Why do Curve Balls Curve?

Even the greenest ping-pong player knows that you can make the ball do remarkable things if you can put spin on it. The ball will always curve in the direction in which it's spinning. Professional tennis players utilize this basic principle during their serve, when they put top-spin on the ball to make it drop sharply after it has passed over the net. Golfers can use it to make their ball hook or slice to the left or right to improve their lie or get around a corner of the fairway (although most duffers regard the tendency as a cursed affliction).

Undoubtedly, though, the area of sports where spin on the ball plays the most important role is in baseball. Now that the season is upon us, we will be hearing much about curves, sliders, knuckleballs and spitters. For the time being, let's just talk about the curve.

For many years, there has been active debate over whether a curve ball actually existed, or whether it was just an optical illusion. In 1870, a pitcher named Fred Goldsmith placed three poles along a chalk line from the pitcher's mound to home plate. He then threw a ball which traveled to the right of the first pole, to the left of the second, and then to the right of the third.

Case proved? Well, not quite. In 1941, LIFE magazine set up a system of strobe lights and took photographs of a curve ball on its way to the plate. Their conclusion was that, if a curve ball moved sideways at all, it was to an insignificant degree.

The difference is that modern pitchers throw balls that curve down, and not to the side. The pitcher's logic for this is evident if you stop to think about it: if a batter swings level and the ball breaks to the side, he still has a good chance of connecting somewhere along the length of the bat. If it breaks sharply downward, he finds himself whiffing the air a foot above the ball.

Goldsmith was throwing his ball sidearm, which meant that the ball was spinning in a horizontal plane and moved laterally on its trip to the plate. Modern pitchers deliver their curves overhand so that they are spinning in a vertical plane and drop sharply with little sideways motion. With a three-quarter delivery, the 30 revolution-per-second rotation of the ball will result in both vertical and horizontal components of motion.

The reason that spinning balls curve at all arises from the same effect that holds an airplane in the sky. A Swiss mathematician and physicist named Daniel Bernoulli announced, in 1738, the unlikely finding that air moving rapidly across a surface exerted less pressure than air moving more slowly. Although this may defy logic, it is still the basic principle upon which all airplanes (and birds) are built.

Because the upper part of the wing on an airplane bulges out while the bottom part is flat, the air flowing over the top must travel faster than the air underneath. This creates a low-pressure zone above the wing, which keeps the plane in the air.

But what has this to do with baseball? Well, when a pitcher puts top-spin on a baseball, the raised stitches carry a layer of air with it. The bottom half of the spinning layer moves with the wind created by the ball's flight, but the top half moves against the wind. This means that the air on the bottom will be moving faster than the air on the top. In a curve, the difference in velocities puts greater pressure on the top, pushing the ball down.

The Bernoulli Principle has been recognized now for over two centuries, but perhaps in no two areas has it been so widely utilized as in baseball and aeronautics.