by Chris Woodford. Last updated: July 24, 2014.
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.
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.
Photo: Sir Isaac Newton (1642–1727) formulated
three basic laws
describing how forces work.
Picture courtesy of US Library of Congress.
How airbags help
An airbag is more correctly known
as a supplementary restraint system (SRS) or supplementary
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
deflate, your head would just bounce back off it and you'd be no better
Who invented airbags?
If you search around online, you'll find quite a few different people are credited with inventing airbags.
Who thought of them first? It appears to have been John W. Hetrick of Newport, Pennsylvania, who
came up with the idea after an accident in which he swerved his car off the road into a ditch to avoid hitting a rock,
almost throwing his daughter through the windshield. Hetrick filed his patent for a Safety cushion assembly for automotive vehicles on August 5, 1952 (it was granted as US Patent #2,649,311 on August 18, 1953). Although a German inventor named Walter Linderer filed an airbag patent several months before Hetrick, it was granted
after Hetrick's, and it seems likely that the two men came up with the same idea independently. Many other inventors have built on the idea since then, notably Allen K. Breed (1927–2000), who developed a variety of different ways of triggering the explosion of gas inside an airbag just before the impact of a crash. According to
Breed's New York Times obituary, he made his first airbag design in 1968, and filed numerous patents for improvements, helping to turn Breed Corporation into one of the world's largest suppliers of car safety systems.
Artwork: John Hetrick's original airbag design from 1953, which I've colored to make it easier to follow. There are three separate drawings here, showing the main mechanism (occupying most of the picture), a driver's perspective view of the steering wheel (bottom right), and a view of the inflated bag from the side (top right). The bag is triggered by a heavy weight (blue) restrained by a spring (yellow) inside the red cylinder on the right. After an impact, the weight pushes the spring to the right, opening a valve inside a pipe (turquoise) that allows compressed air to flow out from a cylinder (green) and inflate the airbag cushion. Artwork courtesy of US Patent and Trademark Office. Read a full description in Safety cushion assembly for automotive vehicles (via Google Patents).
Find out more
On this site
For older readers
- Crash by Nicholas Faith. Channel 4 TV/Boxtree Books, 1998. Why do we resist improvements in car safety so strenuously?
- Mobility Without Mayhem: Safety, Cars, and Citizenship by Jeremy Packer. Duke University Press, 2008. How do we square ideas like safety and caution with a car culture that thrives on freedom and speed? Packer explores contradictions like this in a fascinating historical account.
For younger readers
- Car Science by Richard Hammond. Dorling Kindersley, 2007. A cunning book about science heavily disguised as a book about cars! A great way to learn about the science that makes cars work. If you're interested, I worked as a consultant and contributor on this book. Ages 9–12.
- Eyewitness Car by Richard Sutton. Dorling Kindersley, 2005. One of the many excellent DK Eyewitness books, this one teaches you about the history and technology of cars from early days to the present. Again, most suitable for ages 9–12 though older readers (and even adults) will probably enjoy it too.
- Airbag Deployment in Slow Motion: A side-on view of what happens when an airbag inflates, by Biodynamics Engineering, Inc. Note how the bag automatically deflates at the end, albeit more slowly than it would with a person pressing against it.
- Ford Next-Generation Airbag Crash Tests: Slow-motion crash-test dummy footage showing how airbags inflate at incredibly high speed during a crash.
If you liked this article...
You might like my new book, Atoms Under the Floorboards: The Surprising Science Hidden in Your Home, published worldwide by Bloomsbury.