The science behind the football
The World Cup in Brazil commences in less than three weeks. It is not hard to imagine the preparation undertaken by all the players in the national teams. Just making the final draw is an accomplishment, but come 13 July, only one country will reign supreme in the football world. As easy as it is to visualize the tough road to the World Cup for national teams, it may be a surprise to some readers how much science is employed in making the World Cup happen. Decades of research have helped make clothing assist players’ performances by allowing for ventilation of air and efficient passage of water. Boot designers work to make sure players move about the pitch with as little pain as possible and close to the edge of what physics allows. Even the turf that comprises the pitch is a marvel of great science. But the technological star of a World Cup match is the ball, the basic science of which will be addressed here.
Adidas has supplied the World Cup ball since 1970. Most of us played with a football similar in design to that used in World Cup play all the way up to and including the 2002 World Cup in South Korea and Japan, namely a ball with 20 hexagons and 12 pentagons. Though it may seem counterintuitive, the seams between those 32 panels that roughen the surface of the ball actually help the ball’s aerodynamics. A perfectly smooth ball experiences significantly more air drag compared to a rough ball. Were it not for those six lovely rows of stitches on a cricket ball, reaching the boundary on the fly would be very difficult. We could not enjoy a bowler’s swing if the ball were perfectly smooth.
The science behind modern ball aerodynamics was developed at the turn of the 20th century and our understanding has improved ever since. A kicked football leaves the violent collision with the boot only to be met with air slamming onto its forward side. A thin boundary layer or cushion of air exists near the ball’s surface that prevents the oncoming air from reaching the surface. Look at the dust on the blades of your fan. Turn the fan on high and you will not be able to get the dust off. The air rushing over the blades cannot reach the dust to knock it off. For a football, the boundary layer separates farther back from the part of the ball facing the oncoming air. For a given ball speed, air drag goes down as boundary-layer separation moves farther from the part of the ball first meeting the air.
Compared with a smooth surface, a rough surface actually helps the boundary layer separate farther back on a ball. Beginning with the 2006 World Cup in Germany, the number of panels comprising the football’s surface has been decreasing. The Teamgeist used in Germany had 14 panels. The Jabulani used in the 2010 World Cup in South Africa had eight panels; the Brazuca to be used in Brazil this summer has just six panels. To keep footballs from flying like beach balls because of the smoother surfaces due to panel reduction, Adidas intentionally textured the Jabulani and Brazuca balls, meaning the panels were deliberately roughened.
There is one more, very important, aspect to football aerodynamics: the “drag crises”. All sport balls experience this phenomenon by which air flow around the ball changes from “turbulent” to “laminar” as the ball’s speed decreases. That transition is accompanied by a sharp increase in air drag. If the drag crisis occurs at speeds typical of kicked footballs, strange trajectories, with so-called wobbling “knuckle” effects (on balls with little to no spin), are possible.
What got the Jabulani ball in trouble in 2010 is that its drag crisis occurs at a speed of around 86 kilometres per hour (kph), which is comparable to intermediate-speed kicks. The new Brazuca ball’s drag crisis takes place at a speed near 61kph, which means intermediate-speed kicks should be more stable this time around. How does Brazuca do this with two fewer panels compared with Jabulani? The boomerang-shaped seams on Brazuca actually lead to a total seam length that is 68% longer than the total seam length on Jabulani. The longer total seam length not only means Brazuca’s surface is rougher than Jabulani’s, it means Brazuca’s surface is more uniform than Jabulani’s. Wind-tunnel experiments that I’ve recently published with my colleagues Takeshi Asai and Sungchan Hong at the University of Tsukuba in Japan demonstrate Brazuca’s superior stability over Jabulani. Though high-speed kicks will look similar to goal keepers, intermediate-speed kicks with Brazuca will look different from what goal keepers saw four years ago.
With all the good science in the new ball, it is time to head to the pitch and let the game begin!
John Eric Goff is professor of physics at Lynchburg College (Virginia, US) and the author of Gold Medal Physics: The Science of Sports