Why do footballs curve?
Aerodynamics in Football
Why do footballs curve, spin, and move erratically, when players take a free kick or strike a knuckleball?
Introduction
You may notice that when David Beckham or Wayne Rooney take a free kick, they tend to kick it with a lot of spin. In physics terms, they not only apply a force to kick the ball in the direction they want to, but also impart another force to spin the ball.
But why do they do this? Why does it make the ball curve? And what on earth goes on with a knuckleball? To answer these questions, we’re going to have to dive into the physics, and something called the Magnus Effect!
Bend it like Carlos!
So, let’s begin with an example of a curved freekick! See this freekick from Roberto Carlos of Brazil! You can easily see the incredible deviation of path!
Note, as we said earlier, he imparts a lot spin on the ball. However, to see how this spin would affect the path of the ball we must first consider a ball not spinning being kicked to the left.
As we can see, the air flows around the ball as one might expect. However, let’s see what happens when we add a spin to the ball.
Notice that, unlike before, our sets of arrows are no longer symmetrical and there is less space that the top set of arrows occupy than the bottom set of arrows occupy, this means there is lower pressure on the underside of the ball, as the air moves quicker with the rotation. The air on the top moves slower as it’s against the rotation, and thus has a higher pressure.
Bend it like... Magnus?
Now that we understand the pressure difference, we can begin to explain how the Magnus Effect works. As the air is mostly flowing towards the top of the ball, according to Newton’s Third Law there must be an equal and opposite force on the ball making it curve downwards.
Knuckleballs
Now you understand what makes a spinning ball curve, what's going on here?
Notice anything different to before? Well, for a start, the ball seems to curve in one direction, then another, and then another! Yet, if we look closely, the ball hardly appears to spin! How is this possible? The Magnus effect, as we now know, only applies to spinning balls! Something else must be going on here…
And something else is going on! This is a knuckleball and the lack of spin is actually crucial! Next we need to turn our attention to the ball, notice it is made up of patches and has seams, this means it isn’t smooth. Normally when the ball rotates, the air “sticks” to the ball very well, but it doesn’t stick so well when the ball isn’t spinning. When it isn’t sticking well, the balls seams can disrupt the airflow, and the disrupted air becomes turbulent.
This turbulent air sometimes can turn back on itself and give the ball a small “push” making it change direction seemingly randomly, as seen in the video above! Its unpredictability is caused by the fact it’s impossible to know which seam will cause the turbulence. So next time you’re taking a free kick, whether you want to curl one in the top corner, or hit a unpredictable knuckleball that no keeper will be able to predict, now you know how to do it!