FEATURE ARTICLE
Predicting a Baseball's Path
A batter watches the pitcher's motion plus the spin on the ball to calculate when and where it will cross the plate
A. Terry Bahill, David Baldwin, Jayendran Venkateswaran
And Here's the Pitch …
At last, the moment of truth arrives. The pitcher launches his body
down the mound at you, and his arm suddenly whips out from behind
him. And there is his hand releasing the pitch. The ball is in the
air, spinning and telling you everything you need to know.
As you brace for the pitch, you must evaluate it
quickly—extremely quickly. You must determine its speed and
spin in about one-seventh of a second. In the next one-seventh of a
second, you decide whether to swing and—if you decide
yes—where and when to swing. That leaves just one-seventh of a
second—if the pitch is a fastball—to swing the bat.
In the rest of this article, we will concentrate on the first
roughly 150 milliseconds (about one-seventh of a second) after the
release. In that time, a batter tries to determine the direction and
rate of spin to predict the magnitude and direction of a ball's
deflection. The appearance of the pitch, however, depends on a
pitcher's grip. For example, a two-seam fastball does not look like
a four-seam fastball, although the speed and spin rates of these
pitches are the same.
To prove this, we skewered baseballs on bolts in the four- and
two-seam orientations. The bolts were chucked in electric drills and
rotated at 1,200 revolutions per minute—the typical spin rate
for a fastball—which was measured with a stroboscope. Both
visual observation and photographs show that a four-seam fastball
appears to be a gray blur with thin vertical red lines about
one-seventh of an inch apart and running perpendicular to the spin
axis. These lines are the individual stitches of the baseball, but
even Ted Williams could not see the individual stitches. By
comparison, a two-seam fastball looks different. It exhibits two big
red stripes, each about three-eighths of an inch wide, which are
created by the spinning seams. (For simplicity, our drill-driven
fastballs modeled an overhand delivery. The more common
three-quarter arm delivery would tilt the axis of rotation by 45
degrees.) Those stripes provide easily perceived information for the
batter to determine the angle of the spin and then predict the
direction of the resulting deflection. Therefore, the big difference
between the two- and four-seam fastballs is that—because of
the visibility of vertical red stripes—the batter might be
able to more quickly and easily perceive the spin direction on the
two-seam version.
A video of drills spinning four- and two-seam fastballs shows the
difference even more dramatically. Moreover, the difference in
appearance of the four- and two-seam orientations is even more
apparent for spinning baseballs in our laboratory than it is in this
video. Those differences made us think that something in addition to
the big red stripes distinguishes the two- from the four-seam
pitches. We hypothesized that the difference might relate to the
critical flicker-fusion frequency.
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