The two main aerodynamic parameters used in trajectory analysis are the Reynolds
number, Re, and the spin parameter, Sp. The Reynolds number is a measure of the relative
sizes of the inertial and viscous forces on the ball and is defined as [16] Re = vD/ν, where v
is the ball’s speed, D is the ball’s diameter and ν is the kinematic viscosity, which is the ratio
of viscosity to air density. We studied the Adidas Teamgeist ball, which was introduced in the
2006 World Cup, for which we measured D 0.218 m. With ν 1.54 × 10−5 m2 s−1, the
conversion between theReynolds number and the soccer ball speed is Re×10−5 v/(7m s−1).
Soccer is played essentially in the range 70 000 < Re < 490 000