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Bending the Ball: A New Spin on Soccer Shots
Whenever a soccer ball curves, seemingly changing its orbit in midair, NASA scientists may rush to classify it as a planet. But in fact, the movement is due to yet another physics concept pioneered by Isaac Newton: the Magnus effect, a phenomenon first described in regard to tennis. Newton noted that topspin made the ball dip, whereas backspin made for a straighter trajectory. In other words, the way a ball spins affects its path in the air.
Usually, a ball curves in the same direction in which it’s spinning; air moves with the ball on the side that spins forward, pushing it in the other direction. A ball spinning counter-clockwise, for example, tends to bend to the left. But a ball’s movement may not always be so easy to predict. Its trajectory is also affected by the smoothness of the ball’s surface. In fact, a perfectly smooth (or laminar) ball would likely curve in the opposite direction of a rough-surfaced (or turbulent) ball when kicked in exactly the same manner.
Of course, nobody plays soccer with a perfectly smooth ball, despite the substantial benefit it would have on blooper-reel ratings. Even so, different stitching patterns do alter the roughness of a ball’s surface. This is why there seems to be a controversy every four years over a new ball design—even the coolest new look can have a drastic effect on a ball’s in-flight physics.