|Litell, Mifflin. Integrated Mathematics. Evanston, IL: 1995.||"Ball (tennis), Diam. 6.5 cm, 57 g, hollow interior construction and a cloth covered exterior construction"||57 g|
|Tennis. Americana Online.||"The ball is a cloth-covered rubber sphere that is generally yellow in color, approximately 2½ inches (6.35 cm) in diameter and 2 ounces (56.69 grams) in weight"||56.69 g|
|Sports A to Z: Tennis: Equipment. United States Olympic Committee.||"A tennis ball is hollow and composed of inflated rubber covered with a fabric. It is between 2½"and 2 5/8"(6.35 and 6.67 cm) in diameter and weighs between 2 and 2 1/16 ounces (57.7 and 58.5 grams)."||56.7–58.5 g|
|New Encyclopedia Britannica. 15th ed. 1997.||"The ITF specifies that the ball must be between 2½ inches (6.35cm) and 2 5/8 inches in diameter and between two ounces (56.7 g) and 2 1/16 ounces in weight"||56.7 g|
|World Book Encyclopedia. Chicago: World Book, 1998.||"It must weigh more than 2 ounces (56.7 grams) but less than 2 1/16 ounces in weight"||56.7–58.5 g|
Tennis is an outdoor or indoor game, which is played on a rectangular court by two persons (known as singles) or by four persons (doubles). Rackets are used to hit the ball back and forth across a net. The object of the game is to score points by striking the ball in a way that an opponent can not successfully return it. The origins of the game dates back to the Stone Age, when humans first used their clubs to hit rocks back and forth across barricades of dirt and stone. The tennis court is 78 feet (23.8 meters) long, and its divided into equal sides by a net that stands 3 feet (0.9 meters) high at the center of the court. For singles, the court is 27 feet (8.2 meters) wide. For doubles the width is increased to 36 feet (11 meters). A tennis ball is hollow and composed of inflated rubber, covered with a fabric which must be uniform. Its between 2½ and 2 5/8 inches (6.35 and 6.67 cm) in diameter and weighs between 2 and 2 1/16 ounces (57.7 and 58.5 grams). Yellow and white balls are used in competition. When dropped on concrete from a height of 100 inches (2.5 meters), they must bounce upward from 53 to 56 inches (135 to 147 centimeters). The serve is the most important stroke in tennis. There are no weight restrictions on the racket and it is usually strung with resilient gut or nylon. The surface of the court may be grass, clay, or some kind of hard material which is usually concrete, asphalt, synthetic turf, or wood. The court surface is an important element in tennis because balls bounce higher or lower, faster or slower, depending on the surface they strike. There is a tendency to slide on clay while harder surfaces are ideal for quick starts and stops.
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