Auckland’s David Grey has studied 1,000 goals scored in an English Premier League season with one objective — to learn the art of the curl shot. His research has uncovered the science behind the sorcery … and he shares his findings in this special two-part feature.
By David Grey
Curl shots make for the most spectacular goals in football.
When the ball moves with absurd late curl and surprise dip on its way into the back of the net, the curl shot can seem inexplicable, a piece of footballing sorcery.
How can the ball change direction like that?
How can a curl shot defy logic so openly to sneak into the top corner?
Physics can explain. If you understand the physics at work in a curl shot, you’ll be a step closer to mastering the technique yourself and incorporating a wicked curl shot into your own game.
Let Ashley Young show you how
Here’s an example of Manchester United’s Ashley Young bending one into the top corner:
My research into the goals of the 2021-22 English Premier League (EPL) reinforced the importance of the football curl shot.
It’s the #1 goalscoring technique for shots taken from outside the 18-yard box, and it accounts for a third of all open play EPL goals scored from more than 9 yards.
Even top players who have pulled off breathtaking curling goals can struggle to explain exactly how they managed the feat.
One must wonder; is it more than just striking the ball with a bit of spin? What really happened? Why did the ball move with such an unexpected and improbable curl to beat the keeper and drop into the corner of the net?
It’s all in the physics …
The answers lie In the physics laws of aerodynamics.
Fear not; I will not baffle you with intimidating physics. I won’t bamboozle you with mathematical formulae and abstract concepts. Bear with me, and please read on, because the next time you step on the pitch, you may find yourself grateful for a fresh understanding of the curl shot’s principles.
Firstly, there’s the physics of the strike of the boot on the ball.
Power is imparted from boot to ball as both velocity and spin. For the curl shot, you want a combination of some horizontal spin with the right amount of power to beat the goalkeeper.
More spin will give you more curl effect, but the more you spin the ball, the less power goes into your shot.
Other factors come into play, too, such as the texture of your boot and the compression of the ball as it’s struck. But for curl, it’s the sidespin that’s most important.
The ideal curl shot curves around and just out of reach of the keeper. It’s the imparted horizontal spin from your strike on the ball that must be enough to achieve this curl, yet you must still strike with sufficient power that the keeper doesn’t have time to move across to make a save.
Let Clayton Lewis show you how
Here’s All Whites midfielder Clayton Lewis curling a free kick for a spectacular goal for the Wellington Phoenix:
Main photo: Clayton Lewis lines up the free kick against the Macarthur Bulls. What happens next is a combination of physics, skill and practice.
Gravity, air resistance … and velocity
Secondly, there’s the physics of the ball travelling through the air after your strike.
If you fire a shot in a straight line, without spin, the ball will at first rise at the angle of your strike.
Gravity takes effect to slow the ball’s ascent and, depending upon the power of your shot, it may begin to descend on its path to goal. This is most evident with a chip or a lob, or a more gently struck curl shot, where there’s time for gravity to exert more influence and the ball can drop significantly before reaching the goal.
With a powerful curl shot that reaches the goal in a fraction of a second, this gravitational effect may be insignificant.
In parallel with the effect of gravity, the ball’s frictional resistance as it travels through the air causes the ball to decelerate from your shot’s initial velocity.
In other words, no matter how powerful your shot, air resistance means that the ball will reach the goal at a slower speed than when it first leaves your boot.
Getting the ball to curl and dip
Thirdly, the late curl and dip of the ball in the most spectacular curl shooting can be attributed to the ball decelerating through a critical speed threshold, where the ball’s aerodynamics change.
As the ball slows to a certain speed, the nature of the air resistance changes, and the slowing of the ball becomes more pronounced. This causes the ball to appear to dip and curl more towards the end of its flight.
In technical terms, this phenomenon is known as the ball’s transition between turbulent air flow and laminar air flow.
You don’t need to learn these aerodynamic theories. Just be aware that if you can get the power of your shot just right so that this turbulent to laminar transition takes place just before your shot reaches the goal, the physics will work in your favour, and your shots will gain the spectacular late curl and dip that is so hard for goalkeepers to deal with.
Fourthly, the phenomenon behind a ball’s curling path through the air has a specific name: the Magnus effect.
The Magnus effect describes how lateral spin on a ball will cause a pressure difference between each side of the ball, thereby creating a force that moves the ball sideways, in a curve through the air.
Curl shots aren’t magic. Physics tells us precisely what’s happening, and why.
A well-executed curl shot doesn’t just happen by chance — at least not if you know what you’re doing. Use your physics knowledge of the curl shot to your advantage. Master the curl shot by refining your strike of the ball to be just right to get the curl shot physics working for you.
With the optimal amount of power, lift and spin, you too can generate the most unbelievable, inconceivable, unthinkable curl shot in your next match.
And now you can also explain how you did it.
David Grey is an Auckland-based football researcher who coaches with the 18 Yard Club. He plays as a striker for Northern League club Manurewa.