Protection from forces

It's the stopping that hurts

People say "speed kills", but speed by itself doesn't hurt anyone: in Sydney you are travelling at more than 1000 km/h right now as the Earth spins, and you feel nothing. What causes injuries is stopping too quickly.

To stop a moving body, a force must act on it. The shorter the distance (and time) the body has to stop in, the bigger that force must be. Stop in 1 metre instead of 10 centimetres, and the force is about 10 times smaller.

You already know this. When you catch a fast cricket ball, you let your hands move back with it ("soft hands"). When high jumpers land, they fall onto a thick, squashy mat, not the ground. Both stretch out the stop so the force is smaller.

Safety in cars

• Seat belts stop your body flying forward when the car stops suddenly. Without one, you keep moving at the car's speed until you hit the dashboard, the windscreen or another person, and stop in just a few centimetres. The belt also stretches slightly and spreads the force across the strong bones of your chest and hips. The three-point belt was designed by Nils Bohlin, an engineer at Volvo, in 1959. Volvo shared the design freely with other car makers, and it is estimated to have saved more than a million lives.
• Crumple zones at the front and back of a car are designed to crush in a crash. As the metal folds, the car takes longer to stop, so the people inside feel a smaller force.
• Rigid safety cell: the part around the people is made very strong so it doesn't collapse onto them.
• Airbags inflate in about 1/30 of a second, then let gas out through small holes as your head and chest sink into them, cushioning you over a longer distance.
• Head restraints stop your head whipping backwards in a crash from behind, which prevents whiplash injuries to the neck.

In Australia, new cars are crash-tested by ANCAP, which drives them into barriers with dummies inside packed with sensors, and gives each car a safety rating out of five stars.

Helmets and padding

Bike, motorbike and construction helmets have a hard outer shell that stops sharp objects getting through and spreads a blow over a wider area, and a thick layer of foam inside that crushes on impact so your head stops more gently. Once the foam has been crushed, it can't protect you again, so a helmet must be replaced after a crash, even if it looks fine. Victoria was also the first place in the world to make bike helmets compulsory, in 1990, and the rest of Australia followed.

The same idea is used in shin pads, mouthguards, running shoes, gym mats, the padded posts at football grounds and the crash barriers beside motorways (some are full of water or sand, or are designed to fold up).