The spin racket: Ping-pong champs are intuitive masters of fluid dynamics

Curve balls may help a pitcher strike out batters in baseball; and some nasty spin can make an opponent sweat to return a tennis serve. But more so than in any other ball game, in table tennis -- where the ball is so light and so small -dedicated players must master the physics of spin.

"While [the spin] strategy is also employed in baseball, tennis, cricket, and sometimes soccer, it absolutely dominates the game of ping-pong," says David R. Dowling, professor of mechanical engineering at the University of Michigan in Ann Arbor. "A straight-up fire-throwin' fast ball pitcher may be successful in baseball. However, the equivalent table tennis player, who does not use spin to make the ball's trajectory curve, may easily be defeated by a table tennis player who does use spin."

The reason spin is such a dominating force in ping pong can be seen when you consider that the ball is actually traveling through a fluid, in this case air. In physics terms, when a ball travels without spin, the air moves over the top, bottom, and sides at the same speed and the pressure forces are balanced. However, when a ping pong ball spins as it travels through the air, these pressure forces become imbalanced, causing the ball to veer off to the side or move higher or lower as it flies. Ping-pong players use these imbalanced forces to control a ball's path.

For example, imagine a ping-pong ball somersaulting through the air with top spin. In this case, the top of the ball is moving "into" the wind and the bottom is moving "with" the wind, so the wind's speed is relatively greater on top of the ball than under it. The faster and slower wind speeds translate to higher and lower fluid pressure on the ball. This pressure difference causes a spinning ball to curve toward the side with lower pressure. In the case of a ball with top spin, that means downward.

The phenomenon, which fluid dynamicists call the Magnus effect, can send a ping-pong ball curving up, down, left, or right -- always at a right angle to the direction of motion -- depending on which way the ball is spun.

Fluid dynamics' importance for the sport becomes clearer at high altitudes, where there are fewer air particles for a ball to collide with. Here, the thinner atmosphere reduces an object's air resistance and weakens the Magnus effect, causing a ball with top spin to travel farther than it would at sea level, says environmental fluid mechanics expert Jorge Escobar, an assistant professor at Javeriana University in Bogotá, Colombia, and a competing table tennis player for 15 years. In his city, 9,000 feet (2,745 meters) above sea level, even experienced players might find their returns flying far beyond the edge of the table, Escobar says.

Play depends on more than the density of air; the shape and weight of the ball, as well as the distance it has to travel, also affect how much control a player can exert. A larger or heavier ball could travel the relatively short field of play of a ping-pong table without much spin-induced trickery.

But the smaller and lighter table tennis ball is at the mercy of the Magnus effect -- and inexperienced players are at the mercy of opponents who can skillfully exploit it.

Of course, players don't have to understand the physics behind these effects to take advantage of them in a game. But "if you know a little bit [of] the science behind it, you have a better sense about what you're doing," says Hassan Masoud, mechanical engineer at the Georgia Institute of Technology in Atlanta and a table tennis player himself.

To win at ping-pong, explains Dowling, players have to do something that their opponents are not expecting. Sometimes it's a question of just keeping the ball on the table. Sometimes it's an offensive or defensive move designed to confuse an opponent. Players can "chop," for example, imparting spin to the ball by swinging their rackets so that the ball almost rolls along the surface of the rubber. Or they can "block," stopping a spinning ball and sending it back at the offensive hitter with whatever spin they used themselves. Aggressive players will impose "crazy, crazy spins" on a ball, Escobar says; they might even try to fool their opponents by faking a hand or wrist movement to hide the type of spin they have imparted.

For Olympics spectators interested in watching the fluid dynamics of ping-pong in action, a birds-eye view can reveal the side-to-side curving of a well-spun serve. Dowling also recommends keeping your eyes on the receiver rather than the hitter. In a game that moves so quickly, players must pay attention to how their opponents are hitting the ball and what type of spin they use.

"The player who's about to hit it has already decided what they're going to do," Dowling says. "The receiving player has to be in position and ready, or they're going to be aced."

Dowling and Masoud are both members of the American Physical Society's Division of Fluid Dynamics.