In your article, a spinning coin’s motion is explained by the existence of an air cushion between coin and tabletop. If this is indeed the case, then I would expect coins to do something quite different in a vacuum. What do they do?

Richard Chambers
Charlotte, N.C.

H. Keith Moffatt offers an explanation for the characteristically abrupt collapse of a spinning coin. He attributes the motion to turbulent airflow beneath the coin. How does he explain the identical motion that can be observed in a man’s wedding ring? A spun ring collapses much like a coin. In the wedding ring example, there isn’t much air trapped beneath the edge of the ring. I define collapse here not as a deviation from vertical, but as the characteristic rattling noise immediately prior to the cessation of the object’s motion. The object seems to slap the horizontal surface, rather than roll upon it.

Dave Typinski
High Springs, Fla.

In a vacuum, there would be no air cushion to dissipate energy, so rolling friction between the disk and the surface would play the predominant role in causing the disk to tip and roll more and more quickly before abruptly halting. Rolling friction is also probably responsible for the similar behavior of a spinning ring .–I. Peterson