Friday, October 08, 2004

On the mechanics of the breaking pitch... 

Because the next month or so is marked not just by the run up to the presidential elections (which happen every four years) but also by the basecall playoffs and the chance for the Boston Red Sox to win the World Series (which hasn't happened since 1918) I thought it appropos to spend some time thinking about the mechanics of the breaking ball.

It turns out that although complicated in the realm of popular culture, there's an old chestnut which makes the rounds something to the effect that physics can't describe a curveball, the mechanics of the breaking pitch has been well understood for at least 200 years. At it's heart is something called the Magnus effect (see here for some neat pictures and more on this topic): a description of what happens to a spinning entity as it travels through a fluid.

The basic idea is this. Because a baseball is relatively large relative to air molecules it is imperfectly aerodynamic: air tends to separate on the upstream side of the ball and recombine on the downstream. This means that, for a nonspinning ball moving forward through the air, a high pressure zone is created on the upstream and a low pressure one on the downstream. This pressure difference acts to slow the ball's forward progress and to make it fall to earth much more rapidly than it otherwise might.

The trick to throwing a breaking pitch is to throw the ball and set it spinning at the same time. When that happens the boundary layer of air around the ball is disturbed. No longer does air separate at the front of the ball and the currents recombine exactly at the back. Instead the location where the air recombines is shifted (if the ball has back spin the location is shifted down, if top spin it's shifted up). A incomplete but still possibly useful explanation of why this might be important has routinely been used to explain how an airplane flies. The idea is, basically, that the in the case of, for example, backspin the recombination point is shifted down. This means that air that travels over the top of the ball will travel relatively faster (think of it as getting the 'kick' from the spinning ball) than air that travels below, in the process becoming slightly less dense (think of it getting stretched) and hence giving the ball a small amount of lift. To make the ball sink or move sideways is a analogous.

If you think about it this way it seems like it might be tough to make a ball curve, but it turns out that big league pitchers and serious pitching machines can make a ball move something like 18 inches off the path it might otherwise take as it travels from the mound homeward.


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