Baseball hitters who prefer aluminum bats to wood have a list of reasons: Aluminum bats are more durable, they're more forgiving because they don't break when you hit the ball on the handle, and they have a larger "sweet spot"--the area that produces the longest, hardest drives. Add to that the "trampoline effect," a bit of sports physics that gives the ball an extra boost off the bat.
Energy
When a swung baseball bat comes into contact with a pitched ball, the ball reverses direction in about 1 millisecond, or one-thousandth of a second, according to a paper presented to the International Sports Engineering Association in 2004. During that incredibly brief time, the ball flattens considerably before expanding back to its normal size. Most of the total kinetic energy of the collision goes into compressing the ball, and up to 75 percent of it is lost. Some energy, of course, goes into driving the ball the other way--that is, putting it in play. And some goes into the bat.
Types
A wooden baseball bat, usually made of solid ash or maple, compresses slightly in the collision, but most of the transferred energy simply bends the bat--and breaks it if the bat bends at a weak point. A bat made of aluminum or some other metal, however, is a hollow tube. While it might bend slightly, most of the transferred energy goes into compressing the tube. And just as the compressed ball pops back to its original shape within a fraction of a second, so does the bat.
Effect
When the aluminum bat returns to its original shape, it releases most of the energy that had gone into compressing it. This all happens so fast that the bat is still in contact with the ball--and some of that released energy transfers back into the ball, giving it even greater speed off the bat. This is known as the "trampoline effect." Think about jumping on a trampoline. Energy from the impact of your body causes the flexible surface to stretch; when the surface regains its shape, it transfers some of the energy back to your body, throwing you into the air.
Considerations
The energy from the trampoline effect can make batted balls come back at fielders--particularly pitchers--faster and harder. This is a key reason behind concerns that aluminum bats are more dangerous than wood bats. Because of this, amateur baseball organizations that use metal bats--the major and minor leagues use only wood--have adopted standards based on a calculation called "bat exit-speed ratio," or BESR. To be approved for use, a metal bat must have a BESR comparable to wooden bats. Manufacturers can dampen the trampoline effect and make the ball come off a metal bat more slowly by adjusting the bat's construction.
Misconception
Some players cheat by "corking" their wood bats, or drilling a hole roughly 1 inch wide and 10 inches deep in the bat barrel and filling it with a lightweight material such as cork. Since most of the power of a swing comes from the bat's speed rather than its weight, a lighter, easier-to-swing bat translates into more power. But some also believe that the hollowed-out "corked" barrel produces a trampoline effect. The 2004 paper looked into this possibility and concluded that the wood walls would be far too thick to produce such an effect.
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