What Science Says About The World Cup Ball : Show Me Your Cleats! - World Cup 2010 Blog You'd assume the a new, smoother ball would make be faster and steadier. But you'd be wrong.
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What Physics Says About Smooth Balls

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What Physics Says About Smooth Balls

What Physics Says About Smooth Balls

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Goal scoring is usually accomplished with skill and tenacity. But in this year's World Cup, some people - particularly goalkeepers - say there's been another factor involved: The ball. The players say the ball being used in this year's World Cup doesnt behave like a traditional soccer ball

And as NPR's Joe Palca reports, physicists say the players are right.

JOE PALCA: Before we talk about the new ball, let's talk a little about the old one. You may not realize it but the traditional soccer ball is not really a ball at all. It's an Archimedean solid, or in this particular case...

Professor JOHN ERIC GOFF (Physicist, Lynchburg College): What the mathematicians refer to as a truncated icosahedron.

PALCA: That's John Eric Goff. He's a physicist at Lynchburg College and author of "Gold Medal Physics."

The truncated icosahedron has 32 panels in pentagonal and hexagonal shapes, but there's no law that says a soccer ball must have 32 panels. Goff says for the last World Cup, Adidas introduced a ball with 14 panels. This year's ball, called the Jabulani, has even fewer.

Prof. GOFF: The Jabulani ball uses eight thermally-bonded panels.

PALCA: Goff says the number of panels is important.

Prof. GOFF: Balls that have rougher surfaces have different flight dynamics compared to balls with smooth surfaces.

PALCA: More panels, more seams, more seams, more roughness. And oddly, that means the ball goes faster.

Prof. GOFF: What happens is, as the air goes around a sphere or, you know, one of these sports balls, it forms a little layer near the surface of the sphere called the boundary layer.

PALCA: And a rough surface means the boundary layer breaks down more rapidly when the ball flies through the air.

Prof. GOFF: And what that means is the drag force on the ball, the air resistance, goes down slightly.

PALCA: The Jabulani has fewer seams but Adidas has added grooves that roughen its surface. In fact, the Jabulani has less drag, less air resistance than a traditional ball.

Goff says two Japanese scientists - Takeshi Asai and Kazuya Seo - used a wind tunnel to show that.

Dutch player, Giovanni van Bronckhorst, learned that empirically in the semi-final match with Uruguay.

(Soundbite of cheering)

Mr. IAN DARKE (Announcer, ESPN): Now, van Brockhorst with pace - ohhh...

(Soundbite of cheering)

Mr. DARKE: ...it is an absolute firecracker from Giovanni van Bronckhorst. One-nil, Holland.

(Soundbite of vuvuzelas and cheering)

PALCA: Van Bronckhorst was about 45 yards from the goal when he struck the ball with his left foot.

Prof. GOFF: It was launched at a little over 70 miles an hour coming off of his boot.

PALCA: And because it was traveling so fast and the drag was less than on a traditional ball, it stayed higher in the air longer.

Prof. GOFF: Now, that particular kick was high up in the right corner of the goal.

PALCA: Just inches out of the reach of the leaping goalkeeper.

But speed isnt the only thing goalkeepers hate about the Jabulani: It knuckles funny. Okay, Ill explain. When you kick a ball you can make it spin or not spin. If the ball doesnt spin, physicist Rabindra D. Mehta, of NASA's Ames Research Center, says it will knuckle or wobble as it flies through the air, making it hard for a goalkeeper to know where it's going.

Dr. RABINDRA D. MEHTA (Chief, Experimental Aero-Physics, Ames Research Center, NASA): There is an ideal speed for the maximum knuckling effect. It turns out that for a traditional ball with these 32 panels, an ideal speed was about 30 miles an hour or so.

PALCA: But with the Jabulani, that ideal speed goes up to nearly 45 miles an hour, and there's still some knuckling at higher speeds, too. For goalkeepers this is a nightmare.

Marcus Hahnemann was backup keeper for the U.S. team.

Mr. MARCUS HAHNEMANN (Goalie, Team USA): Well, sometimes it goes that way. Sometimes it goes up. Sometimes it goes down. Sometimes it bends, so no one can read it.

PALCA: Of course, Hahnemann doesnt need to worry about that anymore. Ghana solved his problems by knocking the U.S. out of the tournament.

Joe Palca, NPR News, Washington.

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