Among a baseball pitcher’s arsenal of tricks is the infamous curveball, whereby the baseball takes a sudden dive downward just before it reaches the plate, faking out the batter. A new “History Minute” video from the National Institute of Standards and Technology takes a look at the physics behind this longstanding bane of batters.
The video is a tribute to Lyman Briggs, a physicist who served as director of the National Bureau of Standards (now called NIST) from 1933 to 1945. He was also a lifelong baseball fan, having played outfield on his college baseball team during the 1890s, and conducted several experiments in the 1950s to determine the precise physics at work in a curve ball.
At the time, there was much heated debate about whether a curve ball actually curves, or if it was just a perceptual illusion. Dizzy Dean, a pitcher with the St. Louis Cardinals, believed the former and once boasted, “Ball can’t curve? Shucks, get behind a tree and I’ll hit you with an optical illusion.” But Briggs was a scientist. He wasn’t going to just take Dizzy Dean’s word for it. He was going to do a bunch of experiments.
His hypothesis was that it had something to do with the Magnus effect: when a pitcher throws a curveball with good topspin, this creates a high-pressure zone at the top, and the ball gets deflected downward in an exaggerated drop. That seemed straightforward enough. But Briggs also wanted to know how much that telltale curve depends on spin, versus speed.
First he tried photographing the balls in mid-flight, but they just moved too fast to capture any definitive information about their trajectories. So he mounted a ball on a rubber tree and then shot at it with wooden bullets from an air gun to measure the curve and the speed. Measuring the spin, however, proved to be a bit trickier. For that, Briggs turned to the NSB’s old wind tunnel used to study aerodynamics. As I wrote in 2011:
This allowed him to precisely control most of the variables. He tossed baseballs into the wind tunnel, and let them freefall against the horizontal wind streams, which naturally caused the ball to curve. When a baseball finally hit the ground after the requisite 0.6 seconds, it bounced off a sheet of cardboard treated with lampblack, which left a smear on the ball, indicating point of impact. His conclusion: “An increase in the speed of the pitch beyond 100 feet per second reduced the curve only slightly and the important thing was the spin.”
So how much a ball curves depending more on spin than speed. And it really does curve—a lot. In fact, it can curve as much as 17-1/2 inches between the pitcher’s mound and home plate.