Wired Science

Why Does a Curveball Curve?

What makes a curveball curve and a slider slide? Basic aerodynamics, as manipulated by a skillful pitcher.

Three forces act on a spinning baseball as it flies toward the plate: the downward pull of gravity, the slowing drag of air resistance, and a "lift" force, also known as the Magnus force, caused by the direction of the ball's spin. That final ingredient is the one a pitcher can control.

The spin deflects the flow of air pressure acting on the ball to make it fall more or less slowly or curve to one side. Your basic fastball is launched with backspin; that increases air pressure beneath the ball, making the Magnus force point upward, causing the ball to fall less quickly than it would if only gravity were pulling on it. Curveballs have topspin, which makes them fall faster than they otherwise would. Sliders are spun sideways so that they break to the left or right.

Then there's the much-discussed gyroball, allegedly the specialty of Red Sox pitcher Daisuke Matsuzaka. By turning his wrist away from his body, a pitcher can send the ball spinning not in the same direction it's traveling, but perpendicular to it, like a football or a rifle bullet. That means the air pressure above and below the ball are essentially the same. The result is a sneaky pitch that looks like a fastball as it leaves the pitcher's hand, but drops faster - hopefully leaving a confused batter swinging at the air.

The faster the ball is traveling and spinning, the greater the effect of the Magnus force will be. Greater air density also increases its effect - which makes higher-altitude, thinner-air cities a tougher place to fake out a batter. So if the Rockies are having a tough year, they've always got an excuse.