Every year the newest version of the World Cup soccer ball causes a stir. Now physicists have put those balls to the test, to see just how well they fly.
World Cup soccer balls used to be not much different from regular soccer balls—32 leather panels stitched together by hand. But the balls used in World Cup tournaments have been distinctive since 2006, when Adidas unveiled the Teamgeist, made of just 14 panels that had been glued together rather than stitched. The company has been introducing new World Cup balls ever since: the eight-panel Jabulani ball in 2010 and for the 2014 games—which begin on June 12 in São Paulo, Brazil—the six-panel Brazuca.
Will this change in design make a difference in the game? That's what Sungchan Hong and his colleagues at the University of Tsukuba wanted to find out. Making use of some soccer balls, a wind tunnel, and a robot, they found, according to today's Scientific Reports, that a ball's construction really does affect how it flies through the air.
What's most relevant in a ball's movement is drag, a force that makes it dip and curve in unexpected ways. The smoother the ball, the greater the drag at higher speeds—and with fewer panels that are glued rather than stitched together, the World Cup balls have been getting increasingly smoother.
Which is why this year the Brazuca ball is covered in little nubs—an attempt to cut down on the so-called knuckling effect, which causes the ball to move unpredictably through the air like a knuckleball in baseball.
In the wind tunnel, Hong found, the direction in which the ball is pointed, and thereby the direction in which the panels are oriented, changes the drag. This was a problem for the 8-panel Jabulani ball and the 14-panel Teamgeist ball, each of which traveled more unpredictably than a regular soccer ball, curving more or less depending on which way it was oriented.
But the new 6-panel Brazuca seems to be far more reliable, showing very little difference in behavior no matter how it's rotated. This is likely due to the way the panels of the Brazuca are arranged, along with the rough surface Adidas added to cut down on the drag.
The Real World
Still, there's a big difference between a wind tunnel and a field in Brazil, said Simon Choppin, a researcher at Sheffield Hallam University in Great Britain, who was not involved in the study. "The wind tunnel can re-create very particular spin conditions,"he said, "but for a pass or shot, the spin might be completely different. That's the major gap between the wind tunnel test and reality."
Nobody, Choppin pointed out, has done a wind tunnel simulation of a player trying to hook a 40-yard pass around a defender. He added that most plays won't be affected at all: The drag effects Hong and his team saw in the wind tunnel were under very specific, controlled conditions, in terms of temperature, humidity, and the like—all of which evaporate once players step on the field.
But Jay Cooney, an assistant coach with Sky Blue FC, a team in the National Women's Soccer League, said that even on the field, "there's a massive difference between balls." He added that the quirks are usually most apparent in the first-round games, as players adjust to a new ball, "even though announcers usually chalk it up to nerves."
Yet Cooney wouldn't trade the weirdness of the World Cup balls for more reliability: "That's what I think is charming about soccer—it doesn't have to be perfect."
The players, who already have a bit of a reputation for whining, might disagree. But this year, if Hong is right, the Brazuca might not give them as much cause for complaint.