The Physics of Baseball

Alan Nathan, a physics professor at the University of Illinois at Urbana-Champaign maintains a website devoted to the physics of baseball. In honor of the opening days of the 2009 Major League baseball season, Scientific American recently conducted an interview with him on this subject. An edited transcript of the interview is available on their website. Among the topics discussed are the trajectory of a batted ball:

Typically, if you want to hit a long fly ball, you want to put backspin on that ball. The so-called Magnus force on the spinning baseball will be upward for a ball with backspin, and basically it opposes gravity. It keeps the ball in the air longer so it travels farther. So this leads to batting strategies—you actually undercut the ball. You don't want to hit the ball head-on, which would give you sort of a line drive. You want to undercut it a bit, which gives it more of a vertical takeoff angle and also gives the ball backspin. And the backspin is essential if you want to hit a long fly ball.

On the other hand, if you do the opposite—if you hit on the topside of the ball—generally the ball goes down, so it's going to be a grounder, and it's going to have topspin on it. But topspin itself can lead to interesting things happening. Sometimes a batter can hit a sinking line drive; again, it's another hitting strategy. In this case the ball is hit as more of a line drive, but it's slightly topped, so the ball has topspin on it. Topspin makes the ball fall more rapidly, and that's what you might want to do for a line drive so it falls in front of the outfielder. So there are all sorts of hitting strategies that batters can employ, and I suspect that although they don't think of it maybe quite in these terms, they really do understand what it's all about.

And corked bats:

The idea of corking a bat is that you drill a hole through the barrel end of the bat—maybe a foot deep, maybe about an inch in diameter—and you backfill it with cork. The cork itself doesn't do anything other than hide the fact that you've done this illegal act. The idea is the following: with less weight of the bat at the head of the bat, in the barrel, you're able to swing the bat faster. But it's a trade-off. Because the business end of the bat has less weight, you can swing it faster, but it's less efficient at transferring energy to the ball. And so there's something of a controversy as to whether the net result is positive or negative from the point of view of the batter. My conclusion from the point of view of getting the highest batted ball speed, which is what you want if you want to hit the longest fly ball, is that it's a net loss for the batter. That is to say, what you gain in swing speed does not quite make up for what you lose in the efficiency in transferring energy.

So then you might ask, well, why do batters do this? There's actually a good reason for why they might do it. It's not to get the highest batted ball speed but to have the best bat control. So with less weight of the bat in the barrel, you're much more easily able to manipulate the bat, to get the bat into the strike zone quicker. Batters make the distinction between bat speed—how fast the bat is actually moving when it hits the ball—and bat quickness, which has more to do with the acceleration, the batter's ability to get the bat from where it is initially on his shoulder into the strike zone as quickly as possible. And that means the ability to be able to even change your swing in the middle of the swing, which happens once you get more information on where the ball is going to end up.

So whereas corking the bat very likely does not allow you to hit a longer home run, it might allow you to make good contact more often.

And why a knuckleball can be ridiculously difficult to hit:

The key to understanding the knuckleball is that if a ball were perfectly smooth, the air would flow over it in a fairly smooth fashion. But the stitching on the ball disrupts the flow of air, causing the ball to break. It's not at all the same effect as the effect due to the spinning baseball—it's different altogether. So the key to throwing a knuckleball is that if you throw a ball that's spinning, one way to think about it is that the spinning baseball is averaging over different seam orientations, so there's no net effect due to the air interacting with the seams. But if you throw the ball with almost no spin at all—maybe it rotates a half or even a quarter of a revolution on its way to home plate—then as the air moves over the ball, it's the interaction of the air with those seams that changes the character of the airflow, making it go from a nice, smooth flow to sort of a turbulent flow. That causes local pressure variations which make the ball break, and it breaks in a more or less unpredictable way.

It's really an interesting read and is worth a few minutes of your time. Now if he could only explain what it will take to put the Royals in contention...