## Sunday, September 25, 2011

### Physics of a Free Kick

I was watching YouTube videos on the greatest soccer goals ever scored and found this free kick scored by Roberto Carlos in the 1998 World Cup against France. Roberto’s shot, approximately 30 meters from the goal, curves around the wall, outside the frame of the goal, and off the post. Maybe without knowing it, Roberto is taking advantage of several physics concepts that allow him to “bend” the ball into the goal.

The curve of a soccer ball, or any sport ball, can be explained by fluid dynamics, the Magnus effect, and Bernoulli’s principle. The Magnus effect is a perpendicular force in the direction of low pressure created by a spinning object and fluid. By kicking the ball with a force on the outside of the ball, Roberto makes the ball spin counterclockwise which results in the boundary layer of air being dragged around the ball. Bernoulli’s principle states “where the velocity of a fluid is high, the pressure is low, and where the velocity is low, the pressure is high.” The side of the ball rotating against the air creates a lower velocity of fluid and a higher pressure. The side of the ball rotating with the air creates a higher velocity of fluid and a lower pressure, resulting in a force in that direction. This force helps the ball “bend” around the wall and into the goal.