This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Hula hooping may be literal child's play, but as it turns out, the physics behind the gym-class staple is so complicated that scientists have spent decades trying to understand how the hula hoop defies gravity. Multiple scientific papers have been devoted to this minor mystery: How do one's hips and legs collude with friction and centrifugal forces to keep a simple toy spinning in midair?
The still-unresolved question led a team of UC Berkeley-affiliated engineers to tackle the problem by using an innovative method to analyze motion-tracking data. Their results, recently published in the journal Nonlinear Dynamics, challenge 20 years of conventional wisdom about hula hooping—and suggest a better way to model complex mechanical systems like the hula hoop's twirl, which could be useful in fields like physical rehabilitation and robotics. Previous studies had concluded that two distinct sets of movements, one back-and-forth and the other a vertical bounce, separately controlled the hoop's orbit around the body and its up-down wobble.
To reach these findings, these studies used a mathematical method called principal component analysis to distill a vast quantity of data into its clearest patterns. But the Berkeley team's analysis revealed that the orbit and wobble were inextricably connected—they couldn't be divided into two clean categories because one set of movements impacted the other. Led by Oliver O'Reilly, a professor of mechanical engineering and vice provost for undergraduate education, the Berkeley researchers initially intended to build on previous studies by tracking the motion of the hoop itself with a specialized sensor that gathered 120 snapshots a second of rotational velocity, acceleration, gravitational force and magnetic fields.
When a principal component analysis of the additional data they'd gathered led them to starkly different findings than past teams, they changed tack and employed a nonlinear mathematical method to process their data, which revealed that the thigh, shin and feet bones work together to keep the hoop circling, while thigh movement is responsible for the toy's vertical wobble. Unlike previous studies, they showed that the hoop's rotation and wobble were fundamentally linked. Upending the physics of hula hooping wasn't the study's primary goal.
O'Reilly—who has previously dissected the physics of how shoelaces become untied and Star Wars' spherical droid BB-8 rolls—started by trying to pinpoint which body motions were essential for hula hooping. His collaborators, structural engineering postdoctoral researcher Chrystal Chern and recent Berkeley Ph.D. Theresa Honein, now an assistant professor at the American University of Beirut, devised a way to rig high-tech sensors, the kind Chern normally used to track how model buildings would react to simulated earthquakes, to the hoop, right leg and foot. (The hips didn't get tracked, since the sensor would have hit the hoop.) Then Chern, an erstwhile gymnast, hooped as the sensors gathered a massive trove of data.
When they tried to replicate others' results with their data, the researchers found that the numbers didn't add up. Those previous studies had used principal component analysis, which relies on linear math—in which the relationships among variables are driven by addition and subtraction, and when mapped, become straight lines—to distill a morass of data into the strongest and most consistent patterns. "We were really surprised," said O'Reilly.
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