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The World Cup ball has the aerodynamics of a champion

    The Adidas Al Rihla ball during the international friendly between Japan and the United States at the Merkur Spiel-Arena on September 23, 2022 in Dusseldorf, Germany.
    Enlarge / The Adidas Al Rihla ball during the international friendly between Japan and the United States at the Merkur Spiel-Arena on September 23, 2022 in Dusseldorf, Germany.

    As with every World Cup, the players at the 2022 World Cup in Qatar will use a new ball. The last thing competitors want is for the main piece of equipment in the most important tournament in the world’s most popular sport to behave in unexpected ways, so a lot of work goes into making sure every new World Cup ball feels familiar to players.

    I am a physics professor at Lynchburg University and study the physics of sports. Despite controversies over corruption and human rights issues surrounding this year’s World Cup, there is still beauty in the science and skill of football. As part of my research, I do an analysis of the new World Cup ball every four years to see what went into creating the centerpiece of the world’s most beautiful game.

    The physics of drag

    Between shots on goal, free kicks and long passes, many key moments of a football game take place when the ball is in the air. So one of the most important characteristics of a soccer ball is how it moves through the air.

    At low speeds, the air will only hug the surface of the front half of a soccer ball before releasing in an organized fashion called laminar flow, as seen here in this wind tunnel photo.
    Enlarge / At low speeds, the air will only hug the surface of the front half of a soccer ball before releasing in an organized fashion called laminar flow, as seen here in this wind tunnel photo.
    At high speeds, the air flowing over a soccer ball will travel almost entirely to the back of the ball before breaking up into chaotic vortices called turbulent flow.
    Enlarge / At high speeds, the air flowing over a soccer ball will travel almost entirely to the back of the ball before breaking up into chaotic vortices called turbulent flow.

    As a ball moves through the air, a thin layer of mostly still air called the boundary layer surrounds part of the ball. At low speeds, this boundary layer will cover only the front half of the ball before the flowing air peels away from the surface. In this case, the airflow behind the ball is somewhat regular and is called laminar flow.

    However, when a ball is moving quickly, the boundary layer wraps much further around the ball. When the airstream finally separates from the surface of the ball, it does so in a series of chaotic vortices. This process is called turbulent flow.

    When calculating how much force moving air exerts on a moving object — called drag — physicists use a term called the drag coefficient. For a given speed, the higher the drag coefficient is, the more air resistance an object feels.

    It turns out that the drag coefficient of a soccer ball is about 2.5 times greater for laminar flow than for turbulent flow. Although it may seem counterintuitive, roughening a ball’s surface slows boundary layer separation and keeps a ball in turbulent flow longer. This fact of physics – that rougher balls feel less drag – is why dimpled golf balls fly much farther than if the balls were smooth.

    When it comes to making a good football, the speed at which the airflow transitions from turbulent to laminar is critical. This is because when that transition occurs, a ball begins to slow down dramatically. If laminar flow starts at too high a speed, the ball will begin to decelerate much faster than a ball that sustains turbulent flow for longer.

    Evolution of the World Cup ball

    The Adidas Telstar, which featured in the 1970 and 1974 World Cups, is what many people picture when they think of a football.
    Enlarge / The Adidas Telstar, which featured in the 1970 and 1974 World Cups, is what many people picture when they think of a football.

    Adidas has been supplying balls for the World Cup since 1970. Through 2002, every ball was made with the iconic 32-panel construction. The 20 hexagonal and 12 pentagonal panels were traditionally made of leather and sewn together.

    With the 2006 World Cup in Germany, a new era began. The 2006 ball, called the Teamgesit, consisted of 14 smooth, synthetic panels that were thermally bonded together instead of stitched. The tighter bonded seal kept water out of the ball on rainy and humid days.

    Making a ball from new materials, with new techniques and with a smaller number of panels changes the way the ball flies through the air. Over the past three World Cups, Adidas has tried to balance the number of panels, seam properties and surface texture to create balls with just the right aerodynamics.

    The eight-panel Jabulani ball at the 2010 World Cup in South Africa featured textured panels to make up for shorter seams and a reduced number of panels. Despite Adidas’ best efforts, the Jabulani was a controversial ball, with many players complaining that it slowed down abruptly. When my colleagues and I analyzed the ball in a wind tunnel, we found that the Jabulani was generally too slippery and thus had a higher drag coefficient than the 2006 Teamgesit ball.

    The smoother Jabulani ball from the 2010 South Africa World Cup was widely criticized for being slow in the air.
    Enlarge / The smoother Jabulani ball from the 2010 South Africa World Cup was widely criticized for being slow in the air.

    The World Cup balls for Brazil in 2014 – the Brazuca – and Russia in 2018 – the Telstar 18 – both had six oddly shaped panels. Although they had slightly different surface textures, they generally had the same overall surface roughness and thus similar aerodynamic properties. Players generally liked the Brazuca and Telstar 18, but some complained about the Telstar 18’s tendency to pop easily.