Why a 660-tonne steel ball swings to keep Taipei 101 still

Skyscrapers sway in the wind, and gusts that match the tower's own rhythm can make it sway more and more. Taipei 101's answer is a 660-tonne steel pendulum hanging between the 87th and 92nd floors, tuned to swing against the building's motion.

Play with two towers in the same wind below. Change the weight's size and tuning and see why a tuned mass damper only works when it keeps perfect time with the building it protects.