Correction: An earlier version of this story described the trajectory of a bullet thrown and pulled down by gravity as hyperbolic instead of parabolic.
As the FIFA World Cup kicks off on Thursday, many fans are focusing on the surprising star of the last tournament: the ball. Adidas’ Jabulani ball, designed for the 2010 World Cup in South Africa, was said to be aerodynamically superior to other soccer balls. Many players, however, complained that his theft was not true. Brazilian goalkeeper Julio Cesar called the Jabulani “terrible” and said it looked like it came from a grocery store. This time the Jabulani has been retired in favor of the Adidas Brazuca, and early reviews have been positive.
But all this commotion raises an important question: What governs the theft of a soccer ball?
Of course, there are obvious factors: a player’s foot and gravity. The player’s foot applies force to the ball, throwing it forward and generally upward. Gravity causes the ball to slide to the ground. If that was all that was happening, the ball would follow the parabolic trajectory we all studied in high school: up, forward, and down. But there is much more to it.
Drag is what makes the flight of a ball interesting. Drag is what soccer players use to add the dips and curves that trick goalies, and it’s what soccer ball designers manipulate to make their inventions unique.
There are many forms of drag, but one of the most important in ball flight is known as pressure drag or shape drag. As the ball advances, the front of the ball separates the air particles, which do not immediately collect as the ball passes. This leaves negative pressure directly behind the ball, which slows the ball down and creates turbulence, making the flight unpredictable.
Another type of drag that affects trajectory is called skin friction drag – the interaction of air particles with particles on the surface of the ball. The friction of these particles also creates turbulence. This is of a lower magnitude than the turbulence created by the pressure drag, however.
The Jabulani bullet was meant to remedy some of the unpredictability created by skin friction and pressure drag. One of its innovations was fewer panels – eight instead of the 32 on many soccer balls. Fewer panels means fewer points. Fewer stitches means a smoother, rounder ball. In theory, this should create less friction and reduce turbulence.
But that’s not what happened. A few issues caused the Jabulani to fly all over the place, but straight. First, although the points contribute to the turbulence, they create a fairly even turbulence all around the surface of the ball. In many cases, these bumps seem to balance out, causing the ball to fly on a more predictable path. Because Jabulani’s large surfaces lacked stitches while others were rough with seams, the unbalanced turbulence created a bumpy race between the foot and the goalkeeper. Engineers’ attempts to compensate for the seam problem by creating grooves in the ball failed to improve the problem.
More importantly, the Jabulani dived and swerved at different points in its flight than a normal balloon. A regular soccer ball dives and bends the most when flying at 20-30 mph. Since a good free kick can be thrown at around 70 mph, the ball spends much of its flight traveling a predictable trajectory, allowing the goalkeeper to have a good idea of where he may meet. the ball. The Jabulani, on the other hand, deviated and dived significantly between 45 and 50 mph, according to NASA scientists who examined the problem. It’s a critical time for a goalie. If the ball does not fly predictably in this speed range, there is a good chance it will land in the wrong place.
The Brazuca only has six panels, but early lab tests suggest it flies more faithfully than its predecessor. The key innovation appears to be intentional roughness of the ball’s surface. The Brazuca is covered with small polyurethane nubs that mimic the effect that stitches create on a traditional 32-panel balloon. They’re there to even out the turbulence, reducing what experts call knuckleballing. They can be compared to the dimples that help a golf ball fly straight. The designers of the Brazuca also changed the layout of the panels, which distributed the seams differently, further smoothing the flight path.
More important than lab tests, however, will be how players react to the ball. Anyone who watches football knows that players and managers will use virtually any excuse – the referee, the weather, the playing surface, the crowd, the kick-off time – after a loss. So far, the best players in the world seem to be happy with Brazuca. Argentina’s Lionel Messi called the ball “super”, as did Spain’s goalkeeper Iker Casillas. Of course, both players are in the pay of Adidas. We’ll see what they think of the ball if and when their teams leave the tournament.