Friday, June 08, 2012

The "Perfect" Free Kick and the Magnus Effect

Continuing our theme of sports based posts this week, today we're going to cover the world's most popular sport: football (or soccer for us Yankees). The 2012 UEFA European Football Championship starts today in Poland and Ukraine where top teams from across Europe will face off for the next few weeks.

And physics can explain some of the awe-inspiring plays performed by top players. Bicycle kicks, diving headers, and fingertip saves all exemplify the conservation of momentum, impulse (the change in momentum), and Newton's laws. Perhaps one of the most interesting physics phenomena in soccer, however, explains the curve behind long distance free kicks.

In 1997, Brazilian player Roberto Carlos rendered a French keeper stunned and speechless after curving a 30 meter strike off the post and into the goal. Although the top Youtube comment for Carlos' video below reads, "that dude left Einstein puzzled," the 19th century German physicist Gustav Magnus wouldn't have batted an eye.



The Magnus effect, named for the first physicists to experimentally explore the phenomenon, explains how projectiles can curve when moving through a fluid (like air). Whenever a ball is spinning through the air, the Magnus "force" will push it in a direction perpendicular to the direction of movement.

After Carlos sent the ball flying, the airflow started pushing against the ball in the direction of Carlos. This means that the side of the ball spinning toward Carlos would move with the airflow while the opposite side would move against it. This imbalance is the key to the Magnus effect.

The airflow moving with the spinning direction was dragged around the ball toward the back because of frictional forces. Meanwhile, the airflow moving against the spinning ball would be abruptly stopped. Consequently, there's higher pressure on the side of the ball where the airflow is suddenly halted, and this pushes the ball in the opposite direction. So when Carlos put enough spin on the ball, the right side was fighting against the air and the resulting high pressure forced the ball to spin back toward the net.

The Magnus effect rears its head in a number of games including baseball, tennis, and my all-time favorite, catapult dodgeball (don't try that at home). In the video below, you can see some great visuals that aid in understanding this effect.



In addition to the Magnus effect, there's plenty of other physics phenomena related to soccer. For a great overview of the physics behind soccer, take a look at this article from Physics World. The authors included plenty of helpful diagrams and explanations of modern research in this subset of sports science.

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