C&B Notes

Chute de Curva

Much like a curveball in baseball, the ability to shape a shot or pass depends on how the seams and surface area of the ball interact with the air while in flight.  The presence of this Magnus Effect can lead to spectacular goals — including one by Messi earlier this week and a legendary free kick by Roberto Carlos in 1997.

“The details of the flow of air around the ball are complicated, and in particular they depend on how rough the ball is,” says John Bush, a professor of applied mathematics at MIT and the author of a recently published article about the aerodynamics of soccer balls.  “If the ball is perfectly smooth, it bends the wrong way.”  By the “wrong way,” Bush means that two otherwise similar balls struck precisely the same way, by the same player, can actually curve in opposite directions, depending on the surface of those balls.  Sound surprising?

It may, because the question of how a spinning ball curves in flight would seem to have a textbook answer: the Magnus Effect.  This phenomenon was first described by Isaac Newton, whtrajectory. A curveball in baseball is another example from so noticed that in tennis, topspin causes a ball to dip, while backspin flattens out its ports: A pitcher throws the ball with especially tight topspin, or sidespin rotation, and the ball curves in the direction of the spin.  In soccer, the same thing usually occurs with free kicks, corner kicks, crosses from the wings, and other kinds of passes or shots: The player kicking the ball applies spin during contact, creating rotation that makes the ball curve. For a right-footed player, the “natural” technique is to brush toward the outside of the ball, creating a shot or pass with a right-to-left hook; a left-footed player’s “natural” shot will curl left-to-right…

“The fact is that the Magnus Effect can change sign,” Bush says.  “People don’t generally appreciate that fact.”  Given an absolutely smooth ball, the direction of the curve may reverse: The same kicking motion will not produce a shot or pass curving in a right-to-left direction, but in a left-to-right direction.

Why is this?  Bush says it is due to the way the surface of the ball creates motion at the “boundary layer” between the spinning ball and the air.  The rougher the ball, the easier it is to create the textbook version of the Magnus Effect, with a “positive” sign: The ball curves in the expected direction.  “The boundary layer can be laminar, which is smoothly flowing, or turbulent, in which case you have eddies,” Bush says.  “The boundary layer is changing from laminar to turbulent at different spots according to how quickly the ball is spinning.  Where that transition arises is influenced by the surface roughness, the stitching of the ball.  If you change the patterning of the panels, the transition points move, and the pressure distribution changes.”  The Magnus Effect can then have a “negative” sign.