Problems with balls
Goalkeepers from several teams have called Jabulani's flight weird and unpredictable and now they have science by their side. Researchers in Japan were the first to independently test the ball's flight characteristics in wind tunnels, finding evidence that the new ball does in fact fly more erratically than the 2006 Teamgeist ball.
The new Jabulani ball, released by Adidas last December in South Africa and in regular use since the beginning of the 2010 Major League Soccer season in March, is not a traditional black-and-white ball of 32 flat pentagon and octagon panels stitched together. The Jabulani is made of eight spherically-moulded panels bonded together without stitches and the ball is, according to Adidas, "perfectly round" to within less than one hundredth of an inch.
But some sports engineers who have taken a look at the ball are inclined to agree with the goalkeepers. Its surface seems suspiciously smooth despite grooves added by the designers that encircle the ball. It sounds counter-intuitive, but rougher balls tend to be more stable when flying through the air.
If you had a golf ball without dimples, you couldn't drive it 100 yards, if a baseball didn't have stitches, you'd never hit a home run. That's because roughness on a surface creates turbulence around a ball moving at high speeds, a pockets of air that stabilize the ball. When a ball slows down to a critical speed, the airflow becomes smooth and that causes the ball to change velocity - this is a nightmare for goalkeeper.
The design of the ball may actually help it to fly farther at fast speeds, according to wind tunnel tests conducted in Japan. When kicked at the typical speeds achieved by professional players he ball feels less drag and actually flies a few meters farther.
This tests also showed that as the ball slows, at just under 60 km/h, the ball suddenly feels heavy drag forces that "put on the brakes". So, the fullback who wants to head the ball will have to wait more, and the player who is familiar with the Teamgeist might lose his timing.
And there is another issues: ball's spin. Spinning balls experience a sideward force noticed 300 years ago, and it happens because a spinning ball generates a pressure on one side is greater than on the other. This sideways force on the Jabulani fluctuates more than the forces on the 2006 World Cup ball, which could cause it to bend in unpredictable ways and help to explain bad reactions from goalkeepers.
What all those scientific tests showed? A very simple conclusion: players hit balls millions of times - ball manufacturer should trust their judgement.
Joana Rodeiro ■