## What are the scientific reasons for spin or curve on a soccer ball? What forces are affecting this?

The spin on a soccer ball is determined by how it is kicked. If the ball is kicked directly in line with its center of mass, it will not spin. An off center kick will cause it to spin. The amount of spin depends on how strong the kick is and how far off center.

By giving the ball a spin a soccer player can cause its path through the air to curve or bend. The curving is caused by a force, called the Magnus effect, caused by the air moving over the spinning ball. When air moves past a ball, or the ball moves through the air, a thin boundary layer of air clings to the surface of the ball. As the air approaches the back of the ball, it cannot continue to cling to the surface, but breaks away, leaving a series of eddies behind the ball. If the ball is not spinning, the flow of air around the ball is symmetric and the difference in pressure between the front and back of the ball produces a drag force.

If the ball is spinning, the air on the side that is moving in the same direction as the ball’s surface is carried farther along the ball and breaks away later than when the ball isn’t spinning. The air on the other side breaks away sooner. The net result is that the ball exerts a force on the air causing it to move in the direction shown in the diagram. Due to Newton’s 3rd Law, there is reaction force of the air on the ball, labeled F in the illustration, that causes the path of the ball to curve in the direction indicated in the picture. Now go try this on your own.

Watch the famous Wendy Sadler demonstrate the Magnus effect with an experiment that you can try at home!

Answered By: Kenneth S. Mendelson, Marquette University