Why a Curveball Curves

I was recently told that there is a game called 'baseball' being played in this season of the year. Intrigued, I looked into this game of "bases" and "balls," only to discover a physics mystery.

It seems the curveball moves mid-flight, without anything seeming to have touched it, and changed its trajectory. What could cause this strange effect?

We are told, in physics, that objects can only change their motion when acted upon by a force. In our time, we are so well-versed in the forces around us — gravity, friction, Eddie Izzard's personal charisma — that we don't really understand how people once thought that objects could be transported by sorcery. Then people point to something like the curveball, and all our modern rationalism is thrown for a loop. It turns in mid air! Surely this is the work of a true magician!

Actually, it's about air and wake. Although we don't think about the air much, it's all around us and can exert a lot of force on the objects on earth. The secret of a curveball is to not only throw the ball, but throw it so it has a spin. Make it spin, and make the spin fast enough, and the baseball will swerve over seventeen inches from the time it leaves your hand to the time it crosses the plate.

Why a Curveball Curves

When the ball spins, it exerts different forces on the air in different places. One side of the ball is turning forwards into the overall forward motion of the ball, and is going faster, relative to the air, than the rest of the ball. The other side of the ball is moving backwards, relative to the motion of the ball. It's going slower than the rest of the ball.

Since each side is moving through the air at different speeds, the air reacts differently to each side. The fast-moving side is tough to hold onto, so the air passes by. The slow moving side, however, is easy to hold, and so some of the air it passes through clings to it as it turns. Because the ball is spinning the air that clings to it gets moved, just a little, to the other side of the ball.

If you ever clung to a merry-go-round as it spun for a half turn, you'd get tossed to the ground on the other side as well. As you leapt off the merry-go-round one way, you'd kick it the other way. The air does the same. Since the air, or the wake of the ball, is deflected one way, it kicks the ball in the opposite direction, and the ball curves.

Or it's a tenth-level mage. Could be either one.

Via Exploratorium, The Physics Behind Baseball, and you can enter your own revolutions per minute and see the wake change for yourself at the NASA page.