Airbags
Last updated: May 27, 2009.
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: An airbag developed by scientists
at Sandia National Laboratories. Photo by courtesy of US
Department of Energy.
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 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
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.
