TOP tennis players know that the most powerful serves come from near the tip
of the racket. Now a physicist in Australia has discovered that it鈥檚 not a
bouncy 鈥渟weet spot鈥� but a curious 鈥渄ead spot鈥� on the racket that power servers
like Pete Sampras and Greg Rusedski can find every time.
Rod Cross, a physicist at the University of Sydney and an avid tennis player,
took his racket to the lab to measure the bounce at the spot that seems to give
a superfast serve. But instead of finding a place where the ball bounced best,
he found a spot where it didn鈥檛 bounce at all鈥攁 鈥渄ead spot鈥�. 鈥淭hat seemed
to me to be counterintuitive,鈥� says Cross. 鈥淚 expected it to bounce everywhere
on the racket.鈥�
To measure the bounce, Cross clamped the handle of his racket to a bench top,
leaving the other end free to vibrate. He fitted the strings and frame with
devices that measured the ball鈥檚 impact and the racket鈥檚 movement. Cross then
dropped a tennis ball from 50 centimetres above the racket and recorded how high
it bounced.
Advertisement
The ball bounced highest when it hit the strings about 5 centimetres from the
handle end, or throat. At the centre of the racket, one of the most comfortable
points to use, it bounced less. But when the ball hit one part of the racket
about 5 centimetres from the tip, it tended to 鈥渟tick to the strings without
bouncing鈥�, Cross reports in the American Journal of Physics (vol 65, p
754).
Based on his measurements of impact force and racket vibration, Cross
concludes that the ball鈥檚 energy of motion is transferred entirely to the racket
at that point, just as one billiard ball stops abruptly when it strikes another.
Cross says that this energy transfer works both ways. If the racket is in motion
and the ball is still, all the energy from the swing will be transferred to the
ball. This is why the dead spot gives power to the serve, he says.
But the dead spot is useless for a return, since it would sap the energy of a
ball that was already in motion. Cross says that for a powerful ground stroke,
it is best to hit with the bouncy part of the racket near the throat. But there
is no single spot on the racket that returns the ball with significantly more
energy than anywhere else: 鈥淭here is no sweet spot,鈥� he says.
Cross鈥檚 observations make perfect sense, says Howard Brody, a physicist at
the University of Pennsylvania and an expert on the physics of tennis. He says
that power servers like Sampras use the point that Cross has dubbed the dead
spot all the time. 鈥淚 have watched these guys serve, and they鈥檙e pretty close to
颈迟.鈥�
Andrew Coe, technical administrator for the International Tennis Federation,
agrees. But he argues that some of the extra serving power must arise from the
faster movement of the tip relative to the throat in the swing used for
serving.
