#足球话题发明家 #
It's the quadrennial World Cup, appreciate the various skills of athletes, and often see football players when playing corner kicks will use a serving method - rotating serve, which can make the football bend the corner when reaching the goal area, and the trajectory of the ball is similar to a banana, so it is generally called "banana ball". It's not a football made of bananas, you can't eat it.
This kind of flying trajectory football will suddenly turn a corner, so that the players present, especially the goalkeeper, will not have time to react. The pretty, most iconic and most defying free-kick in history is known as the "banana ball". French physicists studied this for several years and finally explained the problem with the "Magnus effect". This trajectory is an effect in fluid mechanics called the Magnus effect.
1. What is the Magnus effect
Magnus originally studied why the rotating shell and bullet were biased to one side. In fact, the arc ball of the football is also this principle, see the figure below, the ball in the figure is rotated, and the axis of rotation is perpendicular to the flow direction of the air flow field. Because the ball rotates, the speed of different parts of the ball relative to the air is not the same, some places are faster, and some places are slower. According to Bernoulli's principle, negative pressure will be induced on the fast side. On the slow side, the effect is reversed. In this way, the force is unbalanced, so that the ball flies deflected. This lateral deflection of the ball in flight is often referred to as the "Magnus effect".
The forces experienced on a rotating ball flying in air are generally divided into two types: lift and drag. Lift is the upward or lateral force corresponding to the Magnus effect. The direction of action of resistance is opposite to the direction of movement of the ball.
The boundary layer near the surface of the sphere is pulled in the direction of rotation, causing the air on that side to delay separation. On the other side of the ball, rotation causes the air to separate earlier. This creates a force that pushes the ball towards the low-pressure area (air from high pressure to low pressure), the Magnus force.
According to the physical formula, the farther the distance, the slower the speed and the greater the angle of deviation of the ball. Therefore, we can see that at the end of the banana ball, a more violent deflection occurs, giving the goalkeeper a huge "shock".
2. Kick the essentials of "banana ball"
The key to kicking the arc of the banana ball is that the landing point is at the position of the deviation from the center of the ball, and the greater the deviation from the center of the ball, the faster the rotation speed of the ball. According to some researchers, a good free-kick player will tilt the rotation axis of the ball more than the side spin, allowing Magnus force to tilt downward, thus kicking the ball "fast, sharply bent and sharply sinking" the ball.
Therefore, there are high requirements for the footwork and ball intelligence of the players. Beckham uses the inside of his right foot to ensure that the foot has a long contact time with the ball when kicking, forming a strong friction, so that the ball rotates violently, and at the same time giving the ball enough momentum so that it has enough speed to fly to the opponent's goal. Carlos, on the other hand, uses the outer instep of his left foot, the principle is the same, but the ball rotates and deviates in the opposite direction.
The Magnus effect is a phenomenon in fluid mechanics that is the force on an object (such as a cylinder) rotating in a fluid. The Magnus effect is very common in ball games.