The dynamics of sports is a subject familiar to many, a rich area of application of mathematical modeling. The dynamics of footballs in flight has been the subject of a recent article, and we have a continuing interest in this class of problems through our interaction with Christophe Clanet.

In the video below we see Karl Suabedissan striking two balls, one smooth, the other identical but for an elastic band wrapped around its equator. Both balls are struck with his instep so as to impart a spin that is counterclockwise as viewed from above, so one expects them to bend from right to left. However, the smooth ball bends in precisely the opposite sense. The presence of the surface roughness in the form of the elastic band is sufficient to change the boundary layer on the ball surface from laminar to turbulent, thus restoring the sign of the expected spin-induced lateral force on the ball. It is thus that all footballs have some surface roughness; otherwise, they would bend the wrong way.

PAPER: Bush (2013)

PRESS: Le Progres – Lyon 2009 , MIT News 2014 , America24 , Wall Street Journal