Spinning, hoop-slinging robots are revealing the physics of a favorite toy — the Hula-Hoop. Scientists found that those robots had to have a particular shape in order to hula-hoop. And that helped explain how the hoops stay up against gravity.
The robots were designed to move in small circles, similar to a person hula-hooping. But only some bots could keep a hoop up.
Those robots had indented middles, like an hourglass. The slope at the bottom of the indent provides an upward force to the hoop as it swings around. That’s what helps counteract gravity. And the curvature of that shape kept the spinning ring from drifting up or down and sliding off.
Researchers shared their new findings in the January 7 Proceedings of the National Academy of Sciences.
Learn about the history of Hula-Hoops and, from 2:34-4:00, how angular momentum and torque affect our ability to successfully spin the hoops. At 7 minutes in, hear how this toy has become an unlikely inspiration for engineers and others.
Leif Ristroph is an applied mathematician in New York City at New York University. Inspired by performers near his home in Greenwich Village, he began considering the physics of Hula-Hoops. He was part of a team that realized earlier work hadn’t truly explained how the hoop stays up. (Ristroph has a track record of tackling quirky science 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 the science of toy hoops a whirl. They ran experiments with a series of gyrating robots. (Gyrating means to move around in fast, tight circles.) A robot shaped like a cylinder couldn’t keep its rotating hoop from sliding down. Another robot was shaped like an upside-down ice cream cone. Its slope helped counteract gravity’s downward pull on the hoop. But it had no curvy indent. It, too, couldn’t keep its hoop up. If the hoop began toward the top of the cone, it would migrate up. Then it would be flung away. If the hoop started near the bottom, it would migrate down to the ground.
Only a robot indented in the middle could keep a hoop steadily aloft.
This showed how hoops stay up on robots. But with enough practice, people should be able to keep a Hula-Hoop spinning regardless of body shape.
People can learn to adapt their gyrations based on the position of the hoop. Even the robots could learn this trick. The researchers got the cone-shaped robot to successfully hula-hoop by adjusting its rate of wiggling, depending on how high the hoop slid.
The new findings offer hints for people learning to hula-hoop. First is how to launch their hoop. In the tests, a slow launch prevented robots from keeping the hoop up. So make sure to spin it fast.
Positioning the hoop correctly also helps. In successful experiments, the hoop lined up with the robot’s body so that the hoop and body always shifted in the same direction. Hula-hoopers should try to launch that way, too. If you start with your hips out to the right, start with the hoop out to the right. Then, swing the hoop around quickly. And start swiveling your hips in that same direction.
A final tip gleaned from the research: Bigger hoops are good for beginners — they can be supported with slower gyrations.
