Forces Acting on Pitches
We may play baseball on the moon someday, but it won't be very exciting, especially for pitchers. That's because pitchers rely on air resistance to throw breaking balls like curveballs and sliders and keep batters guessing. Understanding how this works requires a crash course in aerodynamics, a branch of physics that studies the properties of moving air, as well as how solid bodies interact with air as they move through it.
A baseball thrown by a pitcher pushes aside air molecules as it races toward the plate. As they encounter the leading edge of the ball, the molecules stream up and over and back together again to produce a wake behind the ball. For a slowly moving ball, the total pressure acting on the ball remains constant, but the "stickiness" of the air creates a frictionlike force known as viscous drag. As a ball moves increasingly faster, drag becomes more complicated. For such a ball, air doesn't actually reach the surface but forms a smooth, quiet boundary layer around the sphere. At speeds below 50 mph (80 kph), this mini whirlpool of circulating air molecules remains intact, and the air flow is smooth all the way around the ball. As the ball speeds up, however, frictional forces begin to peel away the layer, which creates an area of turbulence -- and lower pressure -- behind the ball. Higher pressure at the front of the ball gets the upper hand and exerts a second retarding force known as pressure drag.
Spin also affects air flow. A baseball leaving a pitcher's hand with significant spin will experience a force, known as the Magnus force, that acts at a right angle to the axis of spin. Consider a thrown ball spinning counterclockwise as viewed from above the mound. The air will flow faster around the third-base side of the ball than the first-base side. On the faster side, the boundary layer will peel away farther upstream, deflecting the trailing wake toward third base. The drag on the third-base side is greater than the drag on the first-base side, so the ball curves toward first base, at a right angle to the axis of spin.
The Magnus force explains how pitchers throw, as Crash Davis would say, "ungodly breaking stuff." For example, to throw a fastball, a pitcher places his middle and index fingers relatively close together and across the laces. When the ball comes out of the pitcher's hand, it has significant backspin, which means its spin axis is parallel to the field. The Magnus force, in this case, pushes up on the ball, acting against gravity.
Hitters often say such a pitch "hops" as it approaches the plate, but it's more likely that the Magnus force simply keeps it from dropping as quickly as it normally would. A good curveball, on the other hand, has more spin but less velocity. Not only that, if it's thrown well, it will have both topspin and sidespin, which places the axis of spin somewhere between horizontal and vertical. The Magnus force also acts on an angle, causing the ball to break down and to the side, sometimes as much as 15 inches (38 centimeters)[source: Nathan]!