Balls are also classified as fast, medium or slow. An important consideration for ball speed is the height and type of fabric on the outside. Balls with more fuzz have more air resistance, travel more slowly, and in rainy conditions, the cover material fluffs and further slows the game. Serious players use different felt thicknesses in different altitudes to increase or decrease the air resistance. If the felt thickness flattens in the middle of the game from intense volleying or wear, the ball will go faster too.
Professional players can hit serves as fast as 135 mph. When a ball is hit with that much force, an engineer must understand what happens during the impact. How does the ball deform and how does that affect its resulting performance characteristics? After considerable deformation, can the ball be used the next day? Will it offer the same spin ability or, more importantly, will it impact the present match?
To answer some of these questions, the United States Tennis Association (USTA) uses a “Stevens machine” to compress the tennis balls. Each ball is squash-tested, or compressed, for 10 seconds and then checked for deformation. If the ball does not return to a round shape, it is rejected by the USTA.
Engineers also often test tennis ball aerodynamics in a wind tunnel, which blows air over the tennis ball to determine how the forces act on it. For example, if the tunnel blows air over the tennis ball at 135 mph it simulates a ball served at 135 mph. Wind tunnels provide engineers with important aerodynamic data that would be close to impossible to obtain any other way.
According to Penn, a tennis ball begins its life as a mound of powder that forms the core. The type of play the ball is made for determines the ingredients in the core. For example, the extended life ball has titanium mixed into the powder to allow it to last longer. The beginner’s ball has a softer core to allow the ball to stay in play longer and give the player more control.
The powder is shaped into pellets and placed in a mold that makes half of the ball. Two halves are glued together, or fused, and a machine injects one atmosphere of air pressure. Finally, the ball cover, made of nylon, cotton, felt and wool, is bonded to the core. The balls are then packaged and shipped to the stores.
Tennis ball manufacturing companies most often hire mechanical, materials, chemical, aerospace and manufacturing engineers.
Celeste Baine is the author of "Is There an Engineer Inside You?: A Comprehensive Guide to Career Decisions in Engineering".