The physics of a topspin return

?When a ball bounces off the court, it bounces with topspin. In order to return the ball with topspin, the direction of spin needs to be reversed. Suppose that the ball bounces off the court with 3000 revolutions per minute (rpm) of topspin. That is a typical amount. The spin is often larger than that. If a player wants to return the ball with say 2000 rpm of topspin, then he or she needs to change the spin by 5000 rpm. If the spin is reduced by only 3000 rpm then the ball will return without any spin at all.

Changing the spin by 5000 rpm is a job for a professional.

The average player would have trouble changing the spin by 3100 rpm, in which case the ball will be returned with 100 rpm of topspin. Another player might manage 3200 rpm, in which case the ball will be returned with 200 rpm of topspin. That’s twice as much topspin as the first player, so it has twice the effect.

Rafael Nadal, who has the heaviest topspin forehand in the game, can change the spin by 5000 rpm.

Those numbers help to explain why many players are so fussy about the strings they use. If one string generates a spin change of three percent more than another, as in the example just given, then the final result could be a doubling of the ball spin coming off the strings.

Rod Cross, a physicist and academic of Sydney University, is the co-author of Technical Tennis (with Crawford Lindsay).