Spin a coin on a flat surface, and it spirals much like a planet orbiting a star — at least until it runs out of steam and rattles to a stop on the table. But spin a wedding ring the same way, and it will make a surprising abrupt turn, following a trajectory more like a boomerang.
That’s the conclusion of a new paper in Physical Review E, which fingers as the culprit an unexpected change in the ring’s air resistance, according to lead author Mir Abbas Jalili, a physicist who studies applied mechanics at the University of California. “All major discoveries of rigid body dynamics had been made in the 18th and 19th centuries, and we thought this is a rare phenomenon that remained unnoticed for decades, and maybe centuries,” Jalili told Gizmodo via email.
Jalili stumbled on this puzzling behaviour while pondering a different research problem in his office. He absent-mindedly removed his wedding ring and spun it on his desk, and was surprised to see that it made a retrograde turn as it spun, more like the flight path of a boomerang than the coins he usually spun while deep in thought. A few weeks later, his Berkeley colleague Reza Alam — a co-author on the new paper — invited Jalili and his wife to dinner, and Jalili demonstrated the effect. It was Alam who suggested they conduct experiments and film various spinning rings to better plot their trajectories.
First, they needed a bunch of wedding rings, so Alam ordered them from Amazon. Apparently the retailer “thought a mass engagement was underway, and wrapped the rings in fancy gift boxes,” Jalili recalled. Then they recorded the spinning motion of the wedding rings with a high-speed camera to verify that the behaviour was consistent. For the actual experiments, they made their own perfectly cylindrical rings of various widths and thicknesses, cut from a tube. You can see sample footage from their experiments below:
See how the path of the spinning ring gradually starts spiralling back? It’s reminiscent of how a boomerang eventually reverses direction to return to the person who threw it. The key to this behaviour in boomerangs is known as gyroscopic precession. One wing moves through the air faster than the other as the boomerang travels forward, creating an imbalance of forces: there is more lift on the top wing, and that difference gives rise to a torque, causing the boomerang to loop back around during flight.
The explanation for the wedding ring’s behaviour is a little different. A spinning coin will trap a thin layer of air between its bottom edge and the surface n which it is spinning. The same thing happens when you spin a ring, except now you have a big hole in the centre. That gives the air an escape route, and it flows through the hole, changing the frictional forces so the spinning ring switches direction.
Maybe this explanation seems obvious in hindsight, but “None of our colleagues were able to predict the origin of the retrograde turn correctly,” Jalili said. “They all thought slippage (the usual suspect) is the main cause.”
It’s often said that scientific discovery begins not with a triumphant cry of “Eureka!” but an unexpected observation punctuated by a puzzled, “huh.... that’s funny”. And it helps to foster a sense of play along with natural curiosity.
Ultimately, Jalili took away his own lesson from the wedding ring experiment. “There are many outstanding problems in nature all around us,” he said. “We do not always need to equip ourselves with advanced facilities to make discoveries, and find applications for them. Even a falling leaf can be inspirational.”
Feynman, Richard (1985). Surely You’re Joking, Mr. Feynman, pp. 157-158
Jalali, M.A., Sarebangholi, M.S., and Alam, M-R. (2015) “Terminal retrograde turn of rolling rings,” Physical Review E, 92: 032913.