A playground toy has kept scientists busy for decades: how do your hips and legs keep a hula hoop spinning in midair instead of sliding to the floor? A team at UC Berkeley now says the answer most researchers settled on about 20 years ago was incomplete.
The study, The symphony of gyrations producing steady motions of a hula hoop, was published this fall in the journal Nonlinear Dynamics by Chrystal Chern, Theresa E. Honein and mechanical engineering professor Oliver O'Reilly, who is also Berkeley's vice provost for undergraduate education.
What changed
Earlier studies used principal component analysis to break hooping into two separate sets of motion, one back-and-forth that kept the hoop orbiting and a vertical bounce that controlled its wobble. The Berkeley team put a sensor on the hoop itself that recorded 120 readings a second of rotation, acceleration, gravity and magnetic fields. When the usual method gave them very different results from earlier teams, they switched to a non-linear network analysis.
According to Berkeley News, that analysis showed the thigh, shin and foot bones work together to keep the hoop circling, while thigh movement mostly drives the hoop's vertical wobble. Orbit and wobble turned out to be linked, not independent.
Why anyone should care
The researchers say the approach could help model other complex body-and-object motions, with possible uses in physical rehabilitation and robotics. It also means the hula hoop at the next school field day is a better physics lesson than it looks.