Physics of Football

Understanding the effect of the small ridges on the Jabulani ball in use this World Cup.

As English goalkeeper Robert Green bent down to save the most ordinary of attempts by US striker Clint Dempsey, the Jabulani ball in use this World Cup would have travelled faster than expected. A fraction of a moment late, he received the ball with his palm bent, gently sliding the ball goalward. As opposed to the ordinary soccer ball we are used to, the Jabulani is rougher, textured with small ridges and aero grooves. In contrast, the ball used in the last World Cup, the Teamgeist, was smoother.

To understand the difference this makes, consider a soccer ball moving through the air. The air flows smoothly over its surface (a phenomenon termed laminar flow) as the ball moves slowly forward, leading to an area of low pressure just behind the ball which serves as a brake—the pressure drag. But if the ball is at speed, air flows in small eddies and vortices (turbulent flow), and it sticks to the surface—thus, there is no zone of low pressure. If you consider the impact spin imparts to a ball that is kicked offcentre, you have to factor in the fact that a slow-moving ball with a lot of spin will have a larger sideways force than a fast-moving ball with the same spin.

When a player kicks the ball at speed with spin, it travels hard and fast in the regime of turbulent flow. As friction slows it down, two things happen: the pressure drag kicks in, and the force of the spin increases. As a result, it suddenly dips and swerves. Top players can control this change of regime. Because the Teamgeist was smoother than an ordinary ball, it would transit earlier to the laminar regime, dipping and swerving more than players expected. In contrast, the Jabulani will transit later than an ordinary ball to the laminar regime. As a result it will be harder and truer for longer, which is what Green did not expect.

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