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Bang! We think of explosions as terrible, dangerous things—but that's not always the case.
Every day, explosions are helping to save people's lives. If you're unlucky enough to be involved in a car accident, a carefully controlled explosion will (hopefully) fire an airbag out from the dashboard, cushioning the impact and helping to reduce the damage to your body.
Airbags are very simple but also amazingly clever, because they have to open up at over 300 km/h (200mph)—faster than a car can crash! Let's take a closer look at how they work.
The trouble with momentum
Like everything else in the world, car crashes are controlled by the laws of physics—and, more specifically, the laws of motion.
Anything that moves has mass (very loosely speaking, this means how much "stuff" an object contains and it's closely related to how heavy it feels) and velocity (loosely, this is the same thing as speed, but strictly it means speed in a certain direction).
Anything that has mass and velocity has kinetic energy, and the heavier your car and the faster you're going, the more kinetic energy it has. That's fine until you suddenly want to stop—or until you crash into something. Then all the energy has to go somewhere.
Even though cars are designed to crumple up and absorb impacts, their energy still poses a major risk to the driver and passengers.
How airbags help
An airbag is more correctly known as a supplementary restraint system (SRS) or supplementary inflatable restraint (SIR). The word "supplementary" here means that the airbag is designed to help the seatbelts protect you rather than replace them (relying on an airbag to protect you without fastening your seatbelt is extremely dangerous).
The basic idea is that the airbag inflates as soon as the car starts to slow down in an accident and deflates as your head presses against it.
That's important: if the bag didn't deflate, your head would just bounce back off it and you'd be no better off.
How effective are airbags?
Several studies found that airbags reduced fatalities by 23–24 percent in head-on crashes and by 16 percent in crashes of all kinds, compared to cars fitted only with manual safety belts.
That's obviously a huge improvement, but it's important to note that airbags are violently explosive things that present dangers of their own.
The biggest risk is to young children, though adults also face a small risk of eye injury and hearing loss.
If an airbag saves your life, you probably consider a slight risk of injury a price well worth paying. Even so, it's clearly important to study the potential dangers of airbags so we can make them as safe and effective as possible.
Modern airbags (installed since the late 1990s) fire with less force than older designs, and there's compelling evidence that this has reduced accidental deaths, especially among children, without compromising passenger safety.
How airbags work
When a car hits something, it starts to decelerate (lose speed) very rapidly.
An accelerometer (electronic chip that measures acceleration or force) detects the change of speed.
If the deceleration is great enough, the accelerometer triggers the airbag circuit. Normal braking doesn't generate enough force to do this.
The heating element ignites a chemical explosive. Older airbags used sodium azide as their explosive; newer ones use different chemicals.
As the explosive burns, it generates a massive amount of harmless gas (typically either nitrogen or argon) that floods into a nylon bag packed behind the steering wheel.
As the bag expands, it blows the plastic cover off the steering wheel and inflates in front of the driver. The bag is coated with a chalky substance such as talcum powder to help it unwrap smoothly.
The driver (moving forward because of the impact) pushes against the bag. This makes the bag deflate as the gas it contains escapes through small holes around its edges. By the time the car stops, the bag should have completely deflated.