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### How do soccer balls Curve?

If you like soccer you probably have enjoyed one of the many times that Roberto Carlos scored an amazing goal while curving the soccer ball, like during the game of Brazil vs. France on 1997. Getting the soccer ball to curve always makes for an incredible kick which is many times unexpected for the opposite team. Curving a soccer ball is not a trick that only professional soccer players can execute, but that anyone can do including you and me. I agree that it takes a lot of practice to learn precisely how to curve a soccer ball exactly the way you want it to. However, if you have ever tried to do this, you know that all it takes is to kick the ball off-center to give it a good spin and the ball will most likely curve. The question is how does this happen? What makes the soccer ball curve under those circumstances is a result of aerodynamics. When a ball is “flying” through the air without spinning the flow distribution around the ball is symmetric. However, if the ball is spinning, the flow distribution around the ball is asymmetric, since there is a preferred direction for the air to go around the ball. This effect can be seen in the following picture:

When the flow around the ball is symmetric (the ball is not spinning) the pressure in both sides of the ball is equal. When the flow around the ball is asymmetric, due to the spinning motion of the ball, the velocity on one side of the ball is faster than on the other side, which makes a lower pressure zone on one side and a higher pressure zone on the other side. This pressure difference generates a net force which forces the ball to curve just like if something was pushing it. This idea is illustrated in the picture below:

Bowling is another sport that takes advantage of the spinning motion of the ball to get it to curve. In the new movie “Wanted”, Wesley Gibson (James McAvoy) curves a bullet around Fox (Angelina Jolie). Even though this is a fiction movie, it is at least theoretically possible to spin and curve a bullet under the same principles as it is possible to spin and curve a soccer or bowling ball.

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