No Soft Serve From This Physicist
As top-ranked tennis players begin competing today at the U.S. Open, they bring to the court years of experience and lucrative endorsement contracts, but probably not much training in physics. Now a report published in this month's issue of the American Journal of Physics offers some scientific backup for what many of these players intuitively know: The best place for a power serve is at a "dead spot" near the tip of the racket.
Rod Cross, a plasma fusion physicist at the University of Sydney in Australia and 50-year tennis veteran, brought his racket into the lab to search for a sweet spot--the supposed place on the racket that will send a ball flying back over the net with utmost oomph. But after clamping the racket handle to the table and dropping balls, he couldn't locate a superior bounce. Instead, he discovered what he calls a dead spot, between the center and the tip of the racket. "It's like dropping a ball of putty," he says. "The ball just stops dead."
To get a better handle on the physics, Cross strapped five piezoelectric accelerometers onto the frame and strings of the racket. By measuring the acceleration of the racket as it recoiled from dropped balls, Cross could calculate the efficiency of the collisions; that is, how much momentum a ball would recover for its bounce. At the dead spot, it turns out, the ball loses much more of its momentum to the racket than if it hits elsewhere. On forehands and backhands, that lost momentum will rattle the racket and your elbow.
The strategy turns around for a serve, though. Because the racket is rotating and the ball is stationary, the dead spot will transfer the racket's momentum efficiently to the ball. Most players learn to serve near the dead spot through trial and error, says Howard Brody, a University of Pennsylvania physicist and author of the book Tennis Science for Tennis Players. But the new work, he adds, has given the dead spot a theoretical grounding. As for Cross, he says that sometimes when he tries to hit with exactly the right part of the strings, "I'm concentrating so hard ... that I mess up the point."