The secret to understanding the curveball is the speed of the air
moving past the ball's surface.
The ball in the photo is spinning clockwise, like a ball thrown with
backspin. As the ball spins, its top surface is moving in the same direction
in which the air is moving (from left to right in the picture). At the bottom
of the ball, the ball's surface and the air are moving in opposite directions.
So the velocity of the air relative to that of the ball's surface is larger
on the bottom of the ball.
What difference does that make? Well, the higher velocity difference puts
more stress on the air flowing around the bottom of the ball. That stress
makes air flowing around the ball "break away" from the ball's
surface sooner. Conversely, the air at the top of the spinning ball, subject
to less stress due to the lower velocity difference, can "hang onto"
the ball's surface longer before breaking away.
As a result, the air flowing over the top of the ball leaves it in a
direction pointed a little bit downward rather than straight back. As Newton
discovered almost three hundred years ago, for every action there is an
equal and opposite reaction. So, as the spinning ball throws the air down,
the air pushes the ball up in response. A ball thrown with backspin will
therefore get a little bit of lift.
Now imagine that the photo was taken looking down from above on a ball
spinning clockwise (rather than being viewed from the side). The ball would
curve toward the right, toward third base, from the pitcher's point of view.
A major league curveball can veer as much as 17 inches from a straight
line by the time it crosses the plate. Over the course of a pitch, the deflection
from a straight line increases with distance from the pitcher. So curveballs
do most of their curving in the last quarter of their trip. Considering
that it takes less time for the ball to travel those last 15 feet (about
16 of a second) than it takes for the batter to swing the bat (about 15
of a second), hitters must begin their swings before the ball has started
to show much curve. No wonder curveballs are so hard to hit.
One important difference between a fastball, a curveball, a slider, and
a screwball is the direction in which the ball spins. (Other important factors
are the speed of the pitch and rate of spin.) Generally speaking, a ball
thrown with a spin will curve in the same direction that the front of the
ball (home plate side, when pitched) turns. If the ball is spinning from
top to bottom (topspin), it will tend to nosedive into the dirt. If it's
spinning from left to right, the pitch will break toward third base. The
faster the rate of spin, the more the ball's path curves.