Q: Why does a curve ball curve?

A: The curved path of a spinning ball is caused by small differences in air pressure which cause it to deviate from its normal trajectory. In the mid eighteenth century Daniel Bernoulli applied Newton's laws of motion to moving fluids to discover that when the speed of air increases its pressure decreases. Bernoulli's principle is responsible for the lift on airplane wings, the ability of a sailboat to sail into the wind, the operation of a carburetor, the spray bottle, and the curve ball. A spinning ball drags a thin layer of air around with it. The air of this layer which is spinning into the oncoming wind has a lower airspeed than that which is spinning away from the wind. Bernoulli's principle predicts that the pressure is lower on the high speed side. This results in a small force being applied across the spinning ball which causes the ball to curve away from its expected line of flight. The direction of the Bernoulli force depends on the orientation of the spin axis of the ball and so the deflection of the ball depends on the kind of spin it has. If it has topspin its spin axis is horizontal, its top is the low speed side and its bottom is the high speed side. This creates a downward push on the ball and causes it to curve downward. The ball may suddenly break after following a normal trajectory for awhile because it takes several revolutions for the spinning layer of air to become organized enough to produce the pressure difference. A ball that is thrown too fast will not have enough time to curve properly. That's why the curve ball is a slow pitch. A ball that is not spinning does not develop the thin, spinning streamline of flow around it. Instead, turbulence rules its trajectory. Turbulence is chaotic, so the path of a knuckeball or a forkball is much less predictable. The spin of a ball in flight acts along with gravity and air resistance to determine the shape of the trajectory. By controlling the spin the pitcher keeps the batter guessing about what the ball might do on its way to the plate.

Richard Brill is assistant professor of science at Honolulu Community College where he teaches earth and physical science. Send questions to him at Honolulu Community College, 874 Dillingham Blvd., Honolulu, HI 96817 or email to rickb@hcc.hawaii.edu

The Breaking Pitch ©1995 Richard C. Brill