Soccer ball

How does a soccer ball deflect? The softness of a ball’s surface, in addition to playing technique, is a critical factor

It happens every four years: the World Cup begins and some of the most talented players in the world carefully line up free kicks, aim and shoot well beyond the goal.

The players all try to bend the ball into an upper corner of the goal, often over a wall of defensive players and out of reach of a rushing goalkeeper. Yet when such shots go wrong at the World Cup, a blame game usually sets in. Players, fans and pundits all suggest that the new official tournament ball, introduced every four years, is the cause.

Many people who say this may be making excuses. And yet, researchers believe that subtle variations between soccer balls affect their flight. Specifically, researchers increasingly believe that one variable really sets soccer balls apart: their surfaces. A smoother ball, such as the much-discussed “Jabulani” used in the 2010 World Cup, is more difficult to control. The new ball used in this year’s tournament in Brazil, the “Brazuca”, has more seams than 50% longer, a factor that makes the ball less smooth and seemingly more predictable in flight.

“The details of the airflow around the ball are complicated, and depend in particular on the roughness of the ball,” explains John Bush, professor of applied mathematics at MIT and author of a recently published article on the aerodynamics of soccer balls. “If the ball is perfectly smooth, it bends the wrong way.”

By “wrong sense,” 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 of those balls. Sound surprising?

Magnus, meet Messi

It’s possible, because the question of how a spinning ball bends in flight seems to have a classic answer: the Magnus Effect. This phenomenon was first described by Isaac Newton, who noticed that in tennis, topspin makes a ball dive, while backspin flattens its trajectory. A curveball in baseball is another example from the sport: a pitcher throws the ball with a particularly tight spin or side spin, and the ball curves in the direction of the spin.

In football, the same thing usually happens with free kicks, corner kicks, wing crosses and other types of passes or shots: the player kicking the ball applies rotation during contact, creating a rotation that causes the ball to curve. For a right-handed player, the “natural” technique is to brush away from the ball, creating a shot or pass with a right-to-left hook; a player’s “natural” shot with the left foot will wrap from left to right.

So far so intuitive: football fans can probably conjure up images of stars like Lionel Messi, Andrea Pirlo or women’s football superstar Marta by doing this. But this type of shot – Brazilians call it the “curva drop” – depends on a ball with a certain surface roughness. Without it, this classic piece of the football player’s arsenal disappears, as Bush points out in his article “The aerodynamics of the beautiful game”, from the volume “Sports Physics”, published by Les Editions de L’Ecole Polytechnique in France. . .

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

Why is it? Bush says this is due to the way the surface of the ball creates movement 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 is gently flowing, or turbulent, in which case you have eddies,” Bush explains. “The boundary layer changes from laminar to turbulent at different places depending on how fast the ball is spinning. Where this transition occurs is influenced by the roughness of the surface, the seam of the ball. If you change the pattern of the panels, the transition points move and the distribution of pressure changes.” The Magnus effect can then have a “negative” sign.

From Brazil: The “wingless dove”

If the reversal of the Magnus Effect has largely escaped detection, of course, it’s because soccer balls aren’t absolutely smooth – but they’ve evolved in that direction over the decades. While other sports, such as baseball and cricket, have strict rules regarding seams on the ball, football does not, and advances in technology have largely given balls sleeker designs and smoother – until the introduction of the Brazuca, at least.

There’s actually a bit more to the story, however, as sometimes players hit balls in such a way that they give them very little spin – the baseball equivalent of a knuckleball. 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 motion of a floating free kick arises because the boundary layer transition points are different on opposite sides of the ball.” Because the ball has no initial spin, the movement of the surrounding air has more of an effect on the flight of the ball: “A striking ball…moves in response to the distribution of pressure, constantly changing.” Indeed, a free-kick Pirlo took in Italy’s game against England on Saturday, which trumped the goalkeeper but hit the crossbar, demonstrated that kind of action.

Bush’s interest in the subject stems from the fact that he’s a lifelong football player and fan – the kind who, sitting in his office, will summon clips of the best free-kick takers 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’ famous free-kick in a 1997 match against France, where the player used the outside of his left foot – but deployed the “positive” Magnus effect ” – to score on an outrageous bending. free kick.

“It was by far the best free kick ever taken,” Bush said. Putting on his professor cap for a moment, he adds: “I think it’s important to encourage people to try to figure it all out. Even in the most mundane things there is subtle and interesting physics.”

Video: http://math.mit.edu/~bush/?p=492