Science Puzzle
Why Are Cars Built to Crumple?
Cars from the 1950s were built like tanks, with stiff steel bodies that hardly dented in a crash. Modern cars are deliberately designed so their fronts and backs fold up like an accordion, and a car that has been in a serious crash can look completely wrecked.
Yet the people inside a modern car are far more likely to walk away. Why does a car that crumples protect them better?
The Answer
Crumpling stretches out the stop. The people inside slow down over a longer distance and a longer time, so the force on their bodies is much smaller.
In a crash, the car and everyone in it must lose all their speed. The faster that happens, the bigger the force needed to do it. A stiff car that stops within 10 centimetres of hitting a wall at 50 km/h puts its passengers through roughly 100 times the pull of gravity. Let the front fold up over 60 centimetres and that falls to about 17 times: still violent, but far more survivable. Seat belts, which stretch slightly, and airbags, which squash, stretch the stop out further still.
Crumpling also soaks up the car's energy. Bending and tearing metal takes a lot of work, and every bit of energy used up that way is energy that doesn't go into the people. Bouncing off the wall would make things worse, because the car would have to stop and then speed up again in the other direction.
The trick is to crumple the right parts. Engineers make the front and back soft and the passenger compartment, the "safety cell", as stiff as possible. The idea was patented in the early 1950s by Béla Barényi, an engineer at Mercedes-Benz, and appeared in their cars from 1959. You do the same thing when you bend your knees on landing from a jump.
The principle: Impulse. Stopping a moving object takes a set change in momentum; spreading that change over a longer time, or a longer distance, means a smaller force.