In football history, few players are well known for their skill in executing free kick. It is amazing to see how they bend the ball so it passes the wall and curves in the middle of the air to trick the goalkeeper. Ronaldo most known “Knuckleball” free kick is one of the most incredible free kick ever, and it is one of the reason that makes him one of the most expensive football player in the world. Unlike other best free kick executors who kicked with a curling shot and bend over in the middle of the air to trick the goalkeeper, Ronaldo “knuckleball” relies on power to lift the ball over the wall and dip inward under the crossbar. However, viewed in physical law, there is nothing but slight difference between Ronaldo “knuckleball” and other player free kick.
The best way to explain the trajectory of the ball is with a law called the Magnus effect.It is named after the German physicist and chemist H.G. Magnus, who first experimentally investigated the effect. The principle said that if an object spins in a fluid, either gas or liquid, it creates a whirlpool around itself, and it makes the object being pulled to one side. To curve successfully, the ball has to go through a process which involves three factors; they are air, velocity, and spin.
When a ball is kicked and travel in the middle of the air, it experiences a friction from the air around it. Friction happens when two opposing surfaces make contact with each other. The friction happens because the air has a viscous force; viscosity is the measure of resistance to relative motion within fluid. The friction causes the ball to curve.Thus, the thing that makes the ball curves is the air, but it cannot make it curve alone without two other factors.
The next factor is the velocity which is the speed at which an object is travelling. When Ronaldo kicked the ball, he put some velocity to the ball. As discussed above that the ball gets friction from the air when it travels, in fact there are two kind of airflow that oppose the motion of the ball. When the ball is moving slowly, the airflow is called laminar, and when the ball is moving fast, the airflow is called turbulent. In order to curve, the ball has to move fast, and then drop its speed, so what really happens is the air cannot make any difference to the path of the ball when it is in high speed, but the turbulent airflow role is to slow down the ball so it can curve afterwards. The turbulent airflow can slow down the ball because the pressure around the ball is imbalance due to its speed. In other words, it is the imbalance pressure which slows down the motion of the ball. As a result, the ball will travel straightly, and when its velocity drops the mechanism called wake deflection force starts to work, and this mechanism is strongly related to the spinning of the ball.
The secret to the free kick is the spinning of the ball. As stated earlier about the Magnus effect that the spinning object makes a whirlpool around itself, the spin is the main reason that the ball curves. The spin makes the force of air friction on both side of the ball imbalance; the force of one side is weaker than the other one. When the velocity of the ball drops the so-called wake deflect force starts to work, and it pulls the ball to the direction of the weak friction of the ball side. Finally the ball curved from its straight path. However, in Ronaldo’s case the spin factor is slightly different. The “knuckleball” does not spin as fast as other free kick technique, but it does not mean that the ball cannot curve. In fact, this makes the wake deflect force works faster and makes the ball dip quicker. The reason is that the air can catch one side very easily but not the other one, and it makes the ball trajectory is difficult to predict since it can come late in the flight of the ball.
In common curly free kick, the technique is to kick one side of the ball with enough power to give it adequate velocity and strong spin, and the velocity drops before the spin stop, so it can curve. While in knuckleball technique, the secret is to kick the center of the ball with the laces of boot, so it can give adequate power and make the ball barely spin. In football, this technique is rarely seen, but in baseball where the name knuckleball originated is a common technique that requires great mastery, and Ronaldo is a talented player who has mastered it.
To sum up, in physic view, the secret of Ronaldo “Knuckleball” free kick are the air, the velocity, and the spin. The law called as the Magnus effect is responsible for this phenomenon. As a result, to make this kind free kick, a football player just has to practice kicking the ball at right angle with adequate power to make it spin and let the nature take care of the rest.
*** Written by TONI NUR ARDIANSYAH–Penulis adalah ketua umum Unit Kegiatan Mahasiswa (UKM)-BAHASA Foreign Language Association (FLAT) 2011-2012..
Editor: Ahmad Nur Saeful Dkk.