Jul 5, 2016 by Ken Bray, Senior Visiting Fellow, University of Bath
The science behind a successful set piece involves three important forces on the soccer ball. Theoretical physicist Ken Bray shows visually how it all works.
Football has seen many innovations during its 150-year history. But few have affected the game as profoundly as technological changes to the aerodynamic properties of the ball. For nearly 40 years the ball’s panel pattern followed the classic hexagon-pentagon format with 32 panels, but in 2006 the design changed radically.
In the World Cup in Germany that year, the Teamgeist ball had only 14 panels. Then in South Africa 2010 the Jabulani ball featured only eight, and in Brazil 2014 there were just six on the Brazuca. The ball being used at Euro 2016 in France, the Beau Jeu, is essentially a derivative of the Brazuca with identical panel design, so for the moment six appears to be the ideal number of panels.
The panel configuration of a ball influences its speed and flight through the air. Problems with the German Teamgeist ball, such as its erratic behavior in flight, have now largely been eliminated in its successors. But what the subsequent technology has produced is a ball with much reduced aerodynamic drag, which means it flies faster, and stays in the air longer. Enhanced speed is highly desirable in penalty kicks, but not for that other important football set piece: the direct free kick. Here the objective is to beat the defensive wall, to get the ball “up and down” to use the jargon of football’s television pundits.
Getting the ball over the wall is not especially a problem, but bringing the modern ball down sufficiently quickly to commit the goal keeper into making a save is another matter, unless a special kicking technique is used. And this is when the kicker needs to produce the right kind of spin.
A ball in flight experiences three important forces: gravity (the ball’s weight); aerodynamic drag caused by air flowing across its surface; and a special force experienced only when the ball spins. This is called the Magnus force after its discoverer, the German physicist HG Magnus. It has the special property that it is always perpendicular to the spin axis of the ball and its forward direction.
The graphic below shows the different kinds of spin a player might impart on the ball, depending on the kicking action adopted.
When backspin is applied, the ball rises quickly. This is the technique used by goalkeepers in kicking for distance, say 60 to 70 meters, but it’s absolutely useless in free kicks, which are typically taken 20 to 30 meters from the goal.
(Types of spin. Courtesy K.Bray.)
Sidespin is the overwhelming preference for the game’s elite kickers, but this is where problems can be encountered. The sideways Magnus force that is produced when perfect sidespin is applied can take the ball beyond a goalkeeper’s diving reach. But crucially, it must descend quickly enough after clearing the defensive wall to force a save. Sidespin does nothing to bring the ball down, which is why so many free kicks of this type are simply ballooned over the crossbar.
Topspin requires a special kicking technique and few players in the modern game can strike a ball, from the ground, in this manner. Even moderate topspin produces a downward-pointing Magnus force, which is very effective in bringing the ball down quickly. There is the further advantage that the ball can be hit harder and with increased initial elevation to ensure that it clears the defensive wall, even though the defenders jump in attempting to block the shot....MORE