# The laws of physics behind the shot

Football is a sport that requires speed, strength, endurance and strategy, but good ball control also requires an understanding of physics.

This article will analyze how the laws of physics are reflected in football. Some things seem very trivial, but their physical logic is very interesting.

According to Newton’s 1st law , if no force is applied to a ball, it will continue to move with the same speed and direction. When we put the ball on the grass, it will stay there (zero motion) because no force is applied. However, after we kick the ball, it will continue to move in the direction we kicked it. Its speed will gradually decrease due to friction, but the direction of motion will remain the same.

According to Newton’s 2nd law , a force acting on an object changes the object’s acceleration – the rate of change of the object’s speed. When we kick the ball, the force acting on the ball will create an acceleration from zero to tens of kilometers per hour. As the ball leaves the foot, it begins to decelerate (negative acceleration) because the frictional force generated is trying to counteract its motion. If we kick a ball in space, where there is no friction, it can accelerate for the duration of the kick, and then continue to move at a constant velocity in the direction we have chosen. kick the ball until it hits another object or another force acts on it.

When the ball leaves the foot, it begins to decelerate because the frictional force generated is trying to counteract its motion.

Two things happen when we kick the ball with the inside cheek. Firstly , the kinetic energy on the ball is quite limited, which prevents the ball from going far. Second , we control the ball with our feet, so the ball will follow the trajectory that the player has aimed for. When a player kicks the ball with the tip of his foot, the ball will go harder but will be less accurate.

The problem with the forefoot shot is that the player gives the ball a lot of force in a small area of contact – that is, the tips of the toes. In addition, if you kick the ball on the beach barefoot, you will experience pain when you touch the ball with the tip of your foot because the force of the impact is concentrated on a small spot. On the contrary, the force applied to the ball in the ball of the foot shot is distributed over a larger area, causing the shooter to be painless.

When kicking the ball with the toe, if the impact point is in the center of the ball, it will follow a straight and strong trajectory, and the ball will roll on the grass until the friction makes it stop. Conversely, if the point of impact is not at the center of the ball, it will rotate on its own axis, depending on the point of impact. A collision at the top of the ball will cause the ball to spin forward. Tapping at the bottom of the ball will turn it backwards. However, in both cases, after a short period of time, the friction caused to the ball by the grass will cause the ball to spin forward.

When the player kicks the ball on the side of the ball, the ball begins to spin in the air depending on the point of the shot.

What is more interesting is that when the player kicks the ball on the side of the ball, the ball starts to spin in the air depending on the point of the shot. When the touch point is outside the ball and drops a little bit near the bottom of the ball, something amazing happens: the ball will rise into the air while spinning on its own axis. At first, the ball will fly exactly in the direction of the shot, and it will slow down due to the friction of the air, the rotation of the ball will cause the air on one side of the ball to move faster than the other, so then the ball will change direction left or right, depending on where the force is applied to the ball. This is known as the Magnus effect, more commonly known as the “banana-shaped” shot (like Roberto Carlos’s famous banana free-kick from Brazil).

Diagram illustrating Roberto Carlos’ incredible shot against France 22 years ago.

Another example in a game, the ball was shot at the corner, at an angle of 1800 to the goal, but the ball still went into the net. In this case, like above, the player touches the ball very hard, with self-spinning, the ball has changed direction almost completely into a banana-shaped trajectory into the goal.

Unfortunately no . No matter how strong and precise the shot is, because the ball is in the air and the friction of the air will slow it down, causing the ball’s turning radius to become smaller. The ball could theoretically fly into a progressively smaller circle, but in order to do so it would need to spin itself extremely fast around its axis, much faster than a player can make with a shot. mine.

According to Newton’s 2nd law, a force acting on an object causes it to accelerate. When the impact force is gone and there is a force in the opposite direction, it will cause the object to decelerate. When a player kicks the ball, he applies a force to the ball causing it to accelerate to high speed. At the moment the ball leaves the player’s feet, it stops accelerating, and from that point on, there are only 2 forces acting on it: the friction of the air slowing the ball down and gravity pulling the ball down. down. When the ball is flying, it slows down and falls lower, in order for the ball to land on goal instead of falling too soon, it needs to be shot high enough. So the next time you see a player kick the ball from half the field into the net, a very complex physical phenomenon has to happen.

Energy can create success or force, create motion or generate heat… Energy is never lost, but transferred from one object to another, and from one form to another (from kinetic energy to heat). ability…) When a ball flies to the goalkeeper at high speed, he has 2 options: catch the ball with 2 hands or clear the ball.

The ball flew towards the goal with a huge kinetic energy. According to the law of conservation of energy, energy cannot be lost, but needs to be transformed from one object to another, from one form to another. Because the energy of the ball is high, hand clearance can cause injury to the goalkeeper because the energy of the ball is completely absorbed into the palm of the goalkeeper’s palm (a small area), and some of the energy is absorbed. amount reflected on the ball as it is pushed in the other direction. Therefore, many goalkeepers choose to break the ball instead of catching the ball because it makes him less stressed. Conversely, when the goalkeeper catches the ball, he stops the ball in his hand and absorbs the energy of the ball, with a strong enough shot, he will be pushed back. Therefore, the goalkeeper often lies on top of the ball or spins with the ball – this will give him good control of the great energy that goes with the ball, preventing the ball from entering the goal.

Unlike other sports, football is played on grass. The grass has a very slippery surface, but it also creates a lot of friction with the ball. As the ball rolls on the grass, the grass rubs the ball and slows it down. However, the players are less affected by the grass, even quite the opposite: they can slip on the grass. The spikes on the boots keep the players on the grass and keep them from slipping. But that’s not the sole role of the spiked shoes. Thanks to the nails standing on the grass instead of the whole foot stepping on the grass, the players can run faster. This is like cheetahs, with their claws stuck in the ground, when they run, completely running on their claws. This reduces friction between cheetahs and the ground, helping them run faster to hunt.

Finally, according to Newton’s 3rd law, any applied force produces a reaction of opposite direction and of equal magnitude. The spikes also help absorb the kicks from the balls when the players kick the ball, making them painless when shooting.

Dr. Erez Garty

Davidson Institute of Educational Sciences

Translated by Thien Tam