## Think you know what makes a soccer ball curve? There’s more to it than that.

I find it terrifically interesting that the same player can kick two different balls in exactly the same way and have the two balls curve in different directions.

“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’s all about the Magnus Effect.

This phenomenon was first described by Isaac Newton, who noticed that in tennis, topspin causes a ball to dip, while backspin flattens out its trajectory. A curveball in baseball is another example from sports: A pitcher throws the ball with especially tight topspin, or sidespin rotation, and the ball curves in the direction of the spin.

But it gets more complicated than that.

“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.

Fascinating article.

• Frank

In cricket they call it “reverse swing”. When the ball goes the “other” way. Atmosphere pressure, humidity etc and use of the ball over time can contribute to this.