Understanding the physics of a soccer ball
The beautiful game more art than science? Many would say a mixture of both. Physics plays a vital part of the game whether we know it or not. Most players have learnt the way a ball moves through experience rather than classroom studies. We can, however, show how everything that happens to a ball during a match, is explained by physics.
There are several laws and effects that occur and influence a ball during a game of football.
Newtons Laws of Motion
Newtons first Law, the law of Inertia, states that everything is at rest or in a constant state of motion, unless acted upon by an external force. The external force in soccer is usually the foot or head of a player. Muscles are used to propel the ball and create forward motion the external forces that slow the ball are wind, gravity, and friction. So in short a ball will not go anywhere unless struck, and once struck will carry on going until gravity, wind or an object such as a player or goal post for example, slows it down.
Newton’s second Law of Motion states, the change in velocity 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. So that means the heavier the object is, the harder you have to hit it to achieve the same speed. This Law is expressed in the equation F=ma . Where F is the net force acting upon the object, a is the acceleration and m is the mass of the object.
So if you want a ball to go faster hit it harder.
Newtons Third Law states that when one body exerts a force upon another body an equal force in strength and opposite in direction is applied back to the first body. Simply put every action has a reaction. This applies when a foot or head strikes a ball, it is hardly felt however as the foot has more mass and inertia, unless the ball is waterlogged and heavy in which case it will be.
The most entertaining element of physics to watch within the game of football is when a ball is curled in a set piece move. We all have favorite corners or free kicks taken by the greats over the years. We look at them as artists in the craft David Beckham, Ronaldinho, Didier Drogba, Roberto Carlos… there are many, many more. The spectacular free kicks and corners taken by these guys, are learnt through many hours of practice on the pitch. They can be explained with physics and math. A 19th century German chemist and physicist, Heinrich Gustav Magnus is credited with explaining the lateral deflection of a spinning object. Magnus was studying this effect in relation to artillery shells, but the science also explains why and how a soccer ball curls in the air.
It all revolves around spin, literally. It is a similar effect to how an aircraft is able to fly, an aerofoil creates lift by using airflow to lower the pressure above the wing to create lift. A spinning ball creates low pressure on one side, which pulls the ball in that direction. This effect can be in any direction depending on the direction of spin. So if you want the ball to curl left you have to make the ball spin left to right i.e kick the ball on the left of center. So lets explain how the effect actually works.
When a ball travels through the air it creates a path through the air while the ball is traveling fast the air is smooth or laminar the ball will travel straight . If however you put spin onto the ball this what happens. If a ball is spinning left to right on the right hand side the air is flowing faster because it is traveling in the same direction as the spin.The air on the left hand side is going against the spin so it creates friction and is slower. This creates the difference in pressure and so pulls the ball in that direction. Once the ball slows the airflow around the spinning ball starts to become more turbulent. Turbulent air sticks to the ball and creates more friction and increases the effect even more. This is why a ball looks like it has a gentle curve on it and then suddenly bends dramatically.
So when you watch Roberto Carlos for Brazil take that memorable free kick against France back in the 90’s, you now know how it was done. He kicked the ball on the far left hand side to create the spin but with just enough force to get past the wall before it slowed and the spin pulled the ball back in.
Another great example was the corner kick taken by Macedonia’s Artim Sakiri against England in 2002. Artim struck the ball with his left foot on the right hand side of the ball, making the ball spin right to left it created low pressure on the right hand side thus curling the ball beautifully into the top corner.
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