Video: Spinning ring puts surprising twist on familiar physics

Nearly everyone is familiar with how a wedding ring or a coin spins and wobbles around on a tabletop. But as the video above shows, in the end the two shapes follow different paths, and physicists have explained why. When a coin or disk spins, its edge circles in the same direction as its center, so it executes a wobbly spiral. A ring starts out making the same sort of spiral, but as it winds down the rotation of its center suddenly changes to opposite that of the edge, so that the ring follows an S-shaped path. According to new research published in Physical Review E, the difference comes down to drag. The disk traps a layer of air between itself and the tabletop—which is key in explaining how the disk suddenly comes to a stop.  But in the case of the ring, the air can escape upward through the hole. This difference in airflow causes the direction of frictional forces perpendicular to the ring’s edge to switch directions, leading to a change in the direction of its spiraling along the tabletop. The authors point out that many spinning objects, like frisbees and boomerangs, are heavily influenced by air resistance; the difference between the ring and the disk appears to be another example.