# Ball Effects

How can you obtain spin shots in soccer?

When a ball is kicked, its trajectory is determined by the forces that affect the ball during flight.
3 forces can be identified:
1 - The propulsive force applied on the ball from the player's foot: the greater the force given to the ball, the longer the distance that it can reach. The ball speed is highest when it is kicked and it gradually diminishes due to air friction.
2 - The friction force given by the flight through the air. The resistance given by the air varies according to the size and shape of the object (a ball in our case) and to the speed. This force is opposite to the propulsive force and causes the progressive slowing down of the ball.
3 - The force of gravity (G) acts downwards and is constant. This force is the responsible for the trajectory arching towards the ground.

In soccer, specific spin effects can be obtained changing in particular two important factors:
- Ball speed
- Ball rotation
The ball speed depends on the force with which it is kicked, while the ball rotation depends on how it is kicked:
- If the ball is kicked applying force directly in its center of mass, the ball moves without rotating
- If the ball is kicked off its center, the ball begins its flight rotating. Rotation speed depends both on the energy applied and on how off the center it was hit.

Players know that by applying rotation to the ball, it is possible to obtain trajectories characterized by specific arched effects: this effect is known as “Magnus effect” in physics.
What not everybody knows is that, during its flight, the ball behaves just like the wing of an aircraft, which can support itself in flight.
Let us see the most frequent cases and the effects that they produce on the ball trajectory.

1° CASE: Trajectory of a ball moving without rotation

CASE: Trajectory of a ball moving without rotation
When the ball is kicked, the air adheres to its surface in the shape of thin concentric air layers.
When the air layer that is closest to the ball (limit layer) reaches the rear part of the ball, it is forced to detach, thus creating a series of vortexes behind the ball.

V = Speed of progress

If the ball is kicked without rotation (as in the picture below), the air flow around the surface is symmetrical and the air pressure is equal on the upper face and the lower face of the ball.
The result of pressures is VOID, so the ball trajectory is determined by the propulsive force (kick) on the one hand and the force of gravity and the forces of friction on the other hand.

P = Air pressure
V = Speed of progress

Regular trajectories are obtained, with a bend only on the vertical axis due to the force of gravity. In the powerful and close-range shots, the influence of the force of gravity is so low with respect to the propulsive force that the shots are basically straight.
(According to the Bernoulli principle, the pressure exerted by a gas (air) on the surface of an object is inversely proportional to the speed the the gas itself on the surface: high speed = low pressure and low speed = high pressure.)
That is how EuroGoal reproduces a powerful shot without rotation: the trajectory is basically straight and very precise.

2° CASE: Trajectory of a ball moving with rotation

If the ball is kicked producing a rotation, its behavior changes radically and spectacularly: the air at its side which moves forward is dragged longer along the surface of the ball itself and detaches later.
The air that is on the opposite side detaches earlier.
The ball rotates and progresses through the air at the same time, so different interactions are created between the progress air flow and the concentric flows produced by rotation.
where flows have the same direction, air speed on the ball surface increases, while where they have opposite directions, they are in contrast and the air speed on the ball surface diminishes.
According to the Bernoulli principle, air pressure on the ball is lower on the side where air flow quickly and higher where it flows more slowly.
The result of these two pressures is an F force (blue arrow) that augments the arching of the trajectory: this phenomenon is called Magnus effect.
P = Air pressure on the ball surface
F = Force resulting from the pressure difference

MAGNUS EFFECT APPLIED IN A PLAY SITUATION
In real play situations, Magnus effect is used numerous times. The most spectacular are those regarding free kicks with wall.
The Magnus effect produces different trajectories according to the more or less inclined rotation axis.
Let us analyze two different situations in which EuroGoal perfectly simulates this kind of trajectories in order to create play situations that are particularly good for training.

- Shot with horizontal or slightly inclined wheels (max. 15°)
The aim is to increase the ball arching on the horizontal axis to reach a target while avoiding opponents.

As can be seen, the ball travels along a trajectory that tends to go back towards the goal.

- Shot with inclined wheels from 15° to 80°.
The aim is to increase the ball arching mainly on the vertical axis to go over the wall and reach the goal.
This kind of trajectory is used in volleyball, tennis and other sports, where it is called TOP SPIN.

This is a particularly spectacular and effective shot that is however difficult to reproduce exactly and constantly.
The advantage of EuroGoal lies in its electronic and mechanical precision that permits to generate any kind of trajectory and speed without ever missing.