Keeping a Hula-Hoop spinning involves more than just skillful movement—it also relies on specific physical principles, including the shape of the body doing the spinning.
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Recent research published in the Proceedings of the National Academy of Sciences explored this phenomenon using gyrating robots to mimic hoop-slinging motions. The findings revealed that successful Hula-Hooping depends on having a shape with two key features: "hips"—a sloped surface to create upward force—and a "waist"—an hourglass-like curve that keeps the hoop from sliding up or down.
Leif Ristroph, an applied mathematician at New York University, was inspired to investigate this after observing street performers near his home in Greenwich Village. Noticing that previous studies hadn’t fully explained how hoops remain stable, Ristroph and his team decided to dig deeper. Known for exploring unconventional physics questions, Ristroph has also examined scenarios like how a lawn sprinkler would behave if it pulled water in rather than spraying it out.
In their experiments, the researchers tested robots with different shapes. A cylindrical robot, lacking a sloped surface, couldn’t keep the hoop from sliding down. A cone-shaped robot, while having the necessary slope, also failed. When the hoop started near the top of the cone, the upward force overpowered gravity, causing it to drift up. Starting near the base, the upward force wasn’t sufficient to counter gravity, and the hoop slipped down. However, an hourglass-shaped robot successfully maintained the hoop at a stable position, demonstrating the importance of both "hips" and "waist."
For humans, the researchers noted that body shape is less critical. People naturally adapt their movements to stabilize the hoop, much like the researchers managed with a cone-shaped robot by adjusting its gyration speed to match the hoop’s position.
A correct launch was also essential in the experiments. Starting with too little motion often resulted in failure, while a properly timed and aligned start, where the hoop and body moved in sync, led to success. For beginners, the study suggested starting with larger hoops, as they are easier to keep aloft due to requiring slower, more forgiving movements.
This research not only sheds light on the mechanics of Hula-Hooping but also highlights how the interplay of physics and body movement ensures success in this seemingly simple activity.
Journal Reference:
Zhu, X., et al. (2024) Geometrically modulated contact forces enable hula hoop levitation. Proceedings of the National Academy of Sciences. doi.org/10.1073/pnas.2411588121