How a Curveball Curves
 Does A Curveball Really Curve?

Curve Ball Physics

The secret to understanding a curveball is the speed of the air
moving past the ball's surface. As the ball spins, its top
surface moves in the same direction in which the air moves. At
the bottom of the ball, the ball's surface and the air move 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? 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.

How far does a Major League Curveball Curve?

A major league curveball can veer as much as 171/2 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 1/6 of a
second) than it takes for the batter to swing the bat (about 1/5
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.

Difference between a Fastball and a Curveball

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.

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