ABSTRACT
When a sporting projectile is set into motion by, say, the impact from racket, club or bat, or the kick from a booted foot, it will travel through some fluid (i.e. a gas or liquid), usually air, as it progresses on its trajectory path. The displacement of the medium (air molecules) as it moves out of the path of the trajectory, produces a reactive force on the projectile which acts to slow it down. As the projectile carves its way through the fluid, the molecules will; (a) change direction as they are pushed out of the way of the projectile, or make a detour around it, and, (b) attain some new mean speed, superimposed on the random Brownian motion of the otherwise undisturbed molecules.
