A big drag
We've learned that a batted ball loses velocity by transferring kinetic energy to the air. But the reality of "air drag" is complicated (they don't call this field of science "fluid dynamics" because it's simple...).

When a spinning ball moves through the air, resistance is greater on the side where the spin increases the relative motion of ball against the air. This increases drag, deflecting the ball away from that side. Information from "The Physics of Baseball" (see bibliography).

Thing is, we need variables (changeable factors that can affect the outcome). Let's ogle some variables that affect the trajectory of a ball moving through air:

Air density: Dense fluids absorb more energy. (Why don't they play baseball under water? Because the game would be even slower....) By varying altitude in the applet on the previous page, you were actually varying air density.

Ball density: Baseball rules allow a bit of variation in ball diameter and mass. A dense ball (one with smaller diameter and/or greater mass) packs more mass in the same volume, so it has more momentum at any given velocity. Thus a dense ball travels further because it loses a smaller proportion of its kinetic energy to the surrounding fluid. Density also explains why sticks and stones can break your bones, but Nerf balls can never hurt you.

Ball surface: A rough ball will travel further than smooth one. Huh? Roughness -- whether from stitching or abrasion -- creates a layer of turbulent air that greatly reduces drag. A smooth golf ball would fly only about half as far as the normal variety, whose dimples create turbulence and reduce drag.

Curves ahead
Now that we've (heh, heh) gotten our feet wet in fluid dynamics, how does a curve ball curve? Look again at those stitches on the ball. They don't just hold the thing together. They also roughen it, disturbing the "boundary layer" that forms around the moving ball. Curve balls -- and other fancy pitches, like knuckleballs -- all depend on the influence of stitches on air.

In 1959, former director of the National Institute of Standards and Technology Lyman Briggs used a wind tunnel to settle a long dispute over how much a baseball could curve during the 18 meter (60 foot) throw to home plate. He found that spin, not speed, determined the amount of curve. NIST.

Resistance is a force on a moving object that causes it to slow down. The force of resistance, or drag, is proportional to velocity -- faster objects experience more drag, all other things being equal (which, the cynics might add, is seldom true in fluid dynamics).

Now consider a pitched ball rotating about a vertical axis and approaching the plate. Due to the rotation, one side is moving considerably faster through the air than the other side. The air will exert a greater force on that side, pushing the ball away from it -- toward the side with the slower relative motion.

The result is a curve ball. It's easier to draw than to explain, which is why we drew it above. That's amazing, but does a curve actually "break" (curve faster) near the plate? Yes. Every second it's in the air, the ball deflects sideways at about the same rate. As a result, it moves on a circular trajectory when seen from above. And that means that most of that curving seems to happen at the end of the pitch -- perfect for confusing batters.

One last thing before we swerve away from curve balls. Because the drag force subsides above 70 miles per hour, fast balls make lousy curve balls. Moreover, because a fastball gets to the plate quicker, the drag force has less time to act on the ball, further reducing the curve.

Forget the pitcher. Will the game be rained out?

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