CURL IT LIKE CARLOS – THE SCIENCE BEHIND THE SPIN THAT GOALKEEPERS DREAD
▶ Top players unwittingly use little-known Magnus effect to put bend and dip into shots,
It appears seemingly out of nowhere and leaves people amazed. Once dismissed as a myth, in recent weeks it has been witnessed around the world and it could decide the outcome of today’s Fifa World Cup final or Wimbledon tennis championship.
Scientists call it the Magnus effect, where spinning objects create gravity-defying forces. Yet while its existence is no longer disputed, arguments still rage over how to explain it.
But sport stars have only one concern – how to conjure it up on demand.
Some have clearly got close to mastering it. During the group stages of the World Cup, Portugal’s Cristiano Ronaldo made headlines with his stunning use of the Magnus effect to complete his hattrick against Spain.
At first, his free kick looked far off target. The ball was seemingly heading wide of the wall of defenders.
But then the force took over and magically curled the ball around the back of the wall and into the goal.
French football fans will hope that defender Benjamin Pavard will today repeat the incredible goal he scored with the help of the Magnus effect in the World Cup against Argentina.
In what was arguably the shot of the tournament, he controlled a tricky cross then struck the ball so that it first moved away from the goalkeeper, then curled and dipped into the net.
Cynics might say Pavard was lucky. But the way he positioned his body and struck the ball suggests he knew exactly how to increase his chances of success.
The trick lies in getting the right mix of three forces – the pull of gravity, the Magnus effect and aerodynamic drag. The action of gravity is most familiar to us and therefore most likely to let us down if other forces interfere with it. And that is exactly what the Magnus effect does.
Named after German physicist Heinrich Gustav Magnus, who investigated it in the 19th century, it appears whenever air flows around a spinning object such as a football that has been kicked off-centre.
As the ball travels, its spin affects the flow of air over the nooks and crannies of its panelled surface, accelerating the air travelling in the direction of the spin and slowing it down on the opposite side.
But how this makes the ball swerve has long been the source of argument.
Many textbooks still claim that the change in speed causes a pressure difference across the ball, changing its direction. The argument is that the air is obeying a bit of school science called Bernoulli’s Law. This states that making fluids travel faster reduces their pressure.
As the air flowing across the ball travels at different speeds, it must therefore also create a pressure difference on opposite sides of the ball, and make it swerve.
But like so much in aerodynamics, including the explanation as to how planes fly, the reality is more subtle.
Air is compressible and slightly “sticky”, properties with which Bernoulli’s Law struggles.
The correct explanation is actually simpler. The air travelling over the ball in the direction of the spin speeds up and changes direction. In other words, it accelerates. Then by Newton’s famous law of action and reaction, this generates a force on the ball, making it swerve.
This then leads to a simple rule: if you want the ball to curl left, then kick it to the right of centre, and vice versa.
But as Pavard’s shot showed, the Magnus effect can also make a ball dip. This requires kicking the ball slightly above-centre. That gives it top-spin, making it drop faster than gravity alone would manage. A kick below-centre gives the ball back-spin, making it travel further and flatter.
Put like that it sounds simple. But because the spin-rate is so important, the ball must be kicked in just the right spot. Missing by just a centimetre can send a shot from the edge of the penalty box wide of its target by a metre or more.
And on top of all this, there’s a third force to deal with: aerodynamic drag. Roughly speaking, the faster the ball, the higher the drag. But as the ball slows below about 30kph, the flow of air over its surface changes, increasing the drag with sometimes dramatic effect.
The wonder is that any football player can achieve the right mix of all three forces with any reliability. Yet there is every chance we will witness a demonstration in Moscow later today.
Tennis fans are much more likely to see the wonders of spin in today’s final at Wimbledon. That’s partly because tennis balls are aerodynamically less complex than footballs with their panelled surfaces.
But another reason is that it’s easier to control the spin rate with a racquet than a boot.
Even amateur tennis players can learn how to “slice” the ball, running their racquet upward against it to produce topspin, or downward for back-spin.
The Magnus effect then does the rest. Top-spin produces a force that makes the ball drop unexpectedly fast; back-spin makes it travel further and flatter.
As a bonus, both also affect what happens when the ball hits the opponent’s racquet or the ground.
Professional tennis players can summon up the Magnus effect to order. But the top stars can also control its strength with exquisite precision, making the ball spin at more than 3,500 revolutions a minute – faster than a washing machine’s spin cycle.
That means the ball completes about 80 rotations when it reaches a rival on the baseline and bends the law of gravity.
For the two teams in Moscow and the two players at Wimbledon, success or defeat could depend on their ability to wield the Magnus effect.
So whoever you support, as they prepare for the start you may want to say quietly: “may the Force be with you”.
The trick lies in getting the right mix of three forces – the pull of gravity, the Magnus effect and aerodynamic drag