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
Get a Grip
A pitcher varies the direction of the deflection by varying the
angle of the spin axis. The spin rate and forward velocity of the
ball determine the magnitude of the deflection. To fine-tune his
abilities, a major-league pitcher practices even when not really
throwing. A pitcher with time to kill tosses a ball into his glove
to practice various grips. The pitcher develops a wrist movement and
a grip that is specific for each of the pitches in his repertoire.
For a fastball, a pitcher snaps the wrist directly forward,
releasing the ball with symmetrical force from the tips of the index
and middle fingers. A fastball delivered with an overhand arm motion
produces backspin. That is, the ball's top surface spins back toward
the pitcher, and the bottom spins forward. The Magnus force will
"lift" such a pitch. More accurately, it decreases the
distance the ball falls due to gravity.
To throw a fastball, a pitcher can grip the ball in different ways,
which are described by the position of the fingers relative to the
ball's seams. Actually, the ball has a single continuous
seam—made up of 108 stitches that hold together two smooth
pieces of leather—but this seam curves to fit the surface of a
sphere. If a pitcher grips the ball across the seams, it appears
that four seams pass in front as the ball makes one revolution.
Hence, this is called a "four-seam" grip. If a pitcher
grips a fastball with the seams, it's called a "two-seam"
grip because only two seams appear on the front during a revolution.
Most pitching coaches recommend a four-seam grip for the fastball.
They presume that a seam perpendicular to the trajectory of the
pitch encounters greater air resistance than the smooth surface of
the ball. Therefore, they speculate that a four-seam fastball
encounters greater air resistance than a two-seam fastball, which
might create a stronger Magnus force on the ball. Pitchers assume
this produces a greater lift on the overhand fastball. Indeed,
pitchers have written that the four-seam grip is more effective than
the two-seam grip in producing rising fastballs. However,
wind-tunnel tests have shown no significant differences in lift
between two- and four-seam orientations. Two of us (Baldwin and
Bahill) have explained that the perceived rise of the four-seam
fastball is probably a perceptual illusion.
A fastball, though, can experience more than an upward deflection.
Any delivery that varies from directly overhand will create some
spin at an angle to horizontal, which generates some lateral
deflection. Moreover, sidearm or "submarine" fastballs
tend to have some topspin so these pitches sink more than they would
due to gravity alone.
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