# Explained how does a soccer ball swerve

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Explained: How does a soccer ball swerve?

16 June 2014, by Peter Dizikes

"Brazuca," has a slightly rougher surface, and may be more predictable.

"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?

Magnus, meet Messi

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, who noticed that in

tennis, topspin causes a ball to dip, while backspin

flattens out its trajectory. A curveball in baseball is

It happens every four years: The World Cup begins another example from sports: A pitcher throws the

and some of the world's most skilled players

ball with especially tight topspin, or sidespin

carefully line up free kicks, take aim--and shoot rotation, and the ball curves in the direction of the

way over the goal. The players are all trying to

spin.

bend the ball into a top corner of the goal, often

over a wall of defensive players and away from the In soccer, the same thing usually occurs with free

reach of a lunging goalkeeper. Yet when such

kicks, corner kicks, crosses from the wings, and

shots go awry in the World Cup, a blame game other kinds of passes or shots: The player kicking

usually sets in. Players, fans, and pundits all

the ball applies spin during contact, creating

suggest that the new official tournament ball,

rotation that makes the ball curve. For a right-

introduced every four years, is the cause.

footed player, the "natural" technique is to brush

toward the outside of the ball, creating a shot or

Many of the people saying that may be seeking excuses. And yet scholars think that subtle

pass with a right-to-left hook; a left-footed player's "natural" shot will curl left-to-right.

variations among soccer balls really do affect how they fly. Specifically, researchers increasingly believe that one variable really does differentiate soccer balls: their surfaces. It is harder to control a smother ball, such as the much-discussed "Jabulani" used at the 2010 World Cup. The new ball used at this year's tournament in Brazil, the

So far, so intuitive: Soccer fans can probably conjure the image of stars like Lionel Messi Andrea Pirlo, or Marta, a superstar of women's soccer, doing this. But this kind of shot--the Brazilians call it the "chute de curva"--depends on a ball with some surface roughness. Without that, this classic piece

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of the soccer player's arsenal goes away, as Bush without wings."

points out in his article, "The Aerodynamics of the

Beautiful Game," from the volume "Sports Physics," In this case, Bush says, "The peculiar motion of a

published by Les Editions de L'Ecole Polytechnique fluttering free kick arises because the points of

in France.

boundary-layer transition are different on opposite

sides of the ball." Because the ball has no initial

"The fact is that the Magnus Effect can change spin, the motion of the surrounding air has more of

sign," Bush says. "People don't generally

an effect on the ball's flight: "A ball that's knuckling

appreciate that fact." Given an absolutely smooth ... is moving in response to the pressure distribution,

ball, the direction of the curve may reverse: The which is constantly changing." Indeed, a free kick

same kicking motion will not produce a shot or pass Pirlo took in Italy's match against England on

curving in a right-to-left direction, but in a left-to- Saturday, which fooled the goalkeeper but hit the

right direction.

crossbar, demonstrated this kind of action.

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.

Bush's own interest in the subject arises from being a lifelong soccer player and fan--the kind who, sitting in his office, will summon up clips of the best free-kick takers he's seen. These include Juninho Pernambucano, a Brazilian midfielder who played at the 2006 World Cup, and Sinisa Mihajlovic, a Serbian defender of the 1990s.

"The boundary layer can be laminar, which is

And Bush happily plays a clip of Brazilian fullback

smoothly flowing, or turbulent, in which case you Roberto Carlos' famous free kick from a 1997

have eddies," Bush says. "The boundary layer is match against France, where the player used the

changing from laminar to turbulent at different spots outside of his left foot--but deployed the "positive"

according to how quickly the ball is spinning. Where Magnus Effect--to score on an outrageously

that transition arises is influenced by the surface bending free kick.

roughness, the stitching of the ball. If you change

the patterning of the panels, the transition points "That was by far the best free kick ever taken,"

move, and the pressure distribution changes." The Bush says. Putting on his professor's hat for a

Magnus Effect can then have a "negative" sign. moment, he adds: "I think it's important to

encourage people to try to understand everything.

From Brazil: The "dove without wings"

Even in the most commonplace things, there is

subtle and interesting physics."

If the reversing of the Magnus Effect has largely

eluded detection, of course, that is because soccer

balls are not absolutely smooth--but they have been Provided by Massachusetts Institute of

moving in that direction over time. While other

Technology

sports, such as baseball and cricket, have strict

rules about the stitching on the ball, soccer does

not, and advances in technology have given balls

sleeker, smoother designs.

There is actually a bit more to the story, however, since sometimes players will strike balls so as to give them very little spin--the equivalent of a knuckleball in baseball. In this case, the ball flutters unpredictably from side to side. Brazilians have a name for this: the "pombo sem asa," or "dove

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APA citation: Explained: How does a soccer ball swerve? (2014, June 16) retrieved 12 October 2021 from

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