It happens
every four years: The World Cup begins and some of the world's most
skilled players carefully line up free kicks, take aim -- and shoot way
over the goal.
The
players are all trying to bend the ball into a top corner of the goal,
often over a wall of defensive players and away from the reach of a
lunging goalkeeper. Yet when such shots go awry in 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 of the people saying that may be seeking excuses. And
yet scholars do think that subtle variations among soccer balls affect
how they fly. Specifically, researchers increasingly believe that one
variable really does differentiate soccer balls: their surfaces. It is
harder to control a smoother 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 "Brazuca," has seams that are over 50 percent longer,
one factor that makes the ball less smooth and apparently more
predictable in flight.
"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 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.
In soccer, the same
thing usually occurs with free kicks, corner kicks, crosses from the
wings, and other kinds of passes or shots: The player kicking the ball
applies spin during contact, creating rotation that makes the ball
curve. For a right-footed player, the "natural" technique is to brush
toward the outside of the ball, creating a shot or pass with a
right-to-left hook; a left-footed player's "natural" shot will curl
left-to-right.
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 of the soccer
player's arsenal goes away, 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 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.
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.
"The
boundary layer can be laminar, which is smoothly flowing, or turbulent,
in which case you have eddies," Bush says. "The boundary layer is
changing from laminar to turbulent at different spots according to how
quickly the ball is spinning. Where that transition arises is influenced
by the surface roughness, the stitching of the ball. If you change the
patterning of the panels, the transition points move, and the pressure
distribution changes." The Magnus Effect can then have a "negative"
sign.
From Brazil: The "dove without wings"
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 moving in that direction over the decades. While other
sports, such as baseball and cricket, have strict rules about the
stitching on the ball, soccer does not, and advances in technology have
largely given balls sleeker, smoother designs -- until the introduction
of the Brazuca, at least.
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 without
wings."
In this case, Bush says, "The peculiar motion of a
fluttering free kick arises because the points of boundary-layer
transition are different on opposite sides of the ball." Because the
ball has no initial spin, the motion of the surrounding air has more of
an effect on the ball's flight: "A ball that's knuckling … is moving in
response to the pressure distribution, which is constantly changing."
Indeed, a free kick Pirlo took in Italy's match against England on
Saturday, which fooled the goalkeeper but hit the crossbar, demonstrated
this kind of action.
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.
And Bush happily plays a clip of Brazilian fullback
Roberto Carlos' famous free kick from 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 outrageously bending free
kick.
"That was by far the best free kick ever taken," Bush says.
Putting on his professor's hat for a moment, he adds: "I think it's
important to encourage people to try to understand everything. Even in
the most commonplace things, there is subtle and interesting physics."
Story Source
http://www.sciencedaily.com
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