Most of the discussion at the World Cup was about the games, the players, the teams, and the countries, but there are few comments about the entity that made it all possible. I am talking, of course, about the ball. For example, the design of the soccer ball is a highly technical endeavor that can dramatically affect the outcome of games depending on whether particular aspects of the way the ball travels when it’s kicked align better with a team’s style of play. Over the years many soccer balls have been designed for World Cup competitions with each type of ball having its defenders and detractors. However, for this post I want to concentrate on some particular spin effects that soccer balls can exhibit that are exploited by players, or that have led to occurrences that have forever been recorded in soccer lore.
When the ball hits an obstacle it may acquire a spin. This fact has sometimes led to curious situations where a premature celebration of a penalty that was missed as a result of the ball hitting the goalpost or being deflected by the goalkeeper, ended in disaster when the ball made its way into the goal as a result of a forward spin.
However, the most remarkable of these situations happens when the ball hits the underside of the upper beam of the goalpost acquiring a backward spin. If the ball hits the beam at just the right angle it will bounce down towards the goal line and then away from the goal. This occurrence has been responsible for some of the controversial goals (or non-goals) which have been dubbed “ghost goals”. One of the most famous ghost goals in history was scored by the British player Geoff Hurst during the World Cup final against Germany in 1966.
German and British fans have not stopped arguing about whether this was a goal or not, and the Germans still think they were robbed of a chance at winning the World Cup. However, sweet revenge came to the German side in 2010 when in a World Cup match a goal eerily similar to Hurst’ 1966 goal was scored by Frank Lampard for the British side. Whereas the television cameras revealed that a goal had indeed been scored, the main referee and the linesman did not see the ball bounce inside the goal and did not call it. Britain eventually lost the game and its chance at winning the World Cup.
As a result of this goal and other similar events the world’s soccer governing body, FIFA (International Federation of Association Football) has adopted a series of technological advances including goal line technology, which has made ghost goals a thing of the past, at least at the world cup level.
The most impressive spin effects on soccer balls do not occur by happenstance. Imparting a spin on the ball when it is kicked, is an action taken by players to bypass a blockade from the other team, to fool the goalie, or to place the path of the ball towards the goal beyond the goalie’s reach. In doing so, the soccer players are exploiting a very well-known scientific principle called the Magnus effect, named after the German scientist Gustav Magnus who studied it.
One way to explain the Magnus effect is to imagine that the soccer ball, as it travels spinning through the air after being kicked, possesses a thin layer of air that spins with it. On the side of the ball that is spinning in the direction of the movement of the ball, the molecules of this thin layer of air collide against the molecules of air blowing past the ball, and slow them down, creating an area of high air density and therefore high pressure. However, on the opposite side (the side of the ball spinning in the direction opposite to the movement of the ball) the air molecules in the thin layer collide with the air molecules blowing past the ball and speed them up creating an area of less air density and therefore low pressure. This pressure gradient creates a force that deflects the ball sideways in the direction of the spin.
Some particularly spectacular applications of the Magnus effect involve free kicks. In the slow motion replays of the kicks below you can clearly see the ball curving through the air on its way to a smashing goal!
But perhaps the greatest application of the Magnus effect involves the so-called “Olympic Goals” which are goals scored from corner kicks. These goals are not very common because of the difficulty of the angle, and the fact that goalkeepers and players usually take positions in front of the goal to defend against these kicks, but when they are scored they are an amazing sight to behold.
Many players have mastered the art of spinning the ball in the right way to score goals that have become the stuff of legend, and names like Roberto Carlos, Beckham, Ronaldinho, Messi, Ronaldo, Maradona, Platini, and others have thrilled or are still thrilling generations of soccer fans.
There are other issues regarding soccer balls that I have not touched in this post including the transition of air flow over the surface of the ball from turbulent to laminar after a kick, or the way the grooves of the ball and the roughness of the ball’s surface can affect its behavior. But I hope that from reading this post you have gained an appreciation for the lore and science behind the spin!
Image by Gang65 modified and used here under an Attribution-Share Alike 3.0 Unported license.