Every four years, people around the world get together to complain about a ball. Aside from which country’s name is the best to chant out loud (U. S. A.! U. S. A.!), the design of the official tournament soccer ball for the World Cup has become one of the biggest sources of contention during the quadrennial series.

Such disputes actually go back to the very first Mundial in 1930, when Argentina and Uruguay couldn’t agree on which soccer ball to use during their match. They eventually decided to split the difference and play the first half of the game with Argentina’s preferred ball and the second with Uruguay’s. But in recent years the arguments have gotten even louder, coming down to the way the ball behaves as it flies through the air. And that has everything to do with physics.

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The Bizarre Liquid That Sometimes Acts Like a SolidThough Isaac Newton first noted the effect while watching tennis players in Cambridge in 1672, it was the 19th century German chemist and physicist Heinrich Gustav Magnus who gave it a name. Magnus was observing the tendency of cannonballs to veer away from their expected flight path and deduced that it had to do with the way they were spinning.

Give a ball a topspin—making it spin in the same direction as it travels—and you will create a difference in the airflow above and below it. As the ball rotates, the air along its bottom edge gets shoved along quicker than it otherwise would. The air acts as a somewhat viscous, or sticky, substance. It travels along the ball’s surface and comes up the back, creating a stream with an upward force. Because every action has an equal and opposite reaction, the ball experiences a corresponding downward force.

A ball with topspin therefore curves down toward the ground faster than it would if gravity alone were acting on it. The opposite happens on a ball with backspin. The air traveling over the top and down the back generates a force in the upward direction, lofting the ball farther than it would otherwise go.

This Magnus Effect gets exploited in a wide variety of sports situations. A baseball pitcher throws a curveball by giving it a topspin or sidespin in the direction they want it to curve. When teeing off, a golfer hits their ball with a backspin to loft it a little longer and get closer to the green. A table tennis player can impart many different spins to try and outmaneuver their opponent, keeping them guessing about exactly where the ball will come down. Some engineers even tried inventing weird airplanes in the early 1900s with rotating cylinder wings that could fly slower than other designs while staying airborne.

So what does this have to do with the World Cup? Scientists who study the aerodynamics of soccer balls (which I’m sure makes for an awesome NSF grant application) have investigated how the Magnus Effect works on different designs. Since the 1970s, when Adidas began providing the ball for every match, soccer ball design has gotten quite high tech. The materials have moved from leather to synthetics while stitching and laces have given way to heat-sealed panels.

Adidas’ 2002 World Cup soccer ball, the Fevernova, was the subject of much wailing because advanced technology allowed it to be much lighter than the balls players were used to training with. The complaints grew even louder with the 2006 Teamgeist, which had fewer stiches to make it more aerodynamic. The kerfuffle reached a vuvuzela-like pitch with the 2010 Jabulani ball, made from thermally bonded three-dimensional panels.

The problem with all these new designs is that they make a soccer ball smoother and therefore change the way the Magnus Effect behaves. Remember how I said the stickiness of the air causes it to curve around the ball and stream up or down the other side? That requires the gas to cling to the ball’s surface. Looking very closely, we would see that a rough ball creates turbulent eddies in the air, and these follow along the surface much more easily.

Now imagine a very smooth ball with topspin. Along the lower edge, the air has trouble forming eddies along the ball’s surface, flying back behind it without being deflected. But the upper edge of the ball is slamming right up against the air, and this will create small bits of turbulent flow. The air then flows down the back of the ball, creating an upward force. The Magnus Effect for a smooth ball works in the opposite way as for a rough ball, which is why soccer players want a ball to have rough stitching and paneling to bend the way they expect.

In 2010, NASA even stepped in and performed tests on Adidas’ Jabulani. They found that the soccer ball’s surface was creating small asymmetric vortices along its surface, causing it to swoop and swerve unpredictably, like a knuckleball in baseball. For this year’s World Cup, Adidas produced the Brazuca, which they rigorously tested in every way they could think. The result? So far, the news has focused on the games and not the ball.