With the World Cup in full swing, everyone has been caught up in football fever, including physicists.
For the 2010 games, Adidas came out with a brand new design for the match ball, named “Jabulani,” meaning, “to celebrate,” in Zulu. But as soon as the teams got to using it, they erupted in complaints, calling it unpredictable, tricky or something you would buy at a supermarket. Goalkeepers and forwards alike claim that the ball has a mind of its own, dipping and swerving as it flies through the air. In one of the first matches of group play, American Clint Dempsey kicked the ball toward the English goal. It skipped along the ground, directly into the hands of keeper Robert Green — and then ricocheted off of his gloves and into the back of the net. The English team blamed not Green, but the Jabulani, for their misfortune. Adidas fought back, insisting that it is the world’s smoothest and most stable ball ever, staying true in its flight path.
So what gives? Just looking at the manufacturing process, the ball is radically different from ones Adidas has brought out for past World Cups. Instead of the traditional 32 hexagonal and pentagonal panels, the Jabulani has a mere eight, which are shaped like concave and convex triangles. These are thermally molded and bonded together without a single stitch on the outside to make the ball as close to a perfect sphere as possible. Raised dimples have also been imprinted on the ball’s surface to add grippy texture and stabilize its flight path, in a similar way the dimples on a golf ball function. Small indentations on an airborne object reduce the separation of air around the ball along its flight path, thus creating a smaller area of low pressure behind it. Less drag means the ball can fly faster and farther. On top of that, many of the match venues are held at very high altitudes, where the decreased air pressure can make the speed of the ball increase by as much as five percent.
But those aren’t the only factors affecting its performance. Because there are a quarter as many panels on the ball than usual, “aero grooves,” indentations that have been added on, fill in for the reduced number of seams, creating more turbulence around the ball. Not only does this make for a faster ball, but it also causes the ball to bend more when kicked. This is a departure from and possibly an improvement on the 2006 World Cup ball, the Teamgeist, which was exceptionally smooth, with12 panels and no grooves. This reduced the mid-flight rotation of the ball, increasing the amount of drag on the ball. At this reduced speed, the drag can act on the ball in erratic ways.
The developers of the ball still stand by it, claiming that after using it for several games, the players will become accustomed to the feel of the ball. They still deny that there are any negative changes to the ball’s design that will significantly affect the play. After all, it’s still round.
—Tiffany Ng, Science & Technology Editor