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In the new movie, Bend it like Beckham, Jess Bhamra is a British teenage girl of Indian descent who dreams of playing soccer the way her hero, David Beckham, plays it. Among other things, Beckham (who happens to be a real-life soccer player with the Manchester United team in England, and one of the game's greatest athletes) has the ability to kick a ball in such a way that it executes a dramatic, sometimes uncanny, curve through the air. The ball's curve, or "bend" in soccer jargon, can be devastatingly effective against an opposing team's defense.
Primarily, Bend it Like Beckham is about the challenges that Jess faces as she struggles with the expectations of her traditional Indian family and with the prejudices of British society. So it isn't terribly surprising that little time is devoted to explaining just what's going on when Beckham bends a ball.
"It's the same physics for soccer balls as for all other curving balls," says Lou Bloomfield, a physics professor at the University of Virginia and author of How Things Work: The Physics of Everyday Life. "Bottom line: a spinning ball deflects the air rushing by it and the air responds by deflecting the ball. This effect shows up in volleyball, it shows up in golf. You can really curve a beach ball without too much trouble."
"Curve balls are usually attributed to the Magnus force," says Bloomfield. "When the ball is spinning, the air tends to follow a longer path around one side than the other, because it's dragged along by the ball's turning surface." Air following the longer path bends more sharply, resulting in a dramatic drop in air pressure on that side of the ball. The ball is pushed toward the low-pressure side. A similar drop in pressure over an airplane's wing is the source of lift that supports the plane. "Although a plane's lift is upward, Bloomfield points out, "for a ball lift can be in any direction, depending on the direction the ball is spinning."
While the Magnus force usually gets all the credit when it comes to explaining curve balls, Bloomfield says that another force may be more important to a spinning ball's path-the wake deflection force. "Most moving balls have turbulent wakes behind them," says Bloomfield. "As it spins, a ball draws the air with it and deflects the wake to one side." The deflection shifts the air stream flowing around the ball and the air stream in turn pushes back on the ball. Both the Magnus force and the wake deflection force tend to push the ball in the same direction, and the two forces combine to lead to the impressive curves produced by skilled players like Beckham.
The key, of course, to bending a ball in a soccer game has less to do with understanding the physics than learning how to put a spin on the ball. Does Jess ever manage to bend it like Beckham? You'll have to watch the movie to find out.
-Inside Science News Service
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