THE curious way a spinning coin turns on its side and rattles on a table
before coming to rest now has a mathematical explanation.
Keith Moffatt of the Newton Institute at Cambridge University was intrigued
by the death throes of coins. 鈥淭he rolling speed goes up as the coin loses
energy鈥攖hat鈥檚 quite paradoxical,鈥� he says. Simple equations even suggest
the rolling speed should increase to infinity.
Moffatt has resolved this paradox by taking the viscosity of air into account
(Nature, vol 404, p 833). His equations describe how energy is
dissipated in the thin layer of air between the coin and the table鈥攎aking
it accelerate, then halt abruptly.
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