The physical laws behind the shot

Football is a sport that requires speed, strength, endurance and strategy, but surely good ball control also requires physical understanding.

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

Newton's laws of motion

According to Newton's law , if no force were to be applied to a ball, it would continue to move at the same speed and direction as before. When we place the ball on the grass, it stays there (moving in zero) because no force is acting. 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 movement will remain the same.

According to Newton's second law , a force acting on an object will change the acceleration of that object - the rate of change of the speed of the object. When we kick the ball, the force acting on the ball will produce an acceleration from zero to tens of kilometers / h. When the ball leaves the leg, it starts to decelerate (negative acceleration) because the generated friction force is trying to counter its movement. 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 speed in the direction we have kick the ball until it hits other objects or another force acting on it.

When the ball leaves the foot, it starts to decelerate because the generated friction force is trying to counter its movement.

Difference between shot with the foot of the foot and with the tip of the foot

Two things will happen when we kick the ball with the inside cheek.Firstly , kinetic energy on the ball is quite limited, which makes the ball does not go far. Secondly , we control the ball with our feet, so the ball will follow the trajectory the player has aimed for. When a player kicks with the toes, the ball will go stronger but will be less accurate.

The problem with a kick with the toes is that the player gives the ball a very strong force at a small contact area - the tip of the toes. In addition, if you play football on the beach with your bare feet, you will have pain when you touch the ball with the tip of your foot due to the impact force is concentrated on a small point. In contrast, the force exerted on the ball during a foot-kick shot is distributed over a wider area, making the kicker painless.

Where on the ball when shooting with the tip of the foot?

When kicking the ball with the tip of the foot, if the impact point is in the middle of the ball, it will follow a straight and strong trajectory, and the ball will roll on the field until the friction force stops it. Conversely, if the touch point is not in the center of the ball, it will rotate itself on the axis, depending on the point of impact. A collision above the ball will cause the ball to spin forward. Touching 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 surface will cause the ball to spin forward.

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

What's more interesting is when the player kicks the ball next to the ball, the ball starts to spin in the air depending on the kick. When the ball touches the outside of the ball and descends a little near the bottom of the ball, a great thing will happen: the ball will fly into the air while spinning itself on its axis. At first, the ball will fly exactly in the direction of the shot, and it will slow down due to air friction, 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 move to the left or right, depending on the point of impact of the force on the ball. This is called the Magnus effect, more commonly known as the "banana" shot (like the famous banana penalty kick by Roberto Carlos - Brazil team).

The diagram illustrates Roberto Carlos's unbelievable shot into the French team 22 years ago.

As another example in a match, the ball is kicked at the corner, at an 1800 corner with the goal, but the ball still flies into the net. In this case, like the above, the player touches the ball very hard, with the rotation of the ball, the ball has changed its direction almost completely into a banana-shaped trajectory and flies into the goal.

Will a banana shot strong enough to make the ball fly back?

Unfortunately no . Regardless of how strong the shot is and how accurate it is, because the ball is in the air and the friction of the air will slow it down, making the ball's turning radius become smaller. The ball theoretically flies into a smaller, smaller circle, but in order to do so, it needs to spin very quickly around the axis, much faster than the case the player can create with a shot. mine.

Why do players shoot free-kicks to score in case they are far from goal?

According to 2 Newton's law, a force acting on the object will make it accelerate. When the force of action no longer exists and there is a force in the opposite direction, the object will be decelerated. When a player takes a shot, he exerts a force on the ball that accelerates to high speed. At the moment the ball leaves the player's foot, it will stop speeding, and from that point onwards, there are only two forces acting on it: the friction of the air causes the ball to slow down and gravity pulls the ball down. down. When the ball is in flight, it flies slowly and falls lower, so that the ball can fly into the goal instead of falling too early, it needs to be shot high enough. So the next time you see a player kicking a ball from the half of the field into the net, a very complicated physical phenomenon is performed.

The difference between breaking the ball and catching the ball on goal

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 .) When a ball flies to the goalkeeper at high speed, he has two options: catch the ball with two hands or break the ball.

The ball flew to the goal with great momentum. According to the law of conservation of energy, energy cannot be lost, but needs to be transferred from one object to another and from one form to another. Because the energy of the ball is high, breaking the ball manually can cause injury to the goalkeeper because the energy of the ball is completely absorbed into the palm of the goalkeeper (a small area), and part of the energy amount of reflection on the ball when it is pushed in the other direction. Therefore, many goalkeepers choose to clear the ball instead of catching the ball because it makes him less able to bear large forces. Conversely, when the goalkeeper catches the ball, he stops the ball in his hand and absorbs the energy of the ball, with a shot strong enough, he will be pushed back. Therefore, the goalkeeper usually lies on the ball or spins with the ball - this will give him good control of the energy that goes with the ball, preventing the ball from entering the goal.

Why do players wear shoes?

Unlike other sports, football is played on grass. Grass has a very smooth surface, but it also creates a lot of friction with the ball. As the ball rolled on the grass, the grass rubbed the ball and slowed it down. However, the players are less affected by the grass, even the exact opposite: they can slip on the grass. The nails in the shoes keep the player standing on the grass and keep them from slipping. But that is not the sole role of the shoe. Thanks to the nails that stand on the pitch instead of the whole foot stepping on the grass, the players can run faster. This is like a cheetah, with its claws plugged in on the ground, as they run, completely running on their claws. This reduces the friction between the cheetah and the ground, making them run faster to hunt.

Finally, according to Newton's third law, any act of force produces a reaction in the opposite direction and of equal magnitude. The studs also help absorb the reflexes from the balls when the players kick the ball, making them painless when shooting.

Dr. Erez Garty

Davidson Educational Science Research Institute

Thien Tam compiles