Football fans in Singapore know that waking up at 3 or 4am for the European tournaments can be brutal. It is a struggle to stay awake when the teams aren’t playing exciting football with lots of goals. At 5.12am on the morning of November 25th though, those tuned in to Liverpool vs Porto were rewarded for their persistence.
Thiago Alcântara, the Spaniard in Liverpool’s midfield, rushed on to the ball as Porto’s defence punted it away from goal. Some 25 meters out, he swung his foot like a wand. Whoosh. The ball glided through a sea of legs into the bottom corner of the net. Close inspection of the slow-motion replays reveals that while the ball kept low to the ground, it never actually bounced. Its trajectory dipped and rose again in mid-air. At the most, the ball skimmed the tips of a few blades of grass.
Magic? No, just really good footballing technique and some physics.
Instead of kicking the ball, Thiago had actually sliced his foot down the back of it. This technique gave the ball a backspin–in other words, the ball was rotating such that its top surface was travelling towards Thiago while the bottom surface travelled towards goal.
Physics took over from there. As the ball flew towards goal, the relative airflow over the ball was backwards, in the same direction the top surface was spinning. This meant airflow over the top of the ball was dragged along by the backspin and deflected. Meanwhile, airflow over the bottom of the ball opposed the spin of the bottom surface and was not deflected. In deflecting airflow over its top, the ball was imposing a force on the air. The air in turn imposed an opposing force on the ball, pushing it upwards. This is the Magnus effect.
The ball’s initial momentum from Thiago’s slice sent it downwards before the Magnus effect kicked in to float it upwards, thus creating the ‘air-bounce’ observed in slow-motion.
The Magnus effect is named after German scientist Heinrich Gustav Magnus, who described it in 1852. He was not the first to do so, however. In the preceding centuries, Isaac Newton and mathematician Benjamin Robins had also described the effect in tennis balls and bullets respectively.
In football, the Magnus Effect is typically observed in direct free kicks, where a player tries to bend a ball over a wall of other players and into the goal. In this scenario, the ball requires topspin to curve it downwards after the kicker has lifted it over the wall – gravity alone is not enough.
One of the most legendary direct free kicks in the sport’s history is Roberto Carlos’ in 1997 for Brazil, which made heavy use of sidespin alongside topspin. His ball had so much sidespin that it veered sideways around the wall and looked to be going out of play before swerving back towards goal like a boomerang.
Thiago’s goal also had a little bit of sidespin that made the ball veer further away from the goalkeeper as it entered the net.
The Magnus Effect also plays a role in several other ball sports, such as golf, the aforementioned tennis, and baseball. Next time you hear the phrase “throw a curveball”, remember that it comes from baseball pitchers making use of the Magnus Effect.
Written by Ellen Ng
Illustrated by Daphne Lim