The Physics Behind Soccer

Soccer is one of the most popular sports in Singapore. Hundreds if not thousands of people enjoy this sport during their leisure time and have established successful careers in it. Physics plays an important role in this sport and the better you understand the forces that come into play, the higher your chances of becoming a professional soccer player.

Today, we will look at the physics behind soccer in detail. Hopefully, this information will help you to appreciate the sport more and increase your understanding of physics. First, we will look at the different forces that affect the ball when its kicked.

Weight and Mass

In the context of science and engineering, weight is defined as the force acting on an object, in this case the ball, due to gravitational force.

On the other hand, mass is the measure of the amount of matter in the ball or any other object. It measures the quantity of matter of the ball regardless of its location in the universe or the gravitational force that is applied on it. It is also important to note that the mass of a soccer ball is the same on moon and on earth.

If the soccer ball is heavy, it will be difficult to kick. In addition, the weight of the player also comes into play. The more the soccer player weights, the harder they can kick the ball. The only downside to such a player is that their ability to run and speed is lower compared to a lighter soccer player who weighs less and more physically fit.

Friction

Simply put, friction is the resistance force to relative motion of fluid layers, solid surfaces, and material elements sliding on each other. There are four different types of friction namely;

  • Static
  • Sliding
  • Rolling
  • Fluid

The first three, static, sliding, and rolling friction occurs between two or more solid surfaces. Fluid friction, as the name suggests, occurs in gases and fluids.

In soccer, friction force prevents the ball from moving the forward or backward direction forever when it is kicked by the player. This is based on the fact that the soccer ball rubs against the ground causing a resistance force (friction) which slows the ball down. This force can work for or against the team as it can prevent the ball from going through the goal post.

Gravitational Force

Gravity is a natural force that acts on all things that have mass or energy. This includes stars, planets, galaxies, and even light. This is the force that gives weight to all physical objects on our planet. Even the moon has gravitational force that is responsible for the ocean tides.

In the context of soccer, this force has a major impact on how the ball flies. If there was no gravity, the soccer ball would just continue flying in the air and never come back down to the football pitch. The players stay grounded because of the gravitational force that acts on them due to the weight. If gravity was altered, it would change how low or high the ball flies when kicked.

Momentum

Based on Newton’s mechanics, linear momentum refers to the product of velocity and mass of an object. Unlike other forces, it is a vector quantity that has a direction and magnitude. Momentum can also be defined as the impetus gained by a moving object (the soccer ball).

Momentum affects how far the soccer ball goes when kicked. If the player has a lot of momentum when they come into contact with the ball and kick it, it will move faster. On the other hand, if there is not enough momentum, the distance that the ball covers will be less irrespective of how hard the ball is kicked.

Momentum is calculated by multiplying the velocity and mass of the ball. For example; 67 mph (velocity) x 400 grams (mass) = 26,800

Velocity

Velocity is defined as the speed of an object (the soccer ball) in a particular direction. If you don’t know the velocity of the ball, you will not be able to get it. Also, if you know the velocity that it will move with when you kick it, you can accurately pass it to other team members accurately.

Knowing the direction that the soccer ball is most likely to go is very important in soccer especially indoor soccer as the field is smaller compared to the conventional soccer field. For example, if you do not want the ball to out of the set bounds, you have to be aware of its velocity when you kick it.

The Magnus Effect

Whenever the ball is kicked off centre, the force causes the ball to spin either in a clockwise or anticlockwise direction. How the ball curves when in the air is dependent on the speed of the spin and the direction. The same principle applies to a curve ball in the popular baseball game. When the player throws the ball, the pitcher creates a fast spin that results in the ball curving during flight.

Magnus effect is defined as the curve of the ball during flight. As the soccer ball spins, the frictional force between the air and surface of the ball causes the air around it to reach in the direction of the spin. At top spin, the ball results in the velocity of the air at the top half/section of the ball to decreases compared to the air velocity at the bottom.

This effect is as a result of tangential velocity of the ball in the top half section that moves in the opposite direction of the airflow. On the other hand, the tangential velocity of the ball in the bottom half moves in the same direction with the airflow.

Because the resultant air speed that acts at the top half of the ball is less compared to one that is on the bottom half, the pressure exerted at the top of the ball increases. This results in a new downward force. As Bernoulli’s principle states, when the air velocity decreases, air pressure increases. The opposite is also true.

Concisely, when a soccer ball is kicked at the centre, it spins counter-clockwise and the magnus force acts on the left side resulting in the ball curving to the left. On the other hand, when its kicked left of the centre, it automatically spins in the clockwise direction and the Magnus force acts on its right side causing it to curve to the right. As a result, the ball can deviate by several feet from the original trajectory by the time it hits the goal post net. Understand how these forces impact the direction of the ball will come in handy when attempting to score as it makes it difficult for the goal keeper to know which direction the ball will take.

Physics Behind Kicking the Ball

Needless to say, soccer is an active sport that involves kicking the ball. In order to understand the forces that come into play when kicking a ball, consider this action as an inelastic collision between the foot of the player and the soccer ball.

The equation below is used to calculate the initial as well as the final velocities

e stands for the coefficient of restitution. In all collision, this value is usually between 0 and 1. If e = 0 the collision is plastic. The two colliding bodies stick together and move in the same direction and in the same velocity after the impact.

If e = 1 the collision is elastic and the kinetic energy is conserved.

Vb1 refers to the velocity of the ball before its kicked

Vf1 refers to the velocity of the foot of the soccer player before kicking the ball

Vb2 refers to the velocity of the soccer ball after being kicked

Vf2 refers to the velocity of the foot after the kick

Another formula that comes into play is conservation of linear momentum for a one-dimensional collision:

mb refers to the mass of the soccer ball

mf refers to the effective mass of the foot

Effective mass refers to the stand-alone mass that would be required to produce the same dynamic effect after colliding with the soccer ball. The player’s foot is subject to many external forces which are generated by the leg muscles when kicking the ball. An equivalent mass is in the other foot and is not acted upon by external forces and produces the same dynamic effect. This is referred to as the effective mass of the player’s foot and its usually larger than the actual mass of the foot.

Physics Behind Soccer Ball Curving or Spinning

Liverpool is one of the English Premier league teams that is best known for having players like Mane and Salah who have mastered the art of curving the ball. So, what is the physics behind a ball spinning? If the player kicks the ball directly in line with its centre of mass, it will not spin but an off the centre kick will cause it to spin.

The curving, spinning, or bending is caused by a force referred to as Magnus Effect which is generated as air waves move over the spinning ball. As the air flows over the surface of the ball, a thin boundary layer of air is created and clings the surface. As the air approaches the back section of the ball, the layer of air ends as its unable to remain clinging on the surface. It breaks away creating a series of eddies behind the ball.

Concisely, when the ball is spinning, the air on the side that is moving goes in the same direction but breaks away when the ball stops spinning. The air on the opposite side breaks away soon. Newton’s 3rd Law comes into play here, the reaction force of air on the ball, which in the picture below is labelled F, changes the path of the ball thereby forcing it to curve in the direction indicated.

Source: Physics Central

When the ball is not spinning, the flow of air around the soccer ball is symmetrical. The difference in pressure between the back and front section of the ball creates a drag force.

 

Final Thoughts

Understanding how physics impact soccer will greatly help you to become a better player and even qualify to play for renowned teams such as Manchester United and Liverpool. Be sure to apply the formulas above in different scenarios and carry out experiments to better understand how the forces impact the movement of the ball.

More importantly, consider registering for group physics tuition at Kung Fu Physics for tailor-made physics tuition lessons. Your comprehension and performance in physics will never be the same again.