What Makes a Clay Court Slow

This piece is pretty much completely about clay court tennis. It's a mere five days until the U.S. Open starts, which makes this piece totally non-topical. Like I said Monday, I've got some karmic debts to repay.

What did it mean, anyway, to say that a court was slow? "Fast" I could figure out with a tennis ball and a lawn--throw the ball along the lawn and notice that it doesn't bounce as high, notice how it kind of slides along the ground. Simple enough.

But no one ever gave an explanation of the physics behind a clay court, and I didn't have a clay court to test things on. I tried bouncing a tennis ball on dirt, but all I ever saw was erratic.

Keep in mind that when I started watching tennis, there was no Google to look things up on. I don't even know how one would have found this information back then. Check out every tennis book in the library and hope that one of them decided to delve into the sport's physics?

Last year, I finally turned the full power of my imagination to the problem, and I came up with an explanation that made sense. When the ball hits the court, it pushes clay particles along with it, which means that friction actually increases as the ball makes contact with the ground, so instead of simply rebounding like on hard courts or sliding like on grass, the ball brakes a little while it's in contact with the clay. Also, because the friction makes the ball bite into the court more, the ball compresses more during its time on the ground, which generates more upward force as it rebounds, which accounts for the higher bounce.

When I finally found a published explanation, it turned out I was close to correct. There's one other thing that contributes to the higher bounce on a clay court: the clay is a mere two millimeters thick, and underneath the clay is a layer of limestone. Limestone returns more of the ball's vertical energy than does the acrylic surface that tops a hard court.

Anyway. Finally equipped with a proper model, and with enough tennis experience to be able to imagine how playing on such a court would be different, I finally began to be able to see, to really see, what set clay courts apart. It no longer seemed stupid that clay court tennis was slower. It just seemed different.

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