Teniska loptica

The Ball Itself

A tennis ball is a small, hollow ball used in games of tennis and real tennis. Tennis balls are covered in a fibrous felt, which modifies their aerodynamic properties. Tennis ball felts comprise wool, nylon, and cotton in a mixture surrounding the rubber edge. The felt delays flow separation in the boundary layer, which reduces aerodynamic drag and gives the ball better flight properties.

The International Tennis Federation defines the official diameter as 6.54-6.86 cm. Balls must have masses in the range 56.0-59.4 g.

Many balls carry a number as well as the brand name, which tells one set from another of the same brand on an adjacent court. Yellow and white are the only colors approved by the International Tennis Federation.

Making the Ball

The switch to the modern method of joining two hemispheres was done to improve uniformity of wall thickness.

Pressure and Testing

Modern regulation tennis balls are kept under pressure, at approximately two atmospheres, until they are first used. Balls meant for high altitudes have a lower initial pressure, and inexpensive practice balls are made without internal pressurization. A ball is tested for bounce by dropping it from a height of 254 cm onto concrete. A bounce between 135 and 147 cm is acceptable at sea level, 20 °C and 60% relative humidity.

Slower Balls

The International Tennis Federation recommends a progression that uses slower balls and smaller courts to introduce the game to adults and children. The slowest balls, marked red or using half-red felt, are oversized and unpressurized or made from foam rubber. The next, in orange, are unpressurized normal-sized balls. The last, in green, are half pressured and normal-sized.

The Sealed Can

Tennis balls are shipped and sold in a pressurized pop-top container. The rubber used isn't airtight, and air can therefore escape from inside the ball to the outside. Putting them in a sealed can forces the ball to retain its internal pressure.

The hollow rubber core is filled with air or gas and pressurized to around 27 pounds per square inch. Sold loose, the balls would quickly lose their internal pressure, whereas sealed in the can they are as pressurized and bouncy on opening as when they were made. Air still finds its way out of the cans, so an old canister may no longer have its bounce. Play speeds the loss, since every hit and bounce forces more air out.

Waste

Explore all seven extraits in the Discovery Set.

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