In this blog post, we’ll explore the principles behind why the “banana kick” (free kick) used by players like Messi and Beckham curves, focusing on the Magnus effect and Bernoulli’s principle.
The Banana Kick: Why the Ball Curves
When watching a free kick, most people exclaim, “That’s a great shot!” or “How does the ball curve like that?” In soccer, a “banana kick” refers to a shot where the ball curves into the goal while spinning in a specific direction. Even if we can’t precisely explain why the ball curves in everyday terms, everyone knows that when spin is applied to the ball, its trajectory curves.
The fundamental reason a “banana kick” curves is the difference in airflow around the ball. For example, let’s assume the ball is moving forward while spinning counterclockwise. On the left side of the ball, the airflow caused by its forward motion and the airflow created by its spin are in the same direction, resulting in a relatively faster airspeed. Conversely, on the right side of the ball, the forward flow and the rotational flow are in opposite directions, causing the airspeed to be relatively slower.
When the airspeed on one side becomes faster and on the other side slower, the pressure on the faster side decreases while the pressure on the slower side increases. As a result, a pressure difference is created, and the ball is pulled toward the area of lower pressure, causing it to curve. This principle explains not only the “banana kick” in soccer but also breaking pitches in baseball. Depending on the direction of spin the pitcher imparts to the ball, the airflow around the ball changes, and consequently, the ball’s trajectory changes as well.
The Magnus Effect and Bernoulli’s Principle
The lateral force generated when a ball spins is called the Magnus effect. The Magnus effect refers to the phenomenon in which an object experiences a force perpendicular to its axis of rotation due to differences in fluid flow around the rotating object. The curving motion of the ball seen in a banana kick and a baseball breaking pitch is a classic example of the Magnus effect.
One of the key concepts for understanding this phenomenon is Bernoulli’s principle. Formulated by Daniel Bernoulli, this principle states that as the velocity of a fluid increases, the pressure in that region decreases relatively. Therefore, when the air velocity on one side of the ball’s surface increases, the pressure in that region decreases, creating a pressure difference with the opposite side that causes the ball’s direction of travel to shift slightly.
However, in modern fluid dynamics, the Magnus effect is not explained solely by Bernoulli’s principle. It is understood that the lateral force is generated by the combined effects of various fluid dynamic phenomena, such as changes in the boundary layer due to the ball’s rotation, air viscosity, and flow separation. Nevertheless, Bernoulli’s principle remains a fundamental concept for understanding the relationship between air velocity and pressure, and it is widely used to explain the Magnus effect.
Fluid Dynamics Is All Around Us
Fluid dynamics is the field that studies the relationship between fluid flow and the motion of objects. As seen in the banana kick and breaking pitches, fluid dynamics is not some distant, difficult theory but a science that is constantly at work all around us.
For example, a water skier sinks in the water when stationary, but once they reach sufficient speed, the force generated by the interaction with the water increases, allowing them to glide across the water’s surface. Similarly, in an airplane, the shape of the wings and the flight speed cause the air flowing over and under the wings to behave differently, generating lift; combined with the wing’s angle of attack and the airflow, this enables the aircraft to fly.
As shown here, the flow of fluids such as air or water can significantly alter an object’s motion. These principles are widely applied not only in sports like soccer and baseball but also in various fields, including aircraft, ships, automobiles, and all kinds of sports equipment. The next time you watch a soccer or baseball game, take a close look at the movement of the ball—you’ll discover the fascinating scientific principles of fluid dynamics in an even more engaging way.
