The following is a guest post by the writer of Science in Real Life. Part Two will be posted later this evening.

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(PART 1)
Greetings, sports fans, and welcome to a very special Science In Real Life about the physics of lobbing the leather. I’d especially like to welcome those of you who are reading this on or from This Purist Bleeds Pinstripes, and thank the Purist herself, who had the balls to allow me to write a guest post for her esteemed virtual publication. Hopefully, for you who are getting an exhibition pass to the way we do things over at SiRL, you will enjoy yourself enough to sign up for a season ticket. Now that I’ve made my pitch, get the wax dug out of your ears, and let’s play some hardball.

The physics of baseball as a whole is of course far too extensive to cover in one essay. Fortunately most of the interesting stuff is concentrated in the mechanics of pitching, which is an aerodynamic spectacle unto itself. It also has the advantage of being extraordinary science hidden behind an ordinary phenomenon – we’ve all watched baseball, and we’ve all thrown spherical objects ourselves. What’s to get, anyway? “It just flies through the air!” you might insist. This is akin to insisting that the economy may be accurately described as “It’s just moving money around!” Sure, you can do it, but you miss all the fun that way.

The fluid dynamics of a baseball pitch, in their full glory, are as complicated as those of a hurricane or an episode of Jerry Springer. But we can dissect some of the basic concepts anyway. First we need the idea of a boundary layer. If you’ve ever had to clean the blades of a rotating fan, you might have wondered why it is that they need cleaning in the first place – shouldn’t the dust get blown away? Would that it were so, but friction’s domain extends even to air. Friction ensures that a thin layer of air hugs the blades as they whir; any dust particles short enough to ride inside that layer get a free pass. The same holds for a baseball (or indeed any object moving through air).

Enter the Magnus Effect. If it sounds imposing and impressive, that’s only because it is. Imagine watching a thrown pitch from overhead. Suppose that as it travels to your right, it rotates counterclockwise. The air hits it head-on (apply directly to the baseball) and splits. The air that follows the ball’s rotation stays a coherent boundary layer slightly longer than the air that goes against it. Check out this handy diagram:

( Image Credit )

We can see that one side of the ball there is more turbulent flow, which is scientific jargon for “a giant mess”. In turbulent flow the air molecules push every which way. Since that push is heavier on one side than the other, there is a net force “into” the direction of the ball’s existing rotation. In full rigorous glory, the Magnus effect is stated as: A rotating body with velocity V relative to the fluid experiences a net force perpendicular to the direction of both the velocity and the axis of rotation.

How does this give us fastballs, curveballs, and sliders? Those of you reading PBP will have to wait until she puts the next segment up. Or, you could just follow this link over to SiRL and keep reading there. END SIDE A