It happens every four years: The World Cup begins and some of the most skilled players in the world carefully line up free kicks, aim and shoot well over the goal.
The players all try to bend the ball into an upper corner of the goal, often over a defensive wall and out of the reach of a goalkeeper. Yet when such blows go wrong at the World Cup, a blame game usually sets in. Players, fans and experts all suggest the tournament’s new official ball, introduced every four years, is to blame.
Many people who say this may be looking for excuses. And yet, researchers believe that subtle variations between soccer balls affect the way they fly. Specifically, researchers increasingly believe that one variable really differentiates soccer balls: their surfaces. It is harder to control a smoother ball, like the much-discussed “Jabulani” used at the 2010 World Cup. The new ball used at this year’s tournament in Brazil, the “Brazuca”, has larger seams. 50% longer, a factor that makes the ball less smooth and apparently more predictable in flight.
“The details of the airflow around the ball are complicated, and in particular they depend on the roughness of the ball,” says John Bush, professor of applied mathematics at MIT and author of a recently published article on 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 hit in exactly the same way, by the same player, may in fact bend in opposite directions, depending on the surface area of those balls. Does that surprise you?
Magnus, meets Messi
This is possible, because the question of how a spinning ball curves in flight seems to have a textbook answer: the Magnus effect. This phenomenon was first described by Isaac Newton, who observed that in tennis, the topspin plunges a ball, while the backspin flattens its trajectory. A curved ball in baseball is another example of the sport: a pitcher throws the ball with a particularly tight topspin, or side spin, and the ball curves in the direction of the spin.
In football, the same thing generally happens with free kicks, corners, crosses from the wings and other types of passing or shooting: the player kicking the ball applies a spin during contact, creating a spin that bend the ball. For a right-handed player, the “natural” technique is to brush outward on the ball, creating a shot or pass with a right-left hook; the “natural” shot of a left-hander will curve from left to right.
So far so intuitive: Football fans can probably conjure up the image of stars like Lionel Messi, Andrea Pirlo or Marta, a women’s football superstar, doing this. But that kind of shot – the Brazilians call it “curvy drop” – depends on a bullet with a certain surface roughness. Without this, this classic piece of the footballer’s arsenal will go away, as Bush underlines in his article “The aerodynamics of the beautiful game”, taken from the volume “Physics of sport”, published by Les Éditions de l’École. polytechnic in France. .
“The point is, the Magnus effect can change signs,” says Bush. “People generally don’t like this fact.” With an absolutely smooth ball, the direction of the curve can be reversed: the same kicking motion will not produce a curved shot or pass from right to left, but from left to right.
In the animation above, a player hits two balls: one smooth and one with a rubber band wrapped around its equator. The two balls are struck with his instep to give a counterclockwise rotation. However, the smooth bullet bends in the opposite direction to that of the ringed bullet. The presence of the elastic band changes the boundary layer on the surface of the ball from “laminar” to “turbulent”. This is why all soccer balls have a certain surface roughness; otherwise, they would bend in the opposite direction to the initial rotation of the ball. (Courtesy of the researchers.)
Why is it? Bush says this is due to the way the ball’s surface creates motion at the “boundary layer” between the spinning ball and the air. The rougher the ball, the easier it is to create the classic version of the Magnus effect, with a “positive” sign: the ball curves in the expected direction.
“The boundary layer can be laminar, which flows smoothly, or turbulent, in which case you have eddies,” says Bush. “The boundary layer changes from laminar to turbulent in different places depending on how fast the ball is spinning. Where this transition occurs is influenced by the roughness of the surface, the stitching of the ball. If you change the pattern of the panels, the transition points move and the pressure distribution changes. The Magnus effect can then have a “negative” sign.
From Brazil: The “wingless dove”
If reversing the Magnus effect has largely gone unnoticed, of course, that’s because soccer balls aren’t absolutely smooth – but they’ve evolved in that direction over the decades. While other sports, like baseball and cricket, have strict rules for seams on the ball, football does not, and advancements in technology have largely given balls smoother and smoother designs – until the introduction of Brazuca, at least.
However, there is actually a bit more to the story, as sometimes players hit balls in a way that gives them very little spin – the equivalent of a knuckleball in baseball. In this case, the ball floats unpredictably from side to side. The Brazilians have a name for it: the “pombo sem asa” or “wingless dove”.
In this case, says Bush, “the particular movement of a floating free kick occurs because the boundary layer transition points are different from the opposite sides of the ball.” Because the ball has no initial rotation, the movement of the surrounding air has more effect on the flight of the ball: “A ball that hits … moves in response to the distribution of pressure, which changes. constantly. Indeed, a free kick taken by Pirlo in Italy’s match against England on Saturday, which cheated the keeper but hit the crossbar, demonstrated this kind of action.
Bush’s interest in the topic stems from the fact that he’s a longtime football player and fan – the kind who, sitting in his office, will put together clips of the best free kickers he’s seen. These include Juninho Pernambucano, a Brazilian midfielder who played in the 2006 World Cup, and Sinisa Mihajlovic, a Serbian defender from the 1990s.
And Bush happily plays a clip of Brazilian full-back Roberto Carlos’s famous free-kick in a 1997 game against France, where the player used the outside of his left foot – but deployed the “positive Magnus effect” “- to score on an outrageously bent free kick. .
“It was hands down the best free kick ever,” said Bush. Putting on his teacher’s hat for a moment, he adds, “I think it’s important to encourage people to try to figure everything out. Even in the most mundane things there is a subtle and interesting physics.