Bending it like BeckhamBy PETE GRATHOFF
The Kansas City Star
It may be the most famous physics lesson in the history of the world.
“Bend It Like Beckham” is a movie, sure, but the way the famous soccer player makes a ball curve is educational as well.
When Beckham (or Houston’s Dwayne De Rosario, Brazil’s Roberto Carlos or someone in this weekend’s MLS Cup) takes a free kick, the goal is scoring a goal by using the Magnus Effect — whether or not the player knows it.
“It’s all about spin,” explained Michael Kruger, a UMKC physics professor.
Beckham kicks the ball and puts spin on it. As the ball travels toward the goal, the velocity of the air that is moving in the same direction as the spin of the ball is faster than the center of the ball.
“The air sticks to the ball a little longer and it ends up getting deflected,” Kruger said. “The thing to remember is Newton’s Third Law: For every action there is an equal and opposite reaction. The ball is somehow deflecting the air to go down. The air was going straight before hitting the ball, after it hits the ball, it’s going down. If the ball makes the air go down, then the air must make the ball go up.”
That is what’s happening on the other side of the ball. The imbalance causes the ball to deflect in one direction, whether it is up, down, left or right.
Check out the movement on kicks by De Rosario and Brazil’s Roberto Carlos. The links are below.
Physics World magazine described how Carlos accomplished the feat.
“Carlos kicked the ball with the outside of his left foot to make it spin anticlockwise as he looked down onto it. Conditions were dry, so the amount of spin he gave the ball was high, perhaps over 10 revolutions per second. Kicking it with the outside of his foot allowed him to hit the ball hard, at probably over 30 ms-1 (70 mph). The flow of air over the surface of the ball was turbulent, which gave the ball a relatively low amount of drag. Some way into its path — perhaps around the 10-meter mark (or at about the position of the wall of defenders) — the ball’s velocity dropped such that it entered the laminar flow regime.
“This substantially increased the drag on the ball, which made it slow down even more. This enabled the sideways Magnus force, which was bending the ball towards the goal, to come even more into effect. Assuming that the amount of spin had not decayed too much, then the drag coefficient increased. This introduced an even larger sideways force and caused the ball to bend further. Finally, as the ball slowed, the bend became more exaggerated still (possibly due to the increase in the lift coefficient) until it hit the back of the net — much to the delight of the physicists in the crowd.”
Perhaps most incredible of all is that the entire play took about three seconds to unfold. That’s longer than it took to read the description.
Carlos showed off his skill in a game against France in 1997. It can be viewed here: http://www.youtube.com/watch?v=W5XpXU8TBoo&feature=related
De Rosario’s kick came while he was with the original San Jose Earthquakes and was against the Galaxy and current Wizards goalie Kevin Hartman: http://www.youtube.com/watch?v=nWO8_nQB4gI