The seams on a baseball play an unexpectedly large role in shifting its trajectory in midair.
How a baseball flies after leaving the pitcher’s hand depends on factors including its speed and spin and even the weather at the ballpark. at Utah State University and his colleagues discovered that the orientation of the baseball’s seams also plays a part, making a bigger difference than expected.
A typical baseball is covered in two saddle-shaped pieces of leather, which are typically stitched together by hand. When it flies, the air molecules close to its surface form a slow-moving layer, called the boundary layer, that surrounds the ball. Later in the throw, this layer separates from the surface and forms a wake behind the baseball.
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To study the effect that the slightly raised stitched seams have on this process, the researchers launched 72 different baseballs with a pneumatic cannon dozens of times each – for a total of more than 3000 throws. They varied the orientations of the seams and applied different amounts of spin to the ball, then analysed the effect these variables had on each ball’s flight by imaging its trajectory as it passed through a smoke-filled container.
When they analysed how the smoke particles moved around each ball, they found that seams can make the process by which the boundary layer separates uneven, which then makes the baseball’s wake – and its motion – shift unexpectedly.
Whenever a baseball’s axis of spin was tilted from the vertical, the boundary layer separated earlier at the seams on one side than the other, and this skewed the ball’s wake and made it trajectory curve. The researchers found the magnitude of this effect to be comparable to the , which is already familiar to pitchers and involves a shift in the ball’s trajectory due solely to its spin. It turns out that the ball’s seams could shift its trajectory an additional several centimetres in any direction over its 18.4-metre journey from the pitcher’s mound to the batter at home plate, Smith said at a presentation of the work at the  in Washington DC on 20 November.
at the University of Sydney in Australia points out that cricket also involves a leather-covered ball with raised seams, and that its trajectory also changes if one part of the ball is smoother than the rest. It is common for the bowler in a cricket match to take advantage of this effect by polishing one part of the ball on their clothing. But the situation is more complex for a baseball because its seams curve, while on a cricket ball they simply follow the equator, says Cross.
At the conference, Smith said the experiments reveal how the seams shift each baseball’s trajectory, but it is difficult to predict how a given ball will fly before it is pitched. The process is stochastic, which means there is some inherent randomness to it. There are also variations in the shape and height of the hand-stitched seams from baseball to baseball, he said.
The way baseballs experience drag, or air friction, is in general something of a physics mystery, says at the University of Illinois Urbana-Champaign. His own prior research has shown that the difference in the number of home runs between seasons can be explained by differing drag properties among baseballs, but it remains unclear why exactly they differ. He thinks that even subtle variations in the seams could be part of the explanation.
