Space & Innovation

Kicking Machine Delivers Unpredictable Zigzag Shots

French scientists built a kicking machine to demystify the soccer shot that stumps goalkeepers.

When a tied soccer match goes to penalty kicks, suddenly it's an entirely different game -- especially when players like famed Portuguese forward Cristiano Ronaldo step up to unleash zigzagging shots on goal.

Much like the knuckleball in baseball, these shots are nearly impossible to predict. Now French researchers are shedding light on this mystery with their unique kicking machine.

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The "knuckleball" characteristic, where a ball that has minimal speed and spin moves in unpredictable ways, is only seen in a few sports: baseball, volleyball, and soccer. To explore the phenomenon, scientists from the École Polytechnique's Hydrodynamic Laboratory and the engineering school ESPCI ParisTech in Paris developed a machine to reproduce it with a soccer ball.

Led by researcher Baptiste Darbois Texier, the team constructed the kicking machine from an electric motor, a steel arm, and a flat plate, they explained in an article published in the New Journal of Physics. The machine allowed them to launch balls with little spin. Each shot's trajectory was captured by a high-speed camera and measured in a wind tunnel.

They found is the presence of unsteady lift forces associated with asymmetric airflow around the ball as it moves. But these forces are seen in other games like bocce and handball that don't have a knuckleball effect. Doing additional calculations from their machine data, the team concluded that distance and velocity are key factors. Seams along the ball may play a role, although a ball without a seam can still zigzag.

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As a teen, I loved watching suspenseful penalty kicks on TV. Now I'll admit to feeling slightly disappointed because it all becomes a game of chance at that point. The most skilled goalkeepers in the world can get faked out. The best players can miss the shot -- like Lionel Messi during this year's dramatic Copa Americana final.

I'm not sure science will ever make penalty kicks more predictable, though, and that's fine by me. While the tie-breaker isn't my favorite, it's a heck of a lot more exciting than a coin toss.

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