In this blog post, we explore the fascinating physics principles hidden within sports through Son Heung-min’s spinless shot and Carlos’s banana shot.
It happened during a World Cup qualifier between Korea and Myanmar long ago. Around the 21st minute of the second half, Son Heung-min’s free kick shook the net. Given Son’s world-class shooting ability, the goal itself might not seem surprising. But the trajectory of this ball was unusual. From a distance, it appeared to be heading straight at the goalkeeper, yet the keeper, seemingly unable to predict its direction, took a dodging stance and was beaten.
Reviewing it in slow motion revealed the ball didn’t rotate even once during its flight, instead moving as if swaying erratically in midair. Such a goal, exhibiting physical movement distinct from typical soccer play, provides us with an intriguing subject worthy of scientific analysis.
A similar case is the free kick goal by Brazil’s legendary soccer player, Roberto Carlos. His shot curved like a banana, sucking into the goal. The ball, which clearly appeared to be heading wide of the goal, bizarrely bent its way inside the post, leaving the goalkeeper utterly helpless.
Such sports moments are often dismissed as mere player skill and overlooked. However, applying a scientific perspective to sports phenomena allows us to gain deeper understanding and interest. Furthermore, science can also be used to advance sports techniques. We call a ball with no spin, like Son Heung-min’s goal, a ‘spinless shot’, and Carlos’s curved free kick is called a ‘banana shot’.
First, let’s examine the spinless shot. Interviews with goalkeepers facing such shots commonly mention, “The ball wobbled wildly as it flew toward me, making it difficult to predict its direction.” Indeed, Son Heung-min’s free kick was aimed directly at the goalkeeper, yet the keeper failed to react because he couldn’t accurately track its trajectory.
So why does the ball wobble erratically when traveling without spin? The cause is the ‘Karman vortex’ phenomenon. This refers to vortices that form around a cylindrical object moving through a fluid, occurring alternately on the left and right sides in a regular pattern. When a ball without spin moves rapidly through the air, irregular vortices form behind it, creating low-pressure areas. Since air flows from high-pressure to low-pressure zones, the ball follows this flow, causing its trajectory to shift slightly. This repeated shift makes the ball appear to wobble, making it difficult for the goalkeeper to predict its path.
Next, let’s examine the banana shot. This shot draws a dramatic curve before being sucked into the goal. Goalkeepers at the time all stated they thought the ball was going wide. This phenomenon can also be scientifically explained through ‘Bernoulli’s Principle’.
According to Bernoulli’s Principle, when a fluid flows through a narrow section, its velocity increases and pressure decreases. Conversely, in a wide section, velocity decreases and pressure increases. For example, when viewed from above, if the ball is kicked with a clockwise spin, the left side of the ball experiences slower airflow due to the opposing direction of rotation, resulting in higher pressure. Conversely, the right side aligns with the rotational direction, increasing speed and decreasing pressure. This pressure difference causes the ball to curve toward the lower-pressure side—the right—resulting in a banana-shaped trajectory.
We often simplistically divide sports as something done with the body and academics as something done with the mind. However, scientifically analyzing physical phenomena in sports, like the spinless shot and the banana shot, allows us to understand sports in a more interesting and profound way. Furthermore, it becomes possible to develop and advance sports techniques by applying scientific principles. Isn’t this the true appeal of sports science?
