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Why the knuckleball takes such a knucklehead path

The new explanation traces to a ‘drag crisis’


A knuckleball refers to how the pitcher grips (seen here) and releases a baseball. Physicists have a new explanation for the seemingly unpredictable trajectory of this pitch.

Justin Skinner / iStockphoto

Knuckleballs baffle baseball hitters. These balls seem to swerve along their path unpredictably. A new study suggests a possible cause of the pitch’s erratic flight. It suggests that the ball experiences sudden changes in air resistance — or the force of drag — on a ball. The scientists describe this as a “drag crisis.”

Scientists described their finding July 13 in the New Journal of Physics.

Their result is at odds with previous research. That had concluded that the zigzags in the ball’s flight path was due to the effect of airflow over the baseball’s seams.

Knuckleballs occur when balls sail through the air with very little spin. This produces unstable flight. Although best known in baseball, similar effects also confound players in soccer and volleyball.

In drag crisis, the thin layer of air surrounding the ball flips between a flow that is turbulent and then smooth. This abruptly changes the drag on the ball. If the transition occurs in an asymmetric pattern, it can push the ball to one side. “This phenomenon is intermittent” and hard to predict, says Caroline Cohen. An author of the new study, she’s a physicist at École Polytechnique in Palaiseau, France.

“We can’t know in advance [to] which side it will go, Cohen says of the balls. Those balls must move at a particular speed to experience a drag crisis. And that may be why knuckleballs tend to be slower than other pitches, her team notes. While the fastest pitches can top 100 miles per hour (160 kilometers per hour) knuckleballs are usually closer to 60 or 70 miles per hour.

For their new study, the scientists built a knuckleball machine. It launched a beach ball without any spin. Then they measured how much the ball veered off course. Afterward, they calculated the ball’s expected motion based on the physics of the drag crisis. Those predictions matched what the ball had done during their tests.

The scientists’ calculations also correctly predict knuckleball-like effects in soccer, volleyball, cricket and baseball. The same was not seen in sports like tennis or basketball. And that’s due to the different properties of their balls, including textures, typical speeds and how far they fly.

Alan Nathan studies the physics of baseball at the University of Illinois at Urbana-Champaign. He describes the new study as “a fine piece of work.” But he is not entirely convinced by the new explanation for knuckleballs. Wind tunnel experiments seem to strongly suggest that the flight path of this pitch is "associated with the seams on the ball,” Nathan says. Those seams can create turbulence that causes a ball to swerve.

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

Power Words

(for more about Power Words, click here)

drag    The slowing force exerted by air or other fluid surrounding a moving object.

force     An influence that tends to change the motion of a body or produce motion or stress in a stationary body.

erratic    An adjective that describes something that happens at unpredictable intervals or a behavior that is unpredictable.

friction     Resistance to motion that arises when one object moves over another.

physics     The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study which emerged later, is a more accurate way of  explaining the motions and behavior of matter. A scientist who works in that field is known as a physicist.

resistance   (as in drug resistance) The reduction in the effectiveness of a drug to cure a disease, usually a microbial infection.

statistics    he practice or science of collecting and analyzing numerical data in large quantities.

symmetry  In geometry, the property of being indistinguishable from a shifted, rotated or reflected image of the same object. For example, the letter X looks the same whether reflected in a mirror or turned upside down — two different kinds of symmetry. The opposite of symmetry is asymmetry.

trajectory    The path traced by a flying object.

turbulence  The chaotic, swirling flow of air. Airplanes that run into turbulence high above ground can give passengers a bumpy ride.

wind tunnel    A facility used to study the effects of air moving past solid objects, which often are scale models of real-size items such as airplanes and rockets. The objects typically are covered with sensors that measure aerodynamic forces like lift and drag. Also, sometimes engineers inject tiny streams of smoke into the wind tunnel so that airflow past the object is made visible.

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Further Reading

S. Ornes. “Baseball: Keeping your head in the game.” Science News for Students. January 22, 2014.

D. Mackenzie. “Cool Jobs: Data detectives.” Science News for Students. December 17, 2013.

S. Ornes. “Baseball: From pitch to hits.” Science News for Students. August 21, 2013.

H. Fields. “Cool Jobs: Sports science.” Science News for Students. July 31, 2013.

J. Cutraro. “Hey batter, wake up!” Science News for Kids. June 18, 2008.

S. Ornes. “Hitting streaks spread success.” Science News for Kids. Jan. 11, 2013.

N. Seppa. “Baseball's resident physicist.” Science News. March 23, 2013.

Original Journal Source: B.D. Texier et al. Physics of knuckleballsNew Journal of Physics. Vol. 18, July 13, 2016, p.  073027. doi:10.1088/1367-2630/18/7/073027.

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