The following essay is reprinted with permission from The Conversation, an online publication about the latest research.
As with every World Cup, the players will use a new ball at the FIFA World Cup 2022 in Qatar. The last thing competitors want is for the most important piece of equipment to behave in unexpected ways at the most important tournament in the world’s most popular sport. As such, a lot of work goes into ensuring that every new World Cup ball feels familiar to players.
I’m a physics professor at Lynchburg University, majoring in sports physics. 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 examine the new World Cup ball every four years to see what went into the development of the heart of the most beautiful game in the world.
The Physics of Resistance
Between shots on goal, free kicks and long passes, many important moments of a football game happen 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.
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 only covers the front half of the ball before the flowing air separates from the surface. In this case, the airflow behind the ball is fairly regular and is called laminar flow.
However, when a ball is moving fast, the boundary layer wraps much further around the ball. When the airflow eventually breaks away from the ball’s surface, 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. The higher the drag coefficient at a given speed, the more drag an object will feel.
It turns out that the drag coefficient of a football in laminar flow is about 2.5 times greater than in turbulent flow. Although it may seem counterintuitive, roughening a ball’s surface delays boundary layer separation and keeps a ball in turbulent flow longer. This physical fact – that rougher balls feel less drag – is why golf balls with studs fly much farther than if the balls were smooth.
In making a good football, the speed at which airflow transitions from turbulent to laminar is critical. This is because a ball begins to slow down dramatically when this transition occurs. If laminar flow starts at too high a velocity, the bullet will begin to slow down much faster than a bullet that maintains turbulent flow longer.
Development of the World Cup ball
Adidas has supplied balls for the World Cup since 1970. Until 2002, every ball was made with the legendary 32-panel construction. The 20 hexagonal and 12 pentagonal panels are traditionally made from leather and sewn together.
A new era began with the 2006 World Cup in Germany. The 2006 ball, dubbed Teamgesit, consisted of 14 smooth, synthetic panels that were thermally bonded rather than sewn. The tighter bonded seal kept water out of the ball’s interior on rainy and muggy days.
Making a ball out of new materials, with new techniques and with fewer panels changes the way the ball flies through the air. In the last three World Cups, Adidas has tried to balance the number of panels, seam properties and surface structure to create balls with just the right amount of aerodynamics.
The eight-panel Jabulani ball used at the 2010 World Cup in South Africa had textured panels to accommodate shorter seams and fewer panels. Despite Adidas’ 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 the wind tunnel, we found that the Jabulani was too smooth overall and therefore had a higher drag coefficient than the 2006 Teamgesite ball.
The World Cup balls for Brazil 2014 – the Brazuca – and Russia 2018 – the Telstar 18 – both had six oddly shaped fields. Although they had slightly different surface textures, they generally had the same overall surface roughness and therefore similar aerodynamic properties. Players generally liked the Brazuca and Telstar 18, but some complained about the Telstar 18’s tendency to pop easily.
Al Rihla Ball 2022
The new Qatar World Cup soccer ball is the Al Rihla.
The Al Rihla is made with water-based inks and glues and contains 20 panels. Eight of these are small triangles with roughly equal sides and 12 are larger and shaped like an ice cream cone.
Rather than using raised textures to increase surface roughness like previous balls, the Al Rihla is covered in dimple-like features that give its surface a relatively smooth feel compared to its predecessors.
To compensate for the smoother feel, the Al Rihla’s seams are wider and deeper – perhaps learning from the mistakes of the overly smooth Jabulani, which had the shallowest and shortest seams on recent World Cup balls and which many players found slow in the air.
My colleagues in Japan tested the four latest World Cup balls in a wind tunnel at Tsukuba University.
When airflow transitions from turbulent to laminar flow, the drag coefficient increases rapidly. When this happens to a flying ball, the ball suddenly experiences a steep increase in drag and abruptly decelerates.
Most of the WM balls we tested made this transition at around 36 mph (58 km/h). As expected, the Jubalani is the outlier with a transition speed of around 82 km/h. Considering that most free kicks start at speeds over 60 mph, it makes sense that players found the Jabulani slow and difficult to predict. The Al Rihla has aerodynamic characteristics very similar to its two predecessors and may even move a little faster at lower speeds.
Every new ball is met with complaints from someone, but science shows that Al Rihla’s players at this year’s World Cup should feel comfortable.
This article was originally published on The Conversation. Read the original article.