The Contentious Physics of Wiffle Ball

Wiffle ball is a variant of baseball played with a plastic perforated ball. Eight three-quarter-inch, oblong holes cover half the ball’s surface area, while the other hemisphere is uninterrupted. Originally designed to relieve the arm of a young baseball pitcher (the son of its inventor, David N. Mullany), the ball achieves a curving trajectory without requiring the pitcher to impart spin or hurl at top speed. Each ball is packaged with instructions for how to release it in order to achieve various effects—with the perforations up for a straight ball, toward the pitcher’s thumb for a curve, and toward the outer fingers for a slider.

The inventor’s grandsons still run the family enterprise, with a product unchanged since its 1953 launch. Their dad, the pitcher for whom the ball was designed, told The Atlantic in 2002 that the Mullany family believed cutting the holes might create a “weight imbalance” that would cause the ball to curve. To this day, the company insists, “we don’t know exactly why it works—it just does!”

That folksy answer is charming, but a scientific one can foster even greater admiration for this curious ball and the sport that makes use of it.

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Before Wiffle ball, uncertainty over the baseball curveball motivated investigation from both the media and the scientific community. Life magazine commissioned photographic studies of curve balls in 1941, to determine whether the phenomenon was real or an optical illusion. The magazine’s editors concluded it was illusory, enraging pitchers of the era.

In 1953, Igor Sikorsky, the inventor of the first viable helicopter, performed experiments demonstrating that a spinning sphere did indeed experience lateral deflection (that is, “curve”). Although Sikorsky didn’t publish the data, his collaborators did share a summary of findings—among them, that four-seam pitches curved more than two-seam. By 1959, the University of Notre Dame’s F.N.M. Brown produced gorgeous wind-tunnel images of smoke streamlines showing the way a ball with backspin deflected its wake. And that same year, the National Institute of Standards and Technology engineer Lyman Briggs published his own study, concluding that a baseball could indeed arc up to 17.5 inches on its way to the plate. The spinning ball lowered air pressures on one hemisphere, pulling the ball in that direction.

And so, physicists confirmed that a curveball really does curve. But even so, the batter’s perception is different. At the plate, a pitch appears to “break”—jumping or dropping suddenly, rather than smoothly arcing. The neuroscientist Arthur Shapiro has shown that this optical illusion may be due to the way our visual system processes information.

That’s for baseballs, which are made from rubber or cork wrapped in yarn and leather. What about Wiffle balls?

Wiffle balls wouldn’t be possible without the ubiquity of plastic. In postwar America, lab-synthesized plastics flooded consumer markets once they were no longer needed for wartime duties in mortar fuses, parachutes, soldiers’ service-issued combs, aircraft components, or in the Teflon containers used for the Manhattan Project’s most volatile gases. The first Wiffle-ball prototypes were made by cutting holes into the plastic packaging for Coty perfume. Today’s mass-produced Wiffle balls begin life as polyethylene pellets, melted and injection-molded into hemispheres that are then pressure-sealed together.

The asymmetric flow field caused by the Wiffle-ball holes might yield the same result as does the effect on a spinning baseball: a trajectory that curves or bends in the direction of the resulting pressure force. Still, whether the ball tends to curve toward, or away from, those holes is a matter of some contention, actively debated in Wiffle chat rooms and on the field.

Robert Adair, a Yale physicist and the author of The Physics of Baseball, has speculated that the holes, like the stitching on a baseball, accelerate turbulence on the perforated side of the Wiffle ball. Faster airflow may lower the pressure and cause the ball to move toward the holes. However, the Brooklyn College professor Peter Brancazio has countered that scuffing a Wiffle ball “essentially takes the holes out of the equation.” If the smooth, unperforated side of the ball were sufficiently roughened, it might disturb the air more than the holes, reversing the pressure asymmetry and causing the ball to curve away from the holes.

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My own study of baseball aerodynamics began as a trap meant to lure my undergraduate students toward the beauty of fluid mechanics. We skewered some baseballs and Wiffle balls and used a wind tunnel to measure the forces—lift, drag, and side or lateral forces—as functions of things like the airspeed and spin rate. For the Wiffle ball, we also varied the orientation of the ball with respect to the airflow, making our own version of the manufacturer’s pitching instructions.

With perforations on either side of the ball, we found that the Wiffle balls experienced a lateral force that generally acted to push the ball toward the position of the holes. Things got more complicated when the perforations were on the upstream portion of the ball. As shown in the first image below, fog traces the airflow over a ball with the holes facing the flow, with a symmetric wake pattern suggesting that if we untethered the ball it would fly straight. The second image shows the flow over a ball with its holes facing up, and a wake that is deflected upward, meaning that the ball is experiencing a downward force.

Unlike a baseball, air can flow through a Wiffle ball. Our results suggested that some airflow is captured within the ball, and that this captured air creates a “trapped vortex” effect that also induces a force on the ball. This effect can either compete with or complement the asymmetric pressure distribution outside the ball due to the perforations. So, we measured the airflow inside the balls in the wind tunnel, and also performed computational fluid-dynamics calculations to confirm this (below).

Which effect—internal or external flow—dominates the ball trajectory depends on the pitching speed, and how much spin the pitcher puts on the ball. Especially at moderate speeds, nuances such as ball scuffing can be decisive.

Publishing a study of Wiffle-ball aerodynamics reveals just how many people care about Wiffle ball, and how deeply. I learned about Wiffle leagues full of adult players, and a tournament in which pitchers hurl lightweight Wiffle balls at 80 to 90 miles per hour, with wicked curves and drops. This isn’t just tweens trying to avoid elbow injuries or broken windows; Wiffle ball is serious. Players sent me modified Wiffle balls with handwritten notes describing the effects the alterations had on their pitches. Some of the notes implied (or insisted on) a challenge: See if your fancy wind tunnel can figure this out.

There’s a hot-rod culture at work in the customization of Wiffle balls. It’s legal in many adult leagues, too. Scuffing will rough up the surface, creating a locally turbulent airflow and a reduced pressure that pulls the ball toward the scuff. Some players use a knife to change the shape of the Wiffle perforations, demonstrating their intuition that the airflow into the ball might be important. Indeed, this alteration can reroute or accelerate the entering air, changing the ball’s eventual trajectory due to the trapped vortices inside the ball. Blocking some of the perforations will have similar effects.

Pitching with top, back, or sidespin—as a former baseball pitcher is likely to do when taking up Wiffle ball later in life—makes things even more interesting. And the unhittable knuckleball resulting from a Wiffle ball lobbed with its perforations toward the batter is a natural effect of the uneasy balancing act being performed by these competing effects of internal and external aerodynamics.

The Mullany family and their company have kept the ball itself above the fray of modifications, never varying the features of the product or marketing any “air effects” kits. Players have full ownership of their modifications, and the company’s lack of involvement lends a sense that the player scuffing or knifing a ball is a bit of an outlaw, rebelling against the rigidity of baseball and its regulations.

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The Wiffle ball was designed to increase access to the thrills of baseball. It made trick pitches available to younger players, and also made the game accessible to adults whose ball-playing glory days are now behind them, offering an experience similar to baseball without the risk of rotator-cuff injury. The Wiffle Ball company has cultivated this all-American, family-run story with their “it just works, who could possibly explain it?” mystique. There is innocent nostalgia baked into the game. And yet, understanding how the ball works its magic takes none of that folk glory away. Instead, it increases respect for the sport, by showing how much is possible within it.