Why do skyscrapers, cars and power lines all hide the same swinging weight?

Because they all fight the same enemy, resonance, with the same trick: a tuned mass damper. It is a weight mounted on springs and dampers inside the thing you want to keep still, and tuned to swing at that thing's own natural rhythm. When the structure starts to sway, the weight swings the other way and soaks up the motion, turning it into heat through friction or fluid, much like a car's shock absorber. The clever part is that the damper weighs far less than what it protects.

In skyscrapers, the dampers are giant blocks of concrete or steel. Wind can push the top of a tall tower more than a metre and make the people inside feel seasick. Taipei 101 hangs a 660-tonne steel sphere across its 87th to 92nd floors; New York's 111 West 57th Street holds the heaviest solid damper in the world, about 730 tonnes. Boston's John Hancock Tower, in 1976, was the first building to get one, and only after it was built.

The same idea hides in smaller places. Almost every modern car has at least one mass damper, some have ten or more, and the little barbell-shaped weights you see on high-voltage cables are dampers too.

It can even be too good. In 2005, Renault put a tuned mass damper in the suspension of its Formula One car and reportedly gained 0.3 seconds a lap. Two weeks later, the sport's appeal court banned it.