Do sluggers swat on spot or swath?

Millions of baseball fans cheer each time Mark McGwire's or Sammy Sosa's bat kisses another home run good-bye. On Sept. 8, the St. Louis Cardinal star broke the 37-year-old, single-season record of 61 home runs with a sizzling drive over the left-field fence. Chicago Cub Sosa matched him a few days later.

While McGwire and Sosa are reaching the bleachers with uncanny ease this season, a physicist half a world away is offering evidence that the fabled sweet spot of their bats is more complex than previously believed.

According to both baseball lore and earlier scientific studies, the sweet spot is composed of one or maybe two points about 15 centimeters from the end of the bat's barrel. Although batters can hit home runs off other parts of the bat, a hit from the sweet spot delivers an exceptionally powerful wallop to the ball without stinging the batter's hands.

Now, Australian physicist Rod Cross of the University of Sydney reports that the sweet spot is really a 3-cm-wide zone, rather than one or two distinct points. Cross measured forces on batters' hands and noted what batters said they felt during impacts at varying distances along the bat. On a bat 84 cm (33 inches) long, the zone extended down from 15 cm from the top, he found.

Struck anywhere, a bat naturally vibrates with distinctive wave patterns, or modes. Previously, physicists identified one sweet spot as the point where the lowest-frequency wave pattern shrinks to zero and where an impact consequently causes minimal vibration in the handle.

Cross reports in the September American Journal of Physics that a higher-frequency wave pattern, which shrinks to zero at a nearby point, is also a sweet spot. "No one had measured these forces before or had identified that there are two equally important modes," he says. Because impacts along the barrel between those two points cause the weakest vibrations in the handle, Cross argues for a sweet zone.

Moreover, the zone concept is attractive, Cross contends, because recoil in the bat handle is minimized during hits near the two points. Such impacts cause the bat's fulcrum to shift to positions under the batter's hands where the torque drops to zero, minimizing the jarring felt in the hands and arms.

Why the interest in baseball Down Under? Although much less popular than cricket, Australian baseball is televised and growing in popularity, Cross says.