The Zigzag Physics of the Knuckleball

What makes a ball zigzag unpredictably in midair?��

The Physics of Baseball lab at the University of Illinois

With the Olympics in its final few days, there have been plenty of opportunities to see one of the weirdest and most mysterious spectacles in sports: the knuckleball. Even if you’ve never heard of a knuckleball, you’ve probably seen it happen right before your eyes. A knuckleball is basically when a spherical projectile is hit or thrown such that the spin of the ball is minimized, creating an unpredictable zigzag trajectory that can catch the opposite team (as well as the players on one’s own team) by surprise.

The name is derived from the way old baseball pitchers in the early days of Major League Baseball (most notably Eddie Cicotte of the Chicago White Sox) used to grip the ball with their knuckles before throwing. The goal is to give the ball as little amount of rotational spin as possible.

Doing so results in a pitch trajectory that’s affected by variations in airflow facilitated by the differences in the smooth surfaces of the ball and the rougher stitching of the seams (or so that’s the idea — more on why this might not be true in a bit). Essentially, you’re forcing the air flow to create an asymmetric drag that creates a zigzag-like pitch. The ball, on its way to its home planet, will basically look like its fluttering side to side or up and down.

Of course, you don’t want to throw a ball that has no spin — just a slight one where the ball covers the distance without completing more than a one-half rotation. You might consider the knuckleball an inverse version of soccer’s curvy free kicks in which the goal is to apply a very vigorous spin on a ball in order to make it move towards a single direction.

Trying to throw a ball in this very narrow rotational threshold is an incredibly arduous task — and it’s the reason so few pitchers who’ve played in the majors have perfected it. Moreover, speed is the top-ranked metric in evaluating pitchers — and because a knuckleball moves slower than every other kind of pitch, there’s less and less incentive these days to be perfect.

Though knuckleballs are most prominently seen in baseball, they also occur in sports like soccer and volleyball — yet are also strangely absent in games like table tennis, squash, and basketball. And in many of these sports, the balls lack seams or a high degree of asymmetry on the surface. So why do we still see knuckleballs in other sports?

That question brings us to a pair of studies on the physics of knuckleballs that fluid dynamics researcher Baptiste Darbois Texier has worked on. In 2012, through a fluid dynamics experiment that dropped steel, glass, and plastic beads of different sizes into water, she and her colleagues found that a sphere in a flow — like a knuckleball — will experience a phenomenon where drag force on the object begins to sharply decrease. A sideways force swoops in that causes “drag crisis” — causing the ball to flutter back and forth and moves forward.

In a newer study published just last month in the New Journal of Physics, Texier and her team used wind tunnel testing to better characterize the behavior of knuckleballs and reproduce their movements in a controlled setting. Using a custom-built “kicking machine” apparatus to recreate knuckleballs in soccer, the team found that “all balls flying in the air at such speed and having no spin may follow a zigzag trajectory, even if they have no seams,” Texier tells Inverse. “This fact proves that a non-rotating and smooth sphere moving in the air experiences fluctuating lift forces which are able to produce non-straight trajectories.”

This is in opposition to a previous notion that explained zigzag trajectories of sport balls with the presence of seams and a small amount of spin. Texier says part of the experiment involved testing the movement of smooth bocce balls dropped into water. “The non-straight trajectories of such dense and smooth balls were really unexpected for us,” she says. “The zigzag wavelength was much larger than the typical shooting distance encountered in a sport field. Such a fact explains why zigzag paths are never observed in bocce.”

A big question remains specific to baseball, however: Would we see knuckleballs if the object was smooth and seamless? Texier doesn’t have an answer to that, but that remains the next question left to answer.

Related Tags