After a half rotation of the coin on the left around the central coin (of the same radius), the coin undergoes a complete rotation. In other words, a coin makes two complete rotations when rolled around the boundary of an identical coin. This fact is readily apparent in the generation of the cardioid as one disk rolling on another.
Coin Paradox
See also
CardioidExplore with Wolfram|Alpha
References
Pappas, T. "The Coin Paradox." The Joy of Mathematics. San Carlos, CA: Wide World Publ./Tetra, p. 220, 1989.Steinhaus, H. Mathematical Snapshots, 3rd ed. New York: Dover, p. 145, 1999.Referenced on Wolfram|Alpha
Coin ParadoxCite this as:
Weisstein, Eric W. "Coin Paradox." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/CoinParadox.html