Football physics: How to take the perfect free kick
The likes of Lionel Messi and Cristiano Ronaldo are known around the world for their expertise in the art of the free kick. Whether it’s a curler into the top corner or a thundering piledriver, free kick taking is a vital part of the modern game. But how does science come into scoring a goal?
The guiding principle is the Magnus effect. Investigated by German physicist Heinrich Gustav Magnus, this law of physics demonstrates that airflow is distorted around any spinning cylinder or sphere in a certain way.
If the ball is spinning anticlockwise, the left side of it will experience less drag as it moves in the same direction as the airflow, while the right side spins against the onrushing air, increasing the drag. This creates a pressure imbalance, with the right side of the ball experiencing higher pressure and the left side experiencing lower pressure. It is this imbalance which forces the ball to move to the area with lower pressure, thus curling to the left.
But the Cristiano Ronaldo style of free kicks is a whole different ball game. The idea behind the immensely powerful, swerving free kick is imparting as little spin as possible. As the air flows over the ball, a boundary layer is produced, which is a cushion of air that sticks very tightly to the surface of the ball. If an imperfection in the ball disrupts this airflow, it will deviate in the air.
A rapidly spinning football won’t deviate much, but a ball hit flatly will, as it will have more time to move in the direction of the disruption. So when Ronaldo strikes the ball with little spin, any minor imperfection in the football will cause it to move during flight and outfox many a poor, bewildered goalkeeper.
Why footballs can be too round
The official match ball for the 2010 World Cup in South Africa, known as the Jabulani, caused consternation with goalkeepers and strikers alike. The lack of panels on the ball and the use of internal stitching made it the roundest ball ever. However, the roundness of the ball caused a lot of confusion among players because of its completely unpredictable swerving. Outfield players didn’t like it because the lack of imperfections meant less grip between ball and foot, meaning that they struggled to impart spin on the ball. Meanwhile, goalkeepers couldn’t anticipate the trajectory because it would have a habit of suddenly slowing mid-flight or ballooning up, a bit like a plastic inflatable ball.
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