In this blog post, we’ll examine the principles behind a soccer ball’s curve, focusing on fluid dynamics and the Magnus force, and explore the science behind the “banana kick.”
“A soccer ball is round.”
This is a phrase often heard when a team considered the underdog in a match pulls off an unexpected victory, or when a dramatic comeback occurs in a situation that seemed all but lost. The reason soccer is loved around the world lies precisely in this unpredictability. Moments where victory doesn’t easily tilt in one direction—moments where the outcome, like a round ball, cannot be easily predicted—are what make soccer so captivating.
Ultimately, the most important thing in soccer is the goal. The moment a goal is scored is filled with countless unpredictable moments. A single strike from a quick counterattack or a header following a sharp cross is thrilling, but the so-called “banana kick”—where the ball literally “curves” past the goalkeeper’s hands and is sucked into the net—delivers a particularly breathtaking moment that defies expectations.
A prime example is the goal scored by Luis Suárez on February 22, 2013, during the second leg of the UEFA Europa League Round of 32 match between Liverpool and Zenit. His shot initially appeared to be heading wide of the goal, but it suddenly curved sharply and was sucked into the corner of the net. With this goal, Liverpool made up for their 0–2 loss in the first leg and kept their hopes of a comeback alive, but they failed to score again and ultimately did not advance to the Round of 16.
So how does a soccer ball curve? Several conditions must be met for a ball to curve. First, the ball must be in motion, and second, it must be spinning. However, these two conditions alone are not enough. The most important factor is the air surrounding the ball. Without air, the ball will not curve no matter how fast it moves or how much it spins. The curve only occurs when the ball moves through the air and creates a flow of air around it.
Newton’s law of action and reaction is at work here. When one object pushes another, the other object exerts an equal and opposite force. The reason the top of your foot hurts when you kick a ball is that the ball pushes back against your foot with an equal force. Since a person is much heavier than a ball, there is simply less wobble; essentially, the same reaction occurs.
The same principle applies to a spinning soccer ball and the surrounding air. As the ball moves forward, the air splits in front of it and flows upward and downward. Because the ball is round, air molecules can flow relatively smoothly along its surface, allowing the ball to travel through the air with little resistance. If the ball were flat, like a plank, it would experience far more head-on collisions with the air, resulting in significantly greater air resistance and a much faster decrease in speed.
Looking at this in a bit more detail, a spinning ball creates differences in airflow due to the rotation of its surface. On the side where the ball’s spin is opposite to the direction of airflow, the spin disrupts the motion of air molecules, causing the airflow to separate from the surface relatively early. Conversely, on the side where the spin is in the same direction as the airflow, the spin aids the airflow, allowing air molecules to flow along the ball’s surface for a longer time before separating. As the points where the airflow separates from the surface differ in this way, the overall distribution of the airflow becomes skewed toward one side.
When the ball pushes the surrounding air in one direction, it experiences a force in the opposite direction as a reaction. This lateral force acting on a rotating object is named the “Magnus force” or “the force generated by the Magnus effect,” after the German physicist Heinrich Gustav Magnus, who was the first to systematically explain it. The direction and magnitude of this force are determined by various factors, including the ball’s speed, rate of rotation, and the density and viscosity of the air.
For example, if the ball’s speed is excessively high relative to its rotation, air molecules will pass by the ball before being sufficiently affected by the rotation, so the Magnus effect may be relatively weak. Conversely, when an appropriate speed is combined with sufficient rotation, the Magnus force becomes much more pronounced. The fact that Suárez’s shot initially appeared to travel in a straight line before curving sharply in the latter half can also be understood as resulting from the ball’s speed gradually decreasing, thereby making the effect of spin relatively stronger. However, the ball’s actual trajectory is the result of multiple factors acting together, including not only the Magnus effect but also air resistance, turbulence, and the ball’s surface texture.
Ultimately, the principles of fluid dynamics are deeply involved in the curving of a soccer ball. Depending on the ball’s speed, the amount of spin, and the state of the surrounding air, the Magnus force manifests in varying magnitudes and directions. When air resistance and turbulence are added to the mix, it becomes extremely difficult to perfectly predict the ball’s exact trajectory during a match. It is precisely this uncertainty that serves as a key factor in creating countless memorable moments and thrilling upsets in soccer.
In other words, the reason a soccer ball curves so beautifully—and unpredictably—is because it is “truly” round. Thanks to its round shape, airflow naturally forms along its surface; the spin creates asymmetry in that flow, generating the Magnus force that alters the ball’s trajectory. There is more to the round shape of a soccer ball than just its design—it is rooted in science. That is why we can still view the saying “a soccer ball is round” not merely as a metaphor, but as a scientifically fascinating statement.
