Why Does a Curve Ball Curve and How Does the Spin of a Baseball Cause It To Curve Sideways?

A curve ball travels in a sideways arc, a very small segment of a very large circle, as Dr. Lyman J. Briggs found in a historic study in 1959.

Then eighty-four, a retired head of the National Bureau of Standards, he settled the question and explained the mechanism, using an overhead camera to show the curvature of the flight path.

He then used smoke streamers in a wind tunnel to study regions of higher velocity and lower pressure around curve balls.

He measured the rate of spin by using major-league pitchers throwing balls with flat tape attached, then counting the number of twists in the tape.

Depending on spin and direction, Dr. Briggs found, the ball can curve downward as well as sideways.

As Dr. Briggs explained it at the time, here is how a ball that is pitched so it spins turns into a curve ball:

“Let us imagine that the spinning ball with its rough seams creates around itself a kind of whirlpool of air that stays with the ball when it is thrown forward into still air.

But the picture is easier to follow if we imagine that the ball is not moving forward, but that the wind is blowing past it. The relative motions are the same.

Then on one side of the ball the motions of the wind and the whirlpool are in the same direction and the whirlpool is speeded up. On the opposite side of the ball the whirlpool is moving against the wind and is slowed down.

Now it is well known from experiments with water flowing through a pipe that has a constriction in it that the pressure in the constriction is actually less than in front of or behind it; the velocity is, of course, higher.

“Hence, on the side of the spinning ball where the velocity of the whirlpool has been increased, the air pressure has been reduced: and on the opposite side: it has been increased.

This difference in pressure tends to push the ball sidewise or to make it curve.

It moves toward that side of the ball where the wind and the whirlpool are traveling together.”