Scientists throw a curve at knuckleball explanation

Knuckleballs baffle baseball players with their unpredictable swerves. A new study suggests a possible cause of the pitch’s erratic flight — sudden changes in the drag force on a ball, due to a phenomenon called a drag crisis.

The result is at odds with previous research that attributed the zigzags to the effect of airflow over the baseball’s seams. Scientists report the finding July 13 in the New Journal of Physics.

Knuckleballs are well known in baseball, but similar phenomena also confound players in soccer and volleyball. Knuckleballs occur when balls sail through the air with very little spin, producing unstable flight.

In drag crisis, the thin layer of air surrounding the ball flips between turbulent and smooth flow, abruptly changing the drag forces on the ball. If the transition occurs asymmetrically, it can push the ball to one side. “This phenomenon is intermittent” and hard to predict, says study coauthor Caroline Cohen, a physicist at École Polytechnique in Palaiseau, France. “We can’t know in advance [to] which side it will go.” Balls must move at a certain speed to experience a drag crisis, which may be why knuckleballs tend to be thrown slower than other pitches, the researchers suggest. While the fastest pitches can top 100 miles per hour, knuckleballs are usually closer to 60 or 70 miles per hour.

The scientists built a knuckleball machine, designed to launch a beach ball without any spin, and measured how much the ball veered off course. Then they calculated the ball’s expected motion based on the physics of the drag crisis and found that the predicted trajectories matched the experiments. The scientists’ calculations also correctly predict knuckleball-like phenomena in soccer, volleyball, cricket and baseball — but not in sports like tennis or basketball, where knuckleballs aren’t seen due to the properties of the ball, including texture, typical speed and how far it flies.

“It’s a fine piece of work,” says Alan Nathan of the University of Illinois at Urbana-Champaign, who studies the physics of baseball (SN: 3/23/13, p.32). But he is not entirely convinced by the explanation of knuckleballs. “Wind tunnel experiments seem to strongly suggest that it’s associated with the seams on the ball,” Nathan says, which can create turbulence that causes the ball to swerve.

So knuckleballs may remain as much of a challenge to explain as it is to hit them.