Not to turn this into a thing, but the speed of grass vs. hard surfaces is a little complicated, and depends on a number of factors.
The most important factor is the “coefficient of friction,” a measurement of the abrasive force between the ground and the tennis ball. Courts with high frictional coefficients interfere with the movement of the ball, disrupting its forward momentum. Think of a sluggish clay court. According to experiments performed by the ITF, a shot hit without spin and traveling at 67 mph will lose about 43 percent of its ground speed after contact with the clay surface, slowing down to a leisurely 38 mph. (The reason clay steals momentum is rooted in the friction of all that loose brick, which clumps around the ball. Each clump is like a little speed bump.) As a result, players have a few extra milliseconds to hit a return.
In contrast, a shot on a fresh grass court — think of Wimbledon on opening day — will maintain a speed around 45 mph, which is 15 to 20 percent faster than clay. Hard courts are usually a smidgen slower than grass, although the speed of the court depends on the amount of sand mixed in with the acrylic paint. (There are at least 45 different kinds of hard court, some of which play slower than clay. The Australian Open, for instance, is played on a Plexicushion surface, which has a slightly higher frictional coefficient than the U.S. Open.) While grass courts become more sluggish over the course of a tournament — the exposed dirt plays more like brick — hard courts actually accelerate, as the soles of shoes wear down the surface friction, especially around the baseline.
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These angles exaggerate the perceived speed of a court. The friction of clay leads to high bounces, which give players even more time to chase down a shot. In contrast, frictionless grass courts have a low “angle of rebound” — the balls maintain a flat trajectory — which leads players to perceive the court as even faster than it is. In fact, these rebound angles are typically more important in shaping the perceived speed of a court than the actual velocity of the ball. (Hard courts and grass courts often generate the same postbounce velocity, but grass courts seem faster because the ball bounces at a lower angle.) As Howard Brody, a physicist at the University of Pennsylvania, notes in his classic Tennis Science for Tennis Players: “The eye and brain are much better at gauging an angle than observing a slight change in ball speed. If the ball comes off the court at a low angle after the bounce, you conclude that the court is fast because you must act faster.”
The Physics of Grass, Clay, and Cement
