Most of the time, soccer is played without much consideration to the actual physics controlling the ball’s movement. Rather, it is our innate and accustomed feel to the ball that calibrates how we play. This is certainly applicable to the power, angle, positioning and curve placed on the ball. However, the physics defining ball movement is rather intricate, with special emphasis being placed on a ball in motion in the air, bringing a range of aerodynamic perspectives into play.
On a surface level, the physics of soccer is fairly simple and could be explained in a basic physics course. These concepts include Newton’s Laws of Motion and basic parabolic trajectories. But a more thorough analysis arises when spin, lift and flow are considered. Most notably, a ball’s spin is due largely in part to a phenomenon called the Magnus Effect, wherein a spinning object drags air faster on one side, which creates a pressure difference that moves the ball in the direction of the lower pressure. More specifically, this is described by Bernoulli’s Principle, which states that an increase in a fluid’s speed (the fluid being the air in this case, which is sped up by the rotation of the ball) corresponds to a decrease in pressure (which is derived from a simple conservation of energy). In essence, the Magnus Effect explains the difference in trajectory between a ball with spin and one without spin, which shows why those with spin move in certain directions. Most notably, this is seen in forms of side spin (when kicking the ball on either side), top spin (when kicking the top of the ball), and back spin (when kicking the bottom of the ball).
It is hard to be on the topic of the physics of soccer without bringing Roberto Carlos into the conversation. A known defier of the laws of the universe, Roberto Carlos’s shots perfectly showcase the Magnus Effect in action. Here, the ball began to spin around its own axis in the direction of the kick, but with air drag (friction), the ball slows down and begins to spin in the other direction, thus changing its trajectory (and easily fooling an unsuspecting Fabien Barthez).
Another particularly interesting consideration in ball movement is not so much with how the ball is struck (though obviously that matters), but what is being struck. Though it may seem like they are somewhat uniform, not all ball designs are created equal. Perhaps the most memorable case in recent history being with the controversial Jabulani ball in the 2010 South Africa World Cup. This ball was specifically designed by Adidas to improve the aerodynamic characteristics of the traditional soccer ball. Specifically, it was made with 8 thermally-bonded 3D panels that have been spherically molded to create as perfect a sphere as possible, without any stitching on the exterior surface. Compared to a traditional 32 hexagon design found on traditional soccer balls, the Jabulani exhibits particularly interesting characteristics. Due to its shape and features, this ball was heavily criticized for its unpredictability and seemingly erratic movement. Notably, important soccer figures have claimed the following:
Diego Maradona: “We won’t see any long passes in this World Cup because the ball doesn’t fly straight.”
Lionel Messi: “The ball is very complicated for the goalkeepers and for us [forwards].”
Gianluigi Buffon: “The new model is absolutely inadequate and I think it’s shameful letting play such an important competition, where a lot of champions take part, with a ball like this.”
Luis Fabiano: “She [the ball] is supernatural.”
Although the ball may seem supernatural, its erratic behavior is due to a phenomenon called knuckling, wherein the airflow over the ball transitions from turbulent (erratic) to laminar (smooth) – as described by Reynolds Number – causing an asymmetrical deflection force that pushes the ball in one direction, making it behave erratically. According to a wind tunnel study conducted by The Fluid Mechanics Lab at a NASA research center, older balls (which have a rougher and less spherical surface) knuckle at a speed of 30 mph. Contrarily, the Jabulani’s physical traits (lack of seams and smooth shape) cause it to knuckle at higher speeds of 45-50 mph. This is of particular significance since that is the speed at which free-kicks are commonly taken, so it is much more likely to observe erratic swoops with a Jabulani than with a traditional ball.
Works Cited
Dunbar, Brian. “NASA Scores Big With Student Soccer Players in the U.S.A. and Canada.” NASA, NASA, 6 Aug. 2010, www.nasa.gov/topics/nasalife/features/soccer_ball.html.
FIFA.com. “Jabulani: The Official Matchball.” FIFA.com, 4 Dec. 2009, www.fifa.com/worldcup/news/y=2009/m=12/news=jabulani-the-official-matchball-1143498.html.
“Forces on a Soccer Ball.” NASA, NASA, www.grc.nasa.gov/www/k-12/airplane/socforce.html.
“The Physics of Soccer.” Real World Physics Problems, www.real-world-physics-problems.com/physics-of-soccer.html.
“Soccer Ball Physics.” Physics, www.soccerballworld.com/Physics.htm.
Soccer is surrounded by physics and as you point out, it’s all focused around doing something to the ball. Making it curve one way or making it go farther. When I was reading this post, I kept thinking of Rory Delap. He was effectively a throw-in master. He could make a throw-in a corner kick. He was that good. I wonder what kind of physics he was able to naturally master to make the ball go so far. Maybe he just had an insane amount of brute force. Something tells me he also figured out how to use backspin to his advantage. I bet those physics are pretty interesting too.