Defying gravity Delve into the physics of the Hula-Hoop It's not the gyrating motion of the hips that keeps the hoop aloft; it's a body type with a sloping surface as "hips." Jennifer Ouellette Jan 3, 2025 1:11 pm | 4 Credit: Warner Bros. Credit: Warner Bros. Story textSizeSmallStandardLargeWidth *StandardWideLinksStandardOrange* Subscribers only Learn more High-speed video of experiments on a robotic hula hooper, whose hourglass form holds the hoop up and in place. Some version of the Hula-Hoop has been around for millennia, but the popular plastic version was introduced by Wham-O in the 1950s and quickly became a fad. Now, researchers have taken a closer look at the underlying physics of the toy, revealing that certain body types are better at keeping the spinning hoops elevated than others, according to a new paper published in the Proceedings of the National Academy of Sciences.We were surprised that an activity as popular, fun, and healthy as hula hooping wasnt understood even at a basic physics level, said co-author Leif Ristroph of New York University. As we made progress on the research, we realized that the math and physics involved are very subtle, and the knowledge gained could be useful in inspiring engineering innovations, harvesting energy from vibrations, and improving in robotic positioners and movers used in industrial processing and manufacturing.Ristroph's lab frequently addresses these kinds of colorful real-world puzzles. For instance, in 2018, Ristroph and colleagues fine-tuned the recipe for the perfect bubble based on experiments with soapy thin films. In 2021, the Ristroph lab looked into the formation processes underlying so-called "stone forests" common in certain regions of China and Madagascar.In 2021, his lab built a working Tesla valve, in accordance with the inventor's design, and measured the flow of water through the valve in both directions at various pressures. They found the water flowed about two times slower in the nonpreferred direction. In 2022, Ristroph studied the surpassingly complex aerodynamics of what makes a good paper airplanespecifically, what is needed for smooth gliding. Girl twirling a Hula-Hoop in 1958 Credit: George Garrigues/CC BY-SA 3.0 And last year, Ristroph's lab cracked the conundrum of physicist Richard Feynman's "reverse sprinkler" problem, concluding that the reverse sprinkler rotates a good 50 times slower than a regular sprinkler but operates along similar mechanisms. The secret is hidden inside the sprinkler, where there are jets that make it act like an inside-out rocket. The internal jets don't collide head-on; rather, as water flows around the bends in the sprinkler arms, it is slung outward by centrifugal force, leading to asymmetric flow.Enter the mini-robotsThere haven't been many prior studies on the physics of the Hula-Hoop. There was one in 1960 when the toy first became a modern fad, another in 1987, and a smattering of more recent papers since 2008 as the Hula-Hoop's popularity has risen again as a form of exercise of performance art. Most studies modeled it as a two-dimensional problem involving a freely hinged extended mass or a ring rolling around a moving circle. This does not account for 3D factors like gravitynamely, how the hoop can remain suspended in rotation for extended periods of time.Hula-Hoop physics has many of the same subtle aspects as a spinning topi.e., a rotating rigid body with "complex couplings between different degrees of freedom, as well as non inertial reference frames and fictitious forces," the authors wrotealong with additional issues involving a rolling point of contact on a body's surface, activated by gyration motions. Ristoph et al. thought that geometry might strongly affect the hoop dynamics and decided to investigate further. We were specifically interested in what kinds of body motions and shapes could successfully hold the hoop up and what physical requirements and restrictions are involved, said Ristroph.The team performed a series of experiments using mini-robots holding different 3D-printed geometric shapes: cones, cylinders, or hourglasses, for example. Built-in motors caused the shapes to gyrate much like a person's hips while using a hula hoop. Then Ristroph et al. launched six-inch mini hoops onto the shapes, filming the resulting movement in high-speed video. Vertical motions of hoops on robotic gyrators of different shapes. Credit: X. Zu et al, 2025 It turns out that the kind of gyrating motion generated is not a significant factor in keeping the hoop aloft and spinning; instead, as Ristroph et al. had suspected, the key is the geometric shape. For instance, all trials involving a cylinder shape were abject failures, unable to keep the hoop aloft. Conical shapes using a circular gyration performed a bit better, but the hoop descended or rose depending on the height at which it was initially released. "The hoop sinks if set free from a low point on the body and rises if released sufficiently high, but it never keeps a level," the authors wrote.The most robust shape for suspending a hula hoop for extended periods turns out to be an hourglass shape, even when using different release heights.People come in many different body typessome who have these slope and curvature traits in their hips and waist and some who dont, said Ristroph. Our results might explain why some people are natural hoopers and others seem to have to work extra hard.The authors ultimately identified two distinct forms of equilibrium that enable steady-state hula hooping: a synchronization process causing the hoop to twirl at the same frequency as the gyration motion, directing its center outward,and the hoop's vertical positioning. The former requires that the hoop be launched sufficiently fast in the same direction as the gyrating motion, while the latter is strongly dependent on body shapewith an hourglass being ideal. More generally, the results "show how the motion and positioning of an object can be controlled through the geometry and kinematics of a surface in which it is in rolling contact," the authors concluded.PNAS, 2025. DOI: 10.1073/pnas.2411588121 (About DOIs).Jennifer OuelletteSenior WriterJennifer OuelletteSenior Writer Jennifer is a senior reporter at Ars Technica with a particular focus on where science meets culture, covering everything from physics and related interdisciplinary topics to her favorite films and TV series. Jennifer lives in Baltimore with her spouse, physicist Sean M. Carroll, and their two cats, Ariel and Caliban. 4 Comments