A ball that is kicked head-on travels with the air flowing past it symmetrically in all directions. Friction with the surface of the ball causes the airflow to initially follow the contour of the ball before forming a turbulent wake that trails behind.
The interactions of this wake with the surrounding air are extremely complex but they form a significant part of the overall aerodynamic drag on the ball. This interaction changes when the ball is initially kicked off-centre, sending it spinning on its own axis as it travels.
The air flowing past the side of the ball rotating towards the direction of travel has a higher relative speed than the air over the opposite side. This deflects the ball’s wake sideways, in the direction of the spin, which creates a reaction force in the opposite direction.
This means that a ball kicked at the right of its centre will spin anti-clockwise and be deflected to the left. This deflection is called the Magnus effect, after the 19th-Century German physicist Heinrich Gustav Magnus.
Although the spin of the ball slows down as it travels due to friction with the air, this is much less significant than the aerodynamic drag that causes the ball to lose forward speed. So the Magnus effect stays fairly constant even as the ball slows down. This causes the curvature to increase noticeably towards the end of the ball’s trajectory and the effect is even more pronounced with very light balls. Table tennis provides the most extreme demonstrations of this with very dramatic deflections achieved by experienced players.
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Asked by: Adrian Flint, via email
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