Curl or bend in association football is spin on the ball which will make it change direction, called a 'screw shot' in the 19th century. When kicking the ball, the inside of the foot is often used to curl the ball, but this can also be done by using the outside of the foot. Similar to curl, the ball can also swerve in the air, without the spin on the ball which makes the ball curl. Curling or bending the ball is especially evident from free kicks, shots from outside the penalty area and crosses. Differences between balls can also affect the amount of swerve and curl: traditional leather footballs were too heavy to curl without great effort, whereas the lighter modern footballs curl with a lower effort threshold. As a general rule, the lighter and smoother the ball the more deviation there is.
Nomenclature
The deviation of a ball from the straight path in the air is known as the curl, or swerve; however, the spin on the ball that causes this is also known as the curl. Shots that curl or swerve are known as curlers, swingers, or in extreme cases, banana shots. The technique of putting curl on a ball with the outside of the foot is known as a trivela, a Portuguese term, with Ricardo Quaresma an exponent. The top spin technique of putting straight curl on a ball is known as a dip or dipping shot.
Usage
Free kicks
Free kick takers often curl and put spin on the ball, to curl it over or around the wall of defending players, out of the reach of the goalkeeper. Goalkeepers usually organize walls to cover one side of the goal, and then stand themselves on the other side. Thus, the free kick taker has several choices, including; either to curl the ball around the wall with finesse, to bend the ball around the wall using power, or to go over the wall. The 1950s Brazilian star Didi invented the folha seca which is nowadays commonly known as the knuckleball free kick, notably used by modern day players such as Cristiano Ronaldo and Juninho, where the ball would be struck with either no or a low amount of spin, causing it to swerve downward unexpectedly at a point near the goal.
Corners
Curling can be an effective technique when taking corners. The ball gradually moves in the air towards the goal. This is referred to as an in-swinging corner. Occasionally, a corner-taker will bend the ball towards the edge of the penalty area, for an attacker to volley, or take a touch and then shoot.
Passing
Curling can be used in passing. Effective passes from midfield to an attacking player are often the result of a curled pass around the defender, or long cross field passes are sometimes aided by the addition of curl or backspin.
Causes
The reason that spin on a football makes it curl is known as the Magnus effect. This causes a rotating ball to form a whirlpool about itself, with one side's air moving with the ball and the other side's air moving against the ball. This creates a difference in air pressure, and the ball deviates from its path as a result of this. The Magnus effect is named after German physicistHeinrich Gustav Magnus, who described the effect in 1852. In 1672, Isaac Newton had described it and correctly inferred the cause after observing tennis players in his Cambridgecollege.
