High-speed video of experiments with a robotic hula hoop player, whose hourglass shape holds the hoop 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 all the rage. Now, researchers have taken a closer look at the toy's underlying physics, showing that certain body types can hold up the spinning hoops better 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 was not even understood at a fundamental level of physics,” said co-author Leif Ristrof of New York University. “As we progressed with the research, we realized that the mathematics and physics involved are very subtle, and that the knowledge gained could be useful in inspiring technical innovations, harvesting energy from vibrations and improving robotic positioners and movers used in industrial processing and manufacturing.”
Ristrof's laboratory regularly tackles these kinds of colorful puzzles from the real world. For example, in 2018, Ristrof and colleagues refined the recipe for the perfect bubble based on experiments with thin soap films. In 2021, the Ristrof lab investigated the formation processes underlying so-called “stone forests” common in certain regions of China and Madagascar.
In 2021, his laboratory 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 different pressures. They found that the water flowed about two times slower in the non-desired direction. In 2022, Ristrof studied the extraordinarily complex aerodynamics of what makes a good paper airplane, specifically what is needed for smooth gliding.
Girl spinning a hula hoop in 1958
Credit: George Garrigues/CC BY-SA 3.0
And last year, Ristrof's lab solved the riddle of physicist Richard Feynman's “reverse sprinkler” problem, concluding that the reverse sprinkler rotates more than 50 times slower than a regular sprinkler, but operates by similar mechanisms. The secret is hidden inside the sprinkler, where there are jets that make it work like an inside-out rocket. The internal jets do not collide head-on; As the water flows around the bends in the sprinkler arms, it is thrown outward by centrifugal force, resulting in asymmetrical flow.
