FIFA World Cup 2018: Tracking the path of a curveball

To produce this curvilinear trajectory of the ball requires not just skill but extreme precision. The direction and speed of the spin is what determines how much the ball curves while in flight.

Written by Bharat Sundaresan | Mumbai | Updated: July 1, 2018 9:12:40 am
Toni Kroos kicks the ball from the inside of his foot.

How do footballers make the ball swerve and curve in while taking free-kicks and corner kicks?

It starts with the footballer striking the ball slightly off-centre, using the inside or outside of his or her foot, to generate spin on it. It’s the spin that eventually dictates the swerve. When it’s the inside of the foot that’s been used, the ball rotates around its axis in an anticlockwise direction while being propelled forward. Now the air that’s flowing from the opposite direction creates friction and drag resistance. To understand this kind of resistance, just stick your hand out of the car window. Since the ball travels faster than air, it cuts through it, making it to part. This creates some turbulence.

Imagine a half-submerged rock in riverbed that parts the flowing water. The air flowing on the left side of the ball, or the side towards which it’s rotating, follows the curve and gets dragged to the opposite direction. The air flowing on the right is unable to rotate around the ball, slows down and creates a drag towards its direction. This distortion also leads to a high pressure build-up on the right and low pressure on the left, making the ball curve inwards towards the goal. It’s a phenomenon that is explained by Physics 101 or Newton’s third law. The late tailing of the ball is called the Magnus effect after German scientist Heinrich Magnus who explained it to improve the accuracy of German artillery in 1852.

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Why do you then need to be a Toni Kroos or Cristiano Ronaldo or Lionel Messi to create this swerve, and not just anyone who knows how to kick a ball?

To produce this curvilinear trajectory of the ball requires not just skill but extreme precision. The direction and speed of the spin is what determines how much the ball curves while in flight. Hit it too fast and the ball bends too late, and if it’s too slow then it bends too soon. The kicker also needs to make sure it starts bending from neither too wide nor too close. There is a wall of defenders in front of him, prepared to thwart the strike after all. Every aspect of the kick, from the intensity to the exact point at which the foot connects needs to be perfect.

How does the Magnus effect impact a cricket ball?

There has been a debate over whether the Magnus effect is as relevant to swing bowling as Bernoulli’s principle-increase in velocity leads to decrease in pressure. With reverse swing, for starters the bowler needs to generate high speeds in the 90 mph+ range, thus ensuring that the air flows faster on the smooth side of the ball-owing to less friction-and creates a low pressure area, pushing the ball in that direction.

Some argue that the Magnus effect is more in play when it comes to a wrist-spinner in particular generating drift in the air-Shane Warne’s ball of the century for example-or bowling a top-spinner and flipper. A top-spun ball dips more, due to the Magnus effect enhancing its downward acceleration, and it pitches shorter than expected and gets it to bounce higher. The backspin imparted while releasing the ball for a flipper, ensures that the Magnus effect works on the opposite direction and reduces the downward acceleration. Therefore, the ball is flatter and also stays lower.

Why is the World Cup ball in Russia moving so much in the air?

It has to do with three factors — it being a perfect sphere, the stitching patterns on it and the seam length. The ideal football is a pimple-covered, perfect sphere, its surface just subtly textured enough to keep the airflow around the ball slightly turbulent. The ridges and pimples on the ball make it more aerodynamic, helping the ball to fly through the air more stably

The Telstar 18 is as close to a perfect sphere as you can get. It has subtle pimples and six thermally bonded panels designed to avoid knuckling, which is the characteristic bobbing and weaving movement when a ball is kicked without spin. Stitching patterns on balls create drag when the ball is in air, helping stabilize its path. Researchers tested balls without stitching, and concluded that they spun more and flew at a higher velocity. This year, the ball doesn’t have stitching. Instead, it is thermally bonded, which is one of the reasons why it’s been flying around the field during matches. The ball was designed to reduce the amount of dip and movement players can put on the ball, especially from set pieces.

Does the seam length have as much an impact on a football as it does on a cricket ball?

Seam length can also have an effect on how the ball flies, which has been more important since 2006 when the balls stopped having the same seam length. The total length of seam of the Telstar 18 is 14.1 feet, 3.28 feet more than on the Brazuca.

With the longer seam, and more symmetric panels, no matter how the ball is turned there is the same amount of seam exposed. There is difference in how the ball flies based on where it’s kicked in relation to the seams. However, researchers have found that the Telstar 18 ball doesn’t have as much variation as they are narrower and shallower compared to the balls used in the past.

Is the Magnus effect exclusive to ball sport?

Some experts suggest that the Magnus effect first came to light thanks to the inaccuracy of the cannonball. Isaac Newton though is widely attributed for its discovery in 1674 while he noticed top-spin being used during a tennis match.

The effect was conceptualized in the Magnus Spherical Airship by Fred Ferguson in 1978, which was a helium-filled sphere that was designed to rotate backwards while propelling the airship into the sky. It can also be seen-if you are the academic kinds-while indulging in a game of paintball, where the barrels impart backspin to ensure the colour-filled ammunition flies farther in a straight line rather than falling prey to gravity before hitting the opponent.

Toni Kroos

Kroos kicks the ball from the inside of his foot and this does two things — it launches the ball and it also makes it spin on its axis in anti-clockwise direction. Had Kroos struck the centre of the ball, the rotations on the ball would be less or insignificant. In that case, it would have missed the target by a distance. It’s the spin that makes the ball to swerve.

The air that flows from the opposite direction of the ball creates friction and drag resistance. This creates some turbulence. The air flowing on the left, follows the curve and gets dragged to the opposite direction. The air flowing on the right is unable to rotate around the ball, slows down and creates a drag towards its direction. This distortion also leads to a high pressure build-up on the right and low pressure on the left, making the ball curve inwards towards the goal.

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