This is from “The Physics of Baseball”
Ever thought about corking your bat?
Batters swinging at baseballs have to swing the big ol’ bat fast, if they want good things to happen. Heavy bats make for hard work. For the last 100 years, players have tried to quicken their bats by breaking the rules. It’s tricky, and it doesn’t work very well. And as if I’ve got to tell you, it’s science.
For years, perhaps as long as the game’s been played, players have tried to add a little more spring to their swing by filling their bats with springy cork, and sealing them up cleanly in hopes that no one would notice.
Why cork? If you’ve ever fingered a flat-headed thumb tack, or muscled a handle-headed push pin into a cork bulletin board, you’ve probably noticed that the material, cork, is soft and spongy. Similarly, if you’ve ever knocked your knees with an ash wood baseball bat, you’ve undoubtedly noticed that it’s not especially soft. It’s bruisingly hard, very hard. So, it might stand to reason that a hard bat isn’t as resilient as a rod of cork. A ball bouncing on cork might rebound more robustly than one knocking on wood.
A cork-lined bat might also be a lighter weight, which would make its swing speed a lot faster than that of a heavy, old, wooden shaft. It’s easy to think that together, the happy springiness and the gossamer lightness, might help a batter drive the ball through the box and beyond. After all, cork is typically around four times springier than hard wood. But that’s not always what you’re swinging for. There are some, uh … other factors.
First of all, modifying the bat by drilling a hole and replacing the wood with another substance is illegal. Secondly, the cork hurts you. It would take a little speed and distance off your hit. Why? Well, the springiness of the cork cannot store energy from the pitch, because it takes too much time. A ball smooches a bat for about millisecond, a thousandth of a second. That’s all the time you get to direct a hit. Hmmm.
Think of it this way: Say you tore an old mattress open and extracted a single bed spring. Then you stapled this spring to your desk, or better yet, someone else’s desk. When you drop a baseball onto the spring, it compresses and springs back. If you do it from the right height, it’ll start oscillating. It will go up and down at what’s called it’s “natural frequency.” Notice that it takes time for the ball to compress the spring and for the spring to lift the ball. It takes time to store energy in the spring. The most efficient speed that any energy stored in the smooshed spring can be returned to the ball is around the spring & ball combo’s natural frequency.
Now, try dropping the ball on something stiffer, say your head. It won’t oscillate, but it will hurt – just ask Jose Canseco. Instead, the ball will thud and roll away. The natural frequency of your head, your skull-brain assembly, is much higher than the spring. Your head is stiff like wood – like a wooden bat.
The natural frequency of wooden bats is around 250 cycles per second, or 250 Hertz. Because the ball leaves the bat so soon (a millisecond), the energy transfer to the ball is not too efficient. If the bat has been hollowed and corked, it’s no longer as stiff, and it will get an even lower natural frequency and an even less efficient transfer of energy to the bat. The baseball bounces off the bat, faster than the cork can store the energy that could be put back in the ball. The cork might deaden the sound of a hollowed out bat, but it doesn’t propel the ball. It can’t. So, balls hit with corked bats don’t go as far.
Now, there is another factor, to be sure. A lighter bat is easier to swing, especially a bat that’s been lightened on the business end. But as we like to say in physics class, “it turns out that” one could get the very same very bearable lightness of bat by choking up about 1 inch (2.5 centimeters). Or, if you want to get tricky, just shave 0.1 inches off (3 box tops wrapped on the bat’s worth) the diameter of a regular bat. This loss of diameter may be a problem for those who want to get out by hitting pop-ups. For the rest of us, it’s the way to go, especially when it’s time to sign your major-league bat maker’s contract.
But, if you worried that choking up will hurt your chances on those outside pitches, there’s still no need to cheat. It is also perfectly legal to lighten one’s bat by hollowing out the end of a wooden bat. I’m sure you’ve seen the bats used by sluggers such as Jay Buhner. The end of the bat looks like it’s been cut through with an ice cream scoop. I recently visited the Louisville Slugger Bat Museum and saw many, many such hollow-ended bats. Most hollowings have a much bigger effect than corking because the manufacturer has removed way more material than is removed during the corking process.
The feel of a bat is a strange and wonderful thing. Ripping it’s heart out and stuffing it with dried bark is almost, uh … unnatural. It won’t push the ball out to the fence. It’s a natural frequency transfer of energy phenomenon. It’s science.
