Why Jabulani ball is missing its mark at FIFA WC
Washington: The Jabulani`s aerodynamic effects coupled with Johannesburg`s altitude may just be the reason why strikers and goalies have been grumbling about the Adidas manufactured ball this football World Cup, says an Indian origin American scientist.
At altitude, the air pressure is lower, and so are aerodynamic effects like drag and lift, which cause balls to travel faster and straighter than they would at lower altitude.
And Johannesburg is 5,500 feet above sea level.
"When they play there, the ball will behave differently because of air density compared to other stadiums," Live Science quoted Rabi Mehta, an aerospace engineer at NASA Ames Research Center in Moffett Field, California, as saying.
He added: "When watching the games recently, you often see long passes that overshoot, and I think that`s because of this effect."
Drag is the force of air resistance pushing against a ball`s motion and slowing it down, while lift is the force that causes a ball to swerve off a straight path.
Both forces are caused by the presence of air, so with less air molecules around, these forces are reduced.
Thus, the same kick in Johannesburg compared with one at sea level would cause a football to travel faster and on a less curved path.
Mehta said: "If they understand what happens in certain situations, that improves their performance.
"When the person is producing a pass he has to realize, `I need to kick it not as hard as I would at sea level, otherwise it`s going to go out of bounds.`"
Mehta has also done some research on the aerodynamics of the new Jabulani football ball being used at the 2010 World Cup.
The ball is made out of eight panels, compared with the previous 14, and has special aerodynamic ridges on its surface.
The Jabulani is an adjustment to a previous new ball called Teamgeist, introduced for the last World Cup in 2006. At the time, Adidas said it was the world`s most accurate football ball, though players said it didn`t perform as they were used to.
Mehta said Jabulani will probably have some of the same quirks as Teamgeist.
"Knuckle-balling" is a primary issue with the ball. A knuckle ball is a ball that swerves or veers in unexpected directions. This happens because the ball is kicked straight, without spin.
And this lack of spin on the ball causes its path to curve because of aerodynamics.
The ball is not perfectly smooth because of the seams between its panels, and now because of the added ridges.
These irregularities cause an asymmetric flow of air around the ball, creating side forces that push the ball into a swerve.
The new ridges and overall design make Jabulani even easier to knuckle-ball at the speeds commonly kicked at during free-kick around the goal area.
However, knuckle-balling is not always bad - sometimes it can help throw a goalie off if a ball swoops at the last minute.
But players like being able to anticipate where their kick will land, and the new ball will take some adjustment, said Mehta.