Science In Real Life

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 agains 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.

From there it’s really just a hop, skip, and a jump to the cornucopia of curves charging into the catchers’ claws. By changing which way the ball is gripped, and the angle through which the arm rotates during the throw, a pitcher can change the axis of rotation. In addition, as with many things in life, controlling the speed of rotation is all in the wrists. Pure backspin on a ball results in an upward force, stabilizing the throw into a speedy straight trajectory – a good old fashioned fastball. A rising fastball does not actually rise, of course. That would require a backspin three times as rapid as what major league pitchers can currently achieve. Instead, it plays off our intuition about falling objects. We are so accustomed to seeing objects fall at 9.81 m/s² that a slowly falling fastball appears to rise.

The rest of the classic tools in a pitcher’s arsenal involve some kind of lateral rotation, and hence a lateral deflection relative to the trajectory of a nonspinning ball. In other words: yes, curveballs really do curve. So do sliders, screwballs, slurves, and any of the innumerable variations on that theme. Here is a handy guide to what rotation the hitter will see for the four most common pitches from a right-handed pitcher:

Every pitch has something like this, with two notable exceptions. Those exceptions look like this:

( Image Credit: Science In Real Life )

The gyroball (recently invented in Japan) looks like a fastball, quacks like a fastball, but spins like a duck. I mean, a bullet. To batters expecting the limited drop of a fastball, this can come as quite a surprise. You will notice that the knuckleball there has no arrows – it is thrown without any deliberate spin. A pitcher throwing a knuckleball puts his or her fate into the hands of Papa Physics. Stray fluctuations in wind speed, air pressure, and density can have this pitch wobbling every which way over, or not over, the plate. Without extensive practice, a knuckleball might, well, strike out.