one chemical reaction and some ingenious physics.

Keywords: steering wheel in a crash, airbag deploying, crash, fender-bender, accident lawyer

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