Tuesday, May 16, 2006

Physics : Here's how Beckham bends it

PARIS (AFP) - Physicists are not known for donning replica shirts and face paint, yet they are among the millions who marvel at David Beckham, Roberto Carlos and other masters of the banana kick.

Backed by wind tunnels, high-speed cameras and computer simulation, scientists have been keen to understand the complex physics that go into one of those dazzling curving kicks from outside the box.

Three big variables combine to determine where the ball will go, according to researchers at Britain's University of Sheffield.

They are the VELOCITY of the ball, determined by the force at which it is struck; the SPIN of the ball, which imparts the curve; and the DRAG, or the braking effect of the air.

The key factor in all this is something called the Magnus Effect, named after a 19th-century German physicist, Gustav Magnus, who was curious to understand why spinning artillery shells deflected to one side.

Magnus found that a spinning ball or cylinder swirls the air around it, causing vortices on both sides of its surface.

Thus, one side of a spinning football experiences lift, while the other side is slowed. It is this imbalance of local air pressures that causes the sideways movement -- the curve.

But the real masters of the banana kick can also get the ball to change angle during its flight.

One of the most memorable place kicks in football history was a 30-metre stunner made by Roberto Carlos in a Brazil-France match in 1997.

The kick was initially so wide that a ballboy, standing to the right of the French goal, instinctively ducked, thinking that he was going to get whacked.

But then the ball suddenly dipped leftwards into the net, delighting Brazilian fans and leaving French keeper Fabien Barthez open-mouthed.

How did Carlos work this magic?

The answer lies in airflow.

Just after a kick, a spinning ball is moving forward at relatively high velocity, and the air flows irregularly over it.

But when the ball slows down -- specifically, when it reaches between eight metres and 10 metres per second (18-23 miles per hour), depending on its surface structure and texture -- the airflow becomes smooth, or "laminar," which instantly increases the drag.

The change is dramatic. In a fraction of a second, drag can be increased by 150 percent. This drastically brakes the forward movement of the ball and thus enhances the curving movement derived by the Magnus Effect.

The British journal Physics World estimates that Carlos kicked the ball at 30 metres per second (70 mph) with the outside of his left foot, impelling an anticlockwise spin of more than 10 revolutions per second.

The force of the kick initially caused the ball to take a relatively straight line, bringing it a metre (yard) to the right of the French defensive wall.

The ball then slowed, causing the laminar airflow to kick in and suddenly boosting the leftward curve, with the outcome that poor Barthez will no doubt remember all his life.

As any footballer will know, bending kicks also have other variables: the angle at which the foot hits the ball, the part of the foot that connects with the ball's surface, the humidity of the air and the ball.

High-speed cameras have shown that you will get maximum spin by kicking the ball with the front of your foot and with the angle between your leg and foot at 90 degrees, and if the ball, the boot and the air are all dry.

What the cameras don't say -- but which everyone knows -- is that it also helps if you are Brazilian.

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