by Chris Woodford. Last updated: June 29, 2018.
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.
Photo: Airbags save lives thanks to the selfless dedication of crash-test dummies, which have been a feature of car design since the very first dummy, Sierra Sam, made his original test drive in 1949. If we couldn't test new safety innovations with dummies, we'd never be able to deploy them in our cars for real. A typical crash-test dummy has over 130 different sensors packed inside. This dummy, whose name is "Adam," is even designed to sweat like a real person so researchers can test the climate conditions inside a car! Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/ NREL).
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.
Chart: The faster you go, the harder it is to stop. That's because your kinetic energy increases with the square of your speed (your speed × your speed). The more kinetic energy you have, the more you need to lose before you come to a stop. If a collision brings your car to a halt in a certain time, the more energy you have, the more violent the collision, and the greater the chance you'll be injured or killed. Airbags help your body stop more slowly, reducing the risk of injury and death. [Chart based on a vehicle of total mass 1500kg.]
The trouble is, people inside a moving car have mass and velocity too and, even if the car stops, they'll tend to keep on going. It's a basic law of physics (known as Newton's first law of motion, after brilliant English physicist Sir Isaac Newton who first stated it) that things that are moving tend to keep on moving until something (a force of some kind) stops them. Cars have had seatbelts for decades, but they're a fairly crude form of protection. The biggest problem is that they restrain only your body. Your head weighs a surprising 3–6kg (6–12lb)—as much as several bags of sugar— and isn't restrained at all. So even if your body is fastened tight, the same basic law of physics says your head will keep on going and smash into the steering wheel or the glass windshield (windscreen). That's where airbags come in.
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?
Airbags sound like they must be a good idea, but scientists like hard evidence: is there any proof that they reduce fatalities? In 1995, Adrian Lund and Susan Ferguson published a major study of road traffic accidents over eight years from 1985 to 1993. They 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.