By Allan Fuller
July's Quiz Question was:
What is the outer layer of a tennis ball made of ?
Tennis balls are filled with air and are surfaced by a uniform felt-covered rubber compound. The felt delays flow separation in the boundary layer which reduces aerodynamic drag and gives the ball better flight properties. Often the balls will have a number on them in addition to the brand name. This helps distinguish one set of balls from another of the same brand on an adjacent court.
Tennis balls begin to lose their bounce as soon as the tennis ball can is opened. They can be tested to determine their bounce. Modern regulation tennis balls are kept under pressure (approximately two atmospheres) until initially used; balls intended for use at 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 (100 inches) onto concrete; a bounce between 135 and 147 cm (53 and 58 inches) is acceptable if taking place at sea-level and 20 °C (68 °F) with relative humidity of 60%; high-altitude balls have different characteristics when tested at sea level.
Congratulations to Theo Lewis, whose name was picked out of the hat to win July's Quiz - please email firstname.lastname@example.org to arrange the means of receiving your bottle of champagne!