"An Introduction to the Physics of Sports" by Dr Vassilios M SpathopoulosMagnus Force

A modified excerpt from the book: "An Introduction to the Physics of Sports" by Dr Vassilios M Spathopoulos.

Some of the most impressive shots taken in a soccer game are produced by applying spin to the ball. The members of the Brazilian national team were the first to use this technique for free kicks, exciting the crowds by scoring difficult goals. Since then almost all free kicks that are taken just outside the penalty box are executed in this way, helping the player to give the ball a curved trajectory, thus clearing the defensive wall and beating the goalkeeper.

So what is this force that is responsible for the deflection of the ball? The first to engage himself with the study of this phenomenon was, who else, but Isaac Newton, who tried to provide an explanation for the curved paths of tennis balls (a sport particularly popular during his time). Those however who reached solid scientific conclusions were Rayleigh and Magnus in 1742, who analyzed the deflection in the path of cannon balls that spun round themselves. What they realised was that the force responsible for their curved path (and also the force that affects a football trajectory), is related to the interaction between the spinning object and the air that surrounds it. So it is yet another aerodynamic force, this time present only when an object is spinning. What essentially happens when a ball is spinning is that the rotational motion of the ball drags the air around it, so changing the direction of its flow. This, once again as a result of the action-reaction law, results in a force being exerted on the ball itself thus deviating from its original path.

About the Author

Dr Vassilios Spathopoulos has a PhD in Aerospace and Electronic Engineering from the University of Glasgow (UK). He has also completed a MSc course in Flight Dynamics at Cranfield University (UK). He currently teaches at the Department of Aircraft Technology, at the Technological Education Institute (TEI) of Chalkis, Greece. See more.

Related Pages