Experiments with gyrating, hoop-slinging robots have revealed how these spinning rings stay up despite the pull of gravity. The shape of the robot’s body is a crucial factor, researchers report in the Jan. 7 Proceedings of the National Academy of Sciences.

The shape needs to have “hips” — a slope that provides upward force to counteract gravity. And a “waist” — curvature like an hourglass — keeps the hoop from drifting up or down and sliding off.

Inspired by performers near his home in Greenwich Village, applied mathematician Leif Ristroph of New York University began considering the physics of Hula-Hoops. Previous studies, he and colleagues realized, hadn’t explained how the hoop stays aloft. (Ristroph has a track record of tackling quirky physics questions. His group recently investigated what would happen if a lawn sprinkler sucked water in instead of shooting it out.)

So Ristroph and colleagues gave it a whirl. In experiments, a gyrating cylindrical robot couldn’t keep a hoop from sliding down. It was missing the essential upward force, generated when a hoop swings over a sloped shape. But a cone-shaped robot, with a slope but no waistlike curve, also failed. If the hoop began toward the top of the cone, the upward force overpowered gravity, and the hoop would migrate up. If the hoop began toward the bottom, the upward force wasn’t enough to keep it aloft, and it migrated down. But an hourglass-shaped robot kept a hoop steadily aloft. 

People should be able to hula-hoop regardless of body shape, by adapting their gyrations based on position changes of the hoop. Indeed, the researchers were able to get a cone-shaped robot to hula-hoop by adjusting the rate of gyration depending on how high the hoop slid.

A correct launch was also essential in the experiments. If the hoop started off too slow, the attempt would fail. In successful sessions, the hoop lined up with the gyrating body, such that the hoop and body always shifted in the same direction. That’s also the best way to launch a hoop, Ristroph says.

A final tip gleaned from the research: Bigger hoops are good for beginners — they can be supported with slower gyrations.