By Tage Olsin (https://creativecommons.org/licenses/by-sa/2.0/deed.en)

A Simple Guide To Pitches and Spin

We hear that the baseball postseason kicked off last night with the Yankees losing to the Astros in the AL wild card game.  Although our baseball coverage may be minimal, we at the Bench Rapport are big fans of science.  And the duel between pitcher and batter in baseball is fundamentally an exercise in high school physics.  So why does a curveball curve, a rising fastball rise (or does it?) or a sinker sink? 

Guest columnist and armchair physicist Zeb Landsman has the answer. 

There are lots of different names for pitches:  curves, slurves, cutters, sliders, splitters, sinkers, etc.  At bottom, each depends on the spin of the ball.  How a pitcher spins the ball and whether youth pitchers should throw breaking pitches in the first place are subjects for another day.  (This comprehensive study implies that curveballs are not the problem but Dr. James Andrews is not so sure.)

But when working on pitches, you should understand the consequences of spin.  Spin is one of the two forces that change the trajectory of a pitched ball.  The other is gravity, which is always downward.  Understanding the interplay between these two forces is a key to understanding pitching.

Spin applies a force that pushes the ball in the direction that the forward face of the ball is spinning—that is, the face of the ball moving towards home plate.  In physics, this force is called the “Magnus Force.”  Imagine a pitched ball with “backspin.”  This is the classic fastball—now called a “four seam fastball.”  The front of the ball—that part of the ball cutting through the air—is spinning upwards.  The force of the spin, therefore, will push the ball upwards.  Will it curve up?  No.  The opposing force—gravity—is too strong.  (There’s no real “rising fastball.”)  But with backward spin, the classic fastball will drop far less than a ball thrown without spin.

Imagine a ball with “topspin”; the front of the ball is moving down.  The force of the spin will push the ball down, joining forces with gravity to create a severe drop.  This is the classic “12-6” curveball; it curves straight down.

Finally, imagine a ball with sidespin.  Typically, for a right-handed pitcher, the spin will travel from the third-base to first-base side of the diamond, causing the ball to curve that way.  (A “screwball” goes the other way.)  The amount of lateral versus vertical spin will determine the pitch’s place on the breaking ball spectrum.  People often confuse the names, but generally a “curve” is more downward, a “slider” is side-to-side, and a “slurve’ is in between.

Adding spin is not the only way to control the curve of the ball.  Removing spin will also affect downward curve.  As we have seen, a typical overhand fastball has backspin, which fights gravity, and keeps the ball from dropping more than it would otherwise.  If the pitcher can eliminate that spin, or slow it down, the ball will drop more; gravity will have a greater consequence.  But you might wonder, why wouldn’t a pitcher just throw a 12-6 curve if he wants downward movement?  Indeed, a good 12-6 curve will drop the most.  Here’s the answer:  batters see the backspin of the ball and anticipate the pitch.  The spin of a classic curve (as well as the arm action) is easy to identify.  But if the pitcher can make the ball look like a fastball, but drop more like a curve, he’s got a great pitch.  That’s the splitter.  The sinker, too.  They’re thrown with backspin—so they look like fastballs—but not enough spin to counteract gravity much.  (I speculate that the mythical Japanese “gyro ball” used this concept, creating an unfamiliar spin that tricked the batter because it induced no Magnus force.)

One final note about “late break.”  It exists, but not exactly the way people think.

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