A coin spinning on a table is a familiar sight. At first it rotates quickly, then it tilts, and its rattling becomes faster and faster. In the last seconds it seems to hover: the rotation slows, but the coin just won't finally lie flat. This final moment is a real puzzle for physicists.
In the new study, scientists filmed such motion with a high-speed camera and varied the conditions: they used disks of different masses and sizes, placed them on different surfaces, and even pumped out the air. It turned out that at the very end, when the coin is almost flat, it's not friction with the table that slows it down, but the air. A thin layer of air under the coin gets compressed and prevents it from falling, like a cushion. The heavier the disk, the harder it presses on this air cushion and the faster it stops.
But at the beginning of the motion, while the coin spins almost vertically, the main brake is rolling friction. It arises when the coin rolls over the surface and slightly sinks into it. On glass, this friction behaved strangely: light disks slowed down more than heavy ones. It seems that light disks stick more strongly to the smooth surface, like a suction cup. This matters for any mechanism where light parts roll over smooth surfaces: from bearings to wheels in precision instruments.
If you pump the air out from under a spinning coin, its final spin lasts longer: without the air cushion it lies down faster, but the spin itself slows down more slowly beforehand.
In Stanisław Lem's novel "Solaris," the protagonist observes the motion of strange objects on the planet's surface: their behavior is as hard to predict as the last seconds of a spinning coin.