What If? 2: Additional Serious Scientific Answers to Absurd Hypothetical Questions

by Randall Munroe

There’s a surprisingly large amount of research into the physics of swing sets, partly because pendulums are really interesting physical systems and probably also because all physicists were once children. Children who play on swing sets quickly learn that they can get themselves going by pumping—kicking out their feet and leaning back, then tucking in their feet and leaning forward. Physicists call this “driven oscillation,” and a series of studies since the 1970s have analyzed exactly how pumping a swing works and what the most efficient way to do it is. What the physicists found, after half a century of research, was that children know exactly what they’re doing. Rhythmically kicking and leaning with their hands on the chains seems to be just about the optimal strategy for powering a swing using the rider’s body. For a while, some physicists theorized that a better strategy for pumping a swing might be to stand on the seat and raise and lower your body, by alternately crouching and standing upright, but further calculation showed that the kids have it figured out.