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Thu 14-Sep-2006 21:25
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The Physics of footballs
Anyone watching the recent World Cup will have noticed the bizarre pattern on the ball with its propeller-shaped panels. Footballs in all their shapes and sizes have come a long way from the various objects used in medieval times.
Early purpose-built balls were made of inflated pigs’ bladders covered in leather. The first major development was the replacement of the pig’s bladder with a rubber inner tube. It was around this time that the leather coat was made of four panels that came to points at the ball’s north and south poles. Since the leather tended to absorb water and the seal wasn’t very airtight, the ball would sag towards an oval shape during a game.
A spherical ball is easier to control in a kicking game, but Rugby, American, and Australian Rules settled on the oval ball that was easier to handle, and the four-panel pattern has stuck in those games. Since soccer and Gaelic required a ball that would stay round, a new layout was developed. This was the 12-panel ball. Two rectangular leather panels stitched together formed each of the six sides. To hold a better shape, the number of panels was increased to 18, i.e. three longer and narrower panels on each side. This is the pattern that remains in the Gaelic football and the volleyball.
By the 1970s, soccer’s ball had diverged from that of the GAA when the ‘Buck Minster’ pattern was introduced, named after the American architect who devised it. It was a 32-panel combination of white hexagons and black pentagons that kept an even better shape. The colored pattern made it clearer which way the ball was spinning, which is important.
In the 1950s a Brazilian midfielder called Didi developed a style of free kick in which the ball would curve through the air. An object moving above a certain speed (12 mph for a football) creates a ‘boundary layer,’ a thin film of air that sticks to the object. When a ball moving away from you spins anti-clockwise, the boundary layer on the right side moves against the oncoming air, compressing against it and creating high pressure. On the left side, the boundary layer moves away from the oncoming air, thinning it out and creating low pressure. High pressure on the right and low pressure on the left combine to push the ball to the left.
That the South American game developed the bending ball is no accident. The European game, played in damp conditions, used leather balls that absorbed water and changed the behavior of the boundary layer making the ball harder to curve. Synthetic materials that repelled water helped bring the bending ball to the European game.
The more seams, the more irregularities on the surface, the thicker the boundary layer, and the more the ball changes course in flight. This can be a bad thing since players generally prefer a ball to behave predictably. In days gone by there would have to be a tradeoff between predictability and the ball staying in shape, so the 32-panels dominated the soccer scene for over fifty years since it was a decent compromise.
Modern materials have changed all of this. New materials mean that water absorption and the changing of the ball’s shape during a game are a thing of the past, so the original reason for the 32 panel layout is gone, hence the pattern used in the recent World Cup.
The Adidas ‘Teamgeist’ ball has only fourteen panels, and hence a lot less stitching and a more even surface. Goalkeepers find this ball disconcerting because of a side effect of it having so little stitching. When kicked in a certain way with little or no spin, it can behave unpredictably. This is because the boundary layer only forms on the sparse seams on the ball’s surface, and different forces acting on different parts of the ball cause it to bob around in the air, like a ‘knuckleball’ in baseball.
Meanwhile, the 16-panel layout soldiers on in the GAA.
According to Antoinette Kelly at O’Neill’s, the white ball was first used in Croke Park in 1924. Up until then, the ball’s skin was a brownish pigskin, and was difficult to see. The balls were made of leather up until 1968/9 when they switched to man-made fiber due to a lack of raw material. At this stage the layout was changed 12 to 18 panels. Croke Park didn’t want a 32-panel ball because it was too soccer-like. Also, the GAA ball is made 3 ounces heavier than the standard soccer ball to stop the wind from blowing it around too much. Very important in a game where the ball spends more time aloft, and indeed, in the middle of high-speed action, than in soccer.
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