Physics of Soccer

Soccer! The most popular sport in the world. It is a beautiful yet
exhilarating game to play. The game is played on a grass field about
115m long and 75m wide [1] . Each team is allowed to have a maximum of
11 players on the field. The point of the game is to get a circular ball,
which is approximately 27 inch in diameter, into a goal that is only ten feet
wide and eight feet high. Players can use any part of their body to score
but they can't use their arms and hands. The game consists of fast paste
movements and intense acrobatic exercise. The game sounds simple
but there is actually a lot of physics behind soccer. Just kicking a ball
relates to the physics of motion, the physic of friction, and even aerodynamics.
So by learning the physics behind soccer, any player can become a much better soccer player.

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Newton's Laws of Motion


Newton's 1st law

The first law of motion is called the Law of Inertia. It states that “any object at rest, will tend to stay at rest, and any object in motion, will tend to stay in motion unless acted on by an unbalanced force.” This unbalanced force could be: gravity, wind, or any moving object. In soccer however, this unbalance force is usually the soccer player’s foot. He or she will use muscle in the body to create a force to move the leg and kick the ball. Because the ball is at rest, it will continue to stay at rest. But once kicked, it will keep moving in a straight line without any intent of stopping. The reason the ball will stop is because of friction and Earth’s gravitational pull.

Newton's 2nd law

Newton’s second law states that “The change in velocity (acceleration) with which an object moves is directly proportional to the magnitude of the force applied to the object and inversely proportional to the mass of the object.” This can be explained by the equation F=ma. The acceleration of the ball (a) is determined by the force applied (F) divided by the mass of the object that is being moved (m). This simply means that if the ball has a lot of mass, it will require more force to accelerate. If the ball has little mass, it will require little force. In soccer, it is important to know this law because if you want the ball to be moving fast, you must apply more force. If you want the ball to move just a little bit, then just apply less force.

Newton's 3rd law


Newton’s final law of motion states that “for every action, there is and equal and opposite reaction.” This literally means that if you kick the soccer ball, it will kick back at you just as hard. You usually don’t realize this because your leg doesn’t seem to move, but this is because your leg has more mass, meaning it has more inertia, which is the resistance to move.


When a soccer players kicks a ball, he transfers his momentum to the ball. Momentum is the velocity of object times its mass. Also when players pass the ball to each other, they use their feet to slow the momentum of the ball by moving with the ball and resisting it slowly. This way, they can have more control over the ball. It's a simple trick. Multimedia by hhchuong

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Magnus Affect

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Probably the most spectacular thing in soccer is seeing a player curving a soccer ball into the back of the net. Many fans will almost automatic choose Roberto Carlos as the best free kick specialist because of his amazing skill in dipping and bending the ball. So, how do soccer players do this? Easy! It is all physics. A soccer ball is simply a projectile that is flying through the air with a initial velocity. The reason the ball curves is because the kicker kicks that ball at a certain angle and velocity. Once the ball is in the air, it is really the air that is curving the ball. Professional soccer players will usually kick the ball and add a little spin on it to neglect as much air resistance as possible [3] . But in a free kick, which is usually 18 to 30 meters away from the goal, players would actually want air resistance because the air will curve and bend the ball in a way to trick the goal keeper. This all sounds easy but it is extremely difficult. Players must hit the soccer ball with a precise velocity and with a particular spin. According to Bernoulli’s principles, the air travels faster relative to the center of the ball where the periphery of the ball is moving in the same direction as the air flow [4] . In a normal kick, the ball will travel at roughly about 65mph. The ball will spin at around 10 revolutions per second. Once the ball travels about 10 meters, its speed will substantially drop and the drag will dramatically increase. Once the ball’s velocity drops the Magnus affect will start to increase. The Magnus affect is the reason the ball curves through the air. When a ball spins through fluid matter,
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t creates a barrier of air, kind of like a force field. This affect will make the ball travels with just fast enough speed and

curve in the air to trick and beat the goal keeper. World class soccer players are best known for their free kicks because it is most exciting when you see physics applied in soccer.


When a soccer ball is kicked along the grass surface, there is always a force parallel, but opposite of the ball [5] . This force is called friction. The friction force is tangental to the surface and opposite to the ball. Friction is caused when two opposing surfaces make contact with each other. The electromagnetic forces within each surfaces resist the relative lateral motion of objects, in this case, it is the ball and grass. However, friction can also happen when the ball is in the air. Because air is matter, friction is always present. Friction is the reason why the soccer ball always slows down. People sometimes confuse gravity as the force that slows down the ball, but really all gravity is doing is increase friction because is forces the ball down against the ground and when that happens friction occurs.

Projectile Motion

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Projectile motion refers to the motion of an object projected into the air at an angle with the horizontal. As you can see in the animation above, when the ball is being kicked by the soccer player the ball reaches it max and come right down (the movement when being drawn out is like a parabola). When the ball reaches the very tip pity top, the velocity is equal to zero. The reason why the ball did not go straight upward when being kick but come down to earth is because the ball is being pulled down by gravity.
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