Number 378 (Story #1), June 24, 1998 by Phillip F. Schewe and Ben Stein|
THE PHYSICS OF FOOTBALL (soccer) is au courant now that the 1998 World Cup has begun in France. The June 1998 issue of Physics World anatomizes a great moment in recent soccer history---the penalty kick taken by Brazilian player Roberto Carlos in a game last year against France. Carlos kicks the ball with high speed (70 mph) and imparts a high spin (10 rev/sec). Airflow past the rocketing ball is at first in the low-drag, turbulent regime. However, about 10 m along its trajectory (just as it shoots wide of a wall of poised defenders) the ball slows enough for it to enter into a smooth-airflow (laminar) phase. This entails an ever increasing degree of drag, which in turns brings the Bernouilli principle (and a hefty sideways force, or "lift") into play, dramatically curving the ball past the goalie into the net.