Like many sports, the gameplay in football can be strongly affected by the ball’s spin. Corner kicks and free kicks can curve in non-intuitive ways, making the job of the goalie much harder. These seemingly impossible changes in trajectory are due to airflow around the spinning ball and what’s known as the Magnus effect. In the animation above, flow is moving from right to left around a football. As the ball starts spinning, the symmetry of the flow around the ball is broken. On top, the ball is spinning toward the incoming flow, and the green dye pulls away from the surface. This is flow separation and creates a high-pressure, low-velocity area along the top of the ball. In contrast, the bottom edge of the ball pulls dye along with it, keeping flow attached to the ball for longer and creating low pressure. Just as a wing has lift due to the pressure difference on either side of the wing, the pressure imbalance on the football creates a force acting from high-to-low pressure. In this case, that is a downward force relative to the ball’s rightward motion. In a freely moving football, this force would curve its trajectory to the side. (GIF credit: SkunkBear/NPR; original video: NASA Ames; via skunkbear)
Fluid dynamics meets Cristiano Ronaldo.