Aerodynamics of a Spinning Baseball

Al Nathan, Physics, UIUC

The trajectory of a baseball in flight differs from a “Physics 101” trajectory because of the aerodynamic effects of drag and lift, the latter applying if the baseball is spinning. Neither of these effects can be calculated from first principles for any sports ball, so one must rely on theoretically inspired models that are constrained both by fundamental principles of physics and by careful experiments. The models are characterized by the so-called drag and lift coefficients, which depend, in principle, on both speed and spin. These coefficients, which are usually measured in wind tunnel experiments, are not sufficiently known at speeds and spins relevant to the game to allow quantitative predictions of the trajectory of a fly ball. A new experiment is described in which high-speed motion-capture cameras are used to measure both the spin and the initial trajectory of a ball projected from a pitching machine. Preliminary results will be presented for the lift coefficient and its dependence on the spin.