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A perfectly happy ball is one that bounces to its original height when dropped on a massive, rigid surface. A completely unhappy ball does not bounce at all. In the former case, the coefficient of restitution ͑COR͒ is unity. In the latter case, the COR is zero. It is shown that when an unhappy ball collides with a happy ball, the COR increases from zero to unity as the stiffness of the happy ball decreases from infinity to zero. The COR is independent of the mass of each ball. The implication of reducing the COR of a tennis ball, as a possible means of slowing the serve in tennis, is also considered. It is shown that ͑a͒ the COR for a collision with a racket varies with the impact point and is a maximum at the vibration node near the center of the strings, and ͑b͒ the serve speed is reduced by only about 20% if the COR for a bounce on the court is reduced to zero.

Measurements are presented on the drag and lift coefficients for three relatively smooth balls launched in air and tracked with two cameras separated horizontally by 6.4 m. The ball spin was varied in order to investigate whether the Magnus force would increase or decrease when the ball spin was increased. For one ball, the Magnus force increased. For another ball, the Magnus force decreased almost to zero after reaching a maximum. For the third ball, the Magnus force was negative at low ball spins and positive at high ball spins. For one of the balls, the ball spin increased with time as it travelled through the air.

The pull tension in a tennis string is always monitored while a racket is being strung, but it is dif®cult to measure the string tension in a racket after it has been strung. In this paper, a simple technique is described based on measurements of the vibration frequency of the string plane. The key to this measurement is the fact that the vibration frequency depends primarily on the area of the string plane and not its shape. It is shown that there is a small loss in tension with time after a racket is strung but there is a large decrease in tension during the stringing process. The tension immediately after stringing is typically about 30% lower than the pull tension. Additional experiments are described, showing that the large drop in tension is due to a combination of factors including stress relaxation, frame distortion and friction between the strings.

No abstract