While watching this year’s baseball playoffs, I remembered something someone told me a while ago. Curveballs don’t really curve. It is an optical illusion. Is this really the case? Also, how many different ways can a pitcher really throw the ball? –Mark J. Costello, via the Internet
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The debate over whether a curveball actually curves began maybe 20 minutes after the pitch was perfected by William “Candy” Cummings in 1867. (I follow the account given by LeRoy Alaways in his 1998 doctoral dissertation for the University of California, Davis, “Aerodynamics of the Curve-Ball: An Investigation of the Effects of Angular Velocity on Baseball Trajectories.”) The matter wasn’t resolved quickly. As late as May 1941, in a mock letter to the editors of the New Yorker, one R.W. Madden quoted a baseball sage as saying, “Now I’ll tell you something, boy. No man alive, nor no man that ever lived, has ever thrown a curve ball. It can’t be done.” This declaration, though clearly tongue-in-cheek, begat much acrimonious discussion. A few months later Life magazine, apparently figuring that for one week it could forgo the usual fluff about Japanese maneuvers in the Pacific, published a photographic analysis purporting to show that “a baseball is so heavy an object…that the pitcher’s spinning action appears to be insufficiently strong appreciably to change its course.”
The reason the ball curves involves something called the Magnus effect. It boils down to this: A pitcher throwing a curve imparts spin to the ball. As the ball flies through the air, it leaves a wake behind it. Were the ball not spinning, the wake would be roughly symmetrical, as shown in the left-hand illustration. Since it does spin, the wake is deflected to one side (the side where the spin is counter to the motion of the air rushing past), as shown in the right-hand illustration. Intuition alone (and that failing, the law of conservation of momentum) should convince us that if the forces acting on the ball are such that they deflect the wake one way, they simultaneously push the ball the opposite way. Thus the curve.
Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.
