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fork(2) writes:

A knuckleball or knuckler is a baseball pitch thrown so as to minimize the spin of the ball in flight, causing an erratic, unpredictable motion. The air flow over a seam of the ball causes the ball to transition from laminar to turbulent flow. This transition adds a deflecting force on the side of the baseball. This makes the pitch difficult for batters to hit, but also difficult for pitchers to control and catchers to catch; umpires are challenged as well, as the ball's irregular motion through the air makes it harder to call balls and strikes." (Wikipedia)

Gizmodo reports on scientific results that attempt to explain the knuckleball's behavior.

In 2012, scientists at the Ecole Polytechnique in France managed to devise a set of laws predicting how much different ball sizes, moving through a fluid (notably air and water), would "knuckle." They conducted a series of experiments that included dropping steel, glass, and plastic beads into a tank of water spiked with fluorescent dye, the better to study their trajectories with ultrafast cameras. All the beads zigzagged in the water, regardless of density, and the less dense the beads, the more they knuckled.

But those findings predicted that soccor balls wouldn't knuckle -- which just happens to be "a specialty of Real Madrid star player Christiano Ronaldo. In Brazil it's known as pombo sem asa ('dove without wings')."

The key is something physicists call the drag crisis. "When a sphere is in a flow, there is a critical velocity at which the wake behind the sphere and the drag force acting on the ball sharply decreases," co-author Caroline Cohen told Inside Science News at the time. That asymmetry in the wake creates a sideways force resulting in the zigzagging motion.

[...] Now Cohen and her Ecole Polytechnique colleagues are back with a new analysis of the knuckleball effect. This time around, the team built their own custom kicking machine to launch balls through the air in a wind tunnel at different speeds, with very little spin.

And they found a more universal culprit for the knuckling effect: unsteady lift forces. However, "Unsteady lift forces are inherent to balls traveling through the air in every sport, so to complete our work we needed to find out why zigzag shots are associated with just a few games, such as soccer or baseball," co-author Baptise Darbois Texier said in a statement.

Once again, the drag crisis proved critical: there is a sweet spot in terms of velocity that produces larger lift forces and more side-to-side movement. The typical shooting distance for any given sport also matters. "In bocce, for example, a zigzag path should occur over a length of 27 meters, but this distance is much longer than the typical shooting length and so the knuckleball effect will be incomplete," said Darbois Texier.

And that's why we typically don't see the knuckling effect in bocce, handball, table tennis, or basketball.

Original Submission