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:
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.
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.
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
Forget the pitcher. Will the game be rained out?