Balls stick, shoes slide: Serving up tennis court physics

August 29, 2001

Not all tennis surfaces are created equal, as Jennifer Capriati found out at Wimbledon in July. After winning the Australian Open on hard courts, and French Open on clay courts, Capriati lost on the grass of Wimbledon in the semi-finals. Now a physics professor at the University of Sydney is looking into some of the peculiar properties of clay tennis courts - which may explain why players like Capriati are able to triumph on clay courts, but may stumble on grass.

"Different surfaces are characterized by their speed and bounce," says Professor Rod Cross. In the September issue of The Physics Teacher, Cross explains that "clay courts are covered with a layer of fine sand that allows a player to slide into and out of a shot much more easily than on grass." But Cross found that while the player can slide more easily, the sand actually causes the ball to "stick" to the court, slowing it down. "In this sense," says Cross, "clay courts can be either fast or slow depending on whether we are referring to the ball or the player."

A player can slide without slipping, says Cross, because the very fine sand on clay tennis courts acts like ball bearings under a player's shoe. "If the grains were perfect spheres," he explains, "the surface would be extremely slippery." But sand actually has pointy bits with rounded ends, so it doesn't end up being too slippery. On the other hand, the cloth of a tennis ball actually captures and drags sand when it hits the court, making it behave like sandpaper, slowing it down.

Players are good at adapting to different surfaces, says Cross. "On a fast grass court, they serve as fast as possible; on clay, they drop the serve speed in order to put more spin on the ball." The result is a fast, low bounce on grass, which is hard to return, and on clay a slower, higher bounce, which is also hard to return.

And hard courts like the ones used in the US Open? They're somewhere in between. "Hard courts are medium speed," says Cross, "and medium to high bounce." And that could be good news for Jennifer Capriati - who has already shown her strength on hard courts this year.
Rod Cross
School of Physics, University of Sydney

American Institute of Physics

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