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In June 2003, the world of baseball was shocked by the revelation that Sammy Sosa of the Chicago Cubs, one of the game’s finest sluggers, had been caught using an illegal bat.

So-called “corked” bats have been hollowed out and filled with a lighter material, such as cork, to disguise the modification. They are illegal because they allow batters to hit the ball further, or so the anecdotal evidence suggestions. The question for science, of course, is whether this effect is real: do corked bats really send balls further?

The reason bats are modified in this way is to make them lighter. This allows the hitter to swing them faster. But if the goal is to give the ball the highest possible velocity as it leaves the bat, lighter is not necessarily better. In fact the collision efficiency, the ratio of ball velocities before and after being hit, is lower for a lighter bat.

There is another factor, the so-called trampoline effect in which the surface of a hollowed out bat deforms and reforms like a trampoline, thereby increasing the elasticity of the collision. This is known to occur in hollow metal bats but whether this holds true for wooden ones is still open.

Whether a corked bat gives and unfair advantage boils down to how these factors even out under the the kind of ball speeds that occur in a real game.

Today, we have an answer thanks to some intriguing work by Alan Nathan at the University of Illinois and a few buddies. They’ve built a cannon capable of firing baseballs in a highly controlled fashion. They’ve used their machine to send balls at baseball bats modified in various ways and then measured the speed at which the balls impact and rebound.

This they say has allowed them to settle the matter.

They have two results. First, they say the trampoline effect is negligible in corked bats. In other words, there is no increase in the elasticity of the bat-ball collision.

Second, they investigated the trade off between higher bat speed and lower collision efficiency and found no benefit to a corked bat.

“We conclude that there is no advantage to corking a bat if the goal is for the batted ball speed to be as large as possible, as is the case for a home run hitter,” they say.

However, there is a caveat. Being able to swing the bat faster allows the hitter to delay the swing for a crucial extra fraction of a second. And this may allow more accurate hits. “So, while corking may not allow a batter to hit the ball farther, it may well allow a batter to hit the ball solidly more often,” say Nathan and co.

That could be a significant effect. The study shows that corked bats don’t allow balls to be hit any further but this has nothing to do with the question of whether corked bats allow home runs to be hit more often.

That’s something that will require a carefully designed study to untangle. In the meantime, corked bats may still confer an advantage, just not in the way everyone thought.

Ref: arxiv.org/abs/1009.2549: Corked Bats, Juiced Balls, and Humidors: The Physics of Cheating in Baseball

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