Physicists take swing at bat battle

Materials: Some say wood is good, while others insist metal is better at putting a hop into a batted ball.

Medicine & Science

April 21, 2003|By Michael Stroh | Michael Stroh,SUN STAFF

Call it the battle of ping versus thwack.

Concerned about the safety of juiced-up metal bats, the Massachusetts Interscholastic Athletic Association is requiring wood bats in tournament play this spring, reigniting a 30-year debate over the role of aluminum and other metal bats in amateur play.

Officials in Massachusetts point to anecdotal evidence that metal bats not only produce more home runs but also more injured pitchers, who can't duck fast enough to dodge whizzing line drives. Metal enthusiasts say their bats are no more dangerous than wood.

The argument has spilled off the diamond and into the laboratory, where a small group of physicists has been struggling for years to quantify the swat strength of each type of bat. (Collegians can use wood or metal bats, though most choose the latter. Major and minor leaguers are required to use wood.)

The physicists publish papers dense with squiggly mathematical equations and terms like "exit velocity" and "coefficient of restitution."

"It's not a trivial business," says Yale physicist Robert K. Adair, author of The Physics of Baseball.

Anyone who takes a few practice swings with both types of bats will quickly notice the physical difference. Wood bats are solid, with most of their weight concentrated away from the hands, in the fat part of the bat known as the barrel. The weight of a hollow metal bat is distributed more evenly, so players can swing it faster than a wood bat of equal weight.

"Higher bat speed generally means the ball comes off the bat faster," notes Alan M. Nathan, a University of Illinois particle physicist who writes and lectures on the physics of the national pastime.

From a pure swat standpoint, that would seem to be strike one against wood.

Not so fast, says Nathan. Because a wood bat's mass is concentrated in the fat end, where bat and ball generally collide, it imparts more energy to the ball.

So, if metal provides more whoosh and wood more wallop, which is better for hitting?

"That is not an easy question to answer," Nathan concluded in a recent paper. He thinks the two factors likely cancel each other.

That leaves what physicists call the "trampoline effect," which helps so-so hitters with metal bats swat the ball out of the park.

To understand it, consider what happens the moment baseball strikes bat. In NCAA games, pitch speeds can reach 90 miles per hour and beyond. A powerful batter can whip a bat around almost 100 mph. The resulting collision can compress the baseball to nearly half its radius, Nathan observes.

The contact -- which lasts less than a thousandth of a second -- dissipates some of the ball's energy in friction, warming the surface of the bat and ball. Where the rest of the energy goes depends on what the bat is made of, scientists say.

When a ball smacks a stiff wooden bat, it squishes, resulting in a loss of energy. When a ball collides with a metal bat, the bat's thin shell compresses and then springs back -- the trampoline effect.

"You have a lot more energy transferred to the collision," says Adair, who calculates that the ball sails off a metal bat up to 10 percent faster than it does off wood.

The trampoline effect is also well known to professional tennis players, who reduce the tension in their racket strings to improve power, scientists say.

Although researchers are closer to understanding the physics of hitting, they haven't settled the debate over whether metal is more dangerous than wood.

In 1999, the NCAA imposed new rules covering the barrel diameter and weight of metal bats in an effort to make them perform more like wood.

But Joseph Crisco, a baseball researcher at the National Institute for Sports Science and Safety, in Providence, R.I., argues that the effect of these changes is questionable. There are too many variables -- such as how fast can a pitcher dive out of the way -- to draw firm conclusions about how safe the new bats are. And analysis of the NCAA's injury data in 2000 showed that serious injuries were rare. "Baseball," he says, "remains one of the safest collegiate team sports."

Metal bats made of aluminum, titanium and exotic alloys are hollow, lightweight and strong.

Bat compresses energy

On impact, a metal bat compresses slightly, storing some of the ball's energy. Then the bat springs back, transferring that energy back to the ball - an advantage over wood.

A metal bat is consistently strong enough to withstand impact on any part of its surface. That makes it unlikely to break.

Typical cost: $100 to $300

A wooden bat is solid and concentrates its mass at the usual point of impact with the ball.

Sweet spot


Ball compresses

On impact, the bat does not deform, but its concentration of weight at the sweet spot can add to distance.

A wooden bat does not have uniform strength and can break if a ball strikes near the handle or on the wrong part of the grain.

Typical cost: $30 to $50

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