Football Wisdoms Physically Challenged

Does a damp pitch make the ball faster?

Do you know the saying “A wet pitch makes the ball faster”?
This is a situation when a player kicks the ball, the ball hits the gate, bouncing onto the pitch in between. If the grass is dry, the ball slows down. If the lawn is wet, the ball will be visibly faster than on a dry lawn. The goalkeeper can easily miscalculate.

Is there truth to the statement?

First off, the ball gets impetus via the player’s kick” The ball then has kinetic energy and has initial velocity v1. This is reduced by the air resistance (which we will ignore here). When the ball is hits the lawn, friction is generated between the lawn and the ball. Force is applied opposite to the direction of movement. Since the force is applied is outside the center of gravity, it forces the ball into rotation. The ball thus gains rotational energy. Where does this energy come from? The friction between lawn and ball converts a part of the kinetic energy into rotational energy and a small part into heat. The ball thus has less kinetic energy after contact with the lawn, i.e. it has slowed down.

Is the pitch wet, friction between grass and ball is less. Therefore, less kinetic energy is converted into rotational energy. The ball does not lose as much speed and does not gain as much rotational energy as on a dry lawn. It is faster than on a dry lawn, but it is not faster than prior to touch down. The ball would have to experience a force in the direction of movement.

How does a banana flank work?

A player runs down the side line and hits the ball in the direction of the penalty area. The flight curve of the ball describes a curve, the famous banana flank, as described e.g. played by Manfred Kaltz.

Or a free kick situation: the goalkeeper positions the wall so that the player can not shoot directly at the goal. The player runs and ‘bends’ the ball in an arc around the wall at the perfect angle. Goal!

How does the player manage to kick the ball in an arc? Why is the ball’s flight path curved?

What the shooter has to do is the same, which tennis or table tennis players do when they play a topspin ball. When the shooter shoots the ball, it is also thrown into a rotation, but in contrast to the topspin in tennis (which rotates around its horizontal axis) the ball rotates around its vertical axis.

In the case of a rotating ball in a moving medium (air), the Magnus effect occurs. The air flows around the ball on all sides. On the inside of the flight curve, the ball rotates with the air flow, on the outer side against the air flow. The air close to the ball is propelled by the ball; on the outer side, slowed down. According to Bernoulli, the pressure drops in a fluid as the velocity increases. In this case, this means that the pressure near the ball on the inside of the flight curve is smaller than on the outside. Thereby, a force is exerted on the ball, which points from the outside to the inside of the flight curve (perpendicular to the flight curve). This results in the curved trajectory (Magnus effect).

In addition, turbulence plays a role here. On the inside of the flight curve, where the ball rotates in the direction of air flow, turbulences form on the ball surface later than on the outside of the flight curve where the ball rotates counter to the direction of air flow. As a result, the flow experiences an impulse in the direction of the outer side of the flight curve. As a counterforce, the ball experiences a force toward the inside of the flight curve.

Or as Horst Hrubesch said: