The Physics of Grass, Clay, and Cement

Shot of a tennis racket and two tennis balls o...

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The following excerpts are from a great article on

how the ball actually reacts to different court surfaces:

From ESPN’s “Grantland”    By Jonah Lehrer (see the author on Colbert Report below)

* * * * * * * * * * * *

There are more than 160 different types of tennis surfaces.

An investigation into how they impact play on the court.

* * * * * * * * * * * *

The official rule book of tennis is 35 pages long. For the most part, the document is a case study in compulsive precision. Here are a few of the exacting guidelines:

  • If a tournament is played above 4,000 feet, pressureless tennis balls must be “acclimatized” for at least 60 days at the local altitude.
  • The cord holding up the net cannot be thicker than ⅓-inch in diameter, and must be covered by a white band between 2 and 2.5 inches wide.
  • The frame of the racket cannot exceed 29 inches in total length.
  • The organizers of tennis tournaments must announce in advance their ball-change policy.

Amid all this punctiliousness, however, the rulebook contains one glaring omission: There are no rules about the surface of the court. While the boundaries of the space are carefully specified — it must be a rectangle, 78 feet by 27 feet, with a one-inch-wide center service line — there are zero references to the different materials on which the game can be played. It’s as if clay, grass, and hard court don’t exist, as if the composition of the playing field doesn’t matter.

This oversight has profoundly shaped the development of the sport. Because there are no rules about court surfaces, modern tennis is played on a stunning variety of materials, from the crushed brick of Roland Garros to the manicured lawns of Wimbledon. In fact, the International Tennis Federation (ITF), the regulatory body overseeing the sport, currently recognizes more than 160 different kinds of tennis courts, including surfaces made of carpet, clay, gravel, concrete, wood, asphalt, and fake grass.

“No other sport is played on so many different materials,” says Jamie Capel-Davies, a senior project technologist at the ITF who oversees the assessment of court performance. “Furthermore, these surfaces really change the game. The same style of play that might work on clay won’t work on grass or acrylic,1 because the ball will behave very differently. As a result, athletes must constantly think about the type of court they are playing on. It’s a variable that can never be taken for granted.”

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.

But friction isn’t just about pure speed: It also influences the angle of the bounce. When a ball impacts a high-friction surface it undergoes a sudden increase in spin, as the bottom of the ball slows down more than the top. This burst of topspin redirects the momentum of the ball, transferring some of that horizontal velocity in a vertical direction. As a result, the ball seems to bounce straight upwards, hanging in the air.

For the complete article – CLICK HERE

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From Wikipedia – Jonah Lehrer (born c. 1981) is an American journalist who writes on the topics of psychology, neuroscience, and the relationship between science and the humanities. He served as a research assistant at Columbia University in Eric Kandel‘s lab.

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