The Physics Of Soccer
The Physics Of Soccer — The Magnus Effect
When a soccer player kicks a ball off-center it causes the ball to spin. The direction and speed of the spin will determine how much the ball curves during flight. It's the same principle as a curve ball in baseball. When throwing the ball, the pitcher imparts a fast spin which causes the ball to curve during flight.
The curve of the ball during flight is known as the Magnus effect
. See the figure below.
As the ball spins, friction between the ball and air causes the air to react to the direction of spin of the ball.
As the ball undergoes top-spin (shown as clockwise rotation in the figure), it causes the velocity of the air around the top half of the ball to become less than the air velocity around the bottom half of the ball. This is because the tangential velocity of the ball in the top half acts in the opposite direction to the airflow, and the tangential velocity of the ball in the bottom half acts in the same direction as the airflow. In the figure shown, the airflow is in the leftward direction, relative to the ball.
Since the (resultant) air speed around the top half of the ball is less than the air speed around the bottom half of the ball, the pressure is greater on the top of the ball. This causes a net downward force (F) to act on the ball. This is due to Bernoulli's principle which states that when air velocity decreases, air pressure increases (and vice-versa).
Therefore, when a soccer player kicks the ball right of center the ball spins counter-clockwise and the force acts left, causing the ball to curve left. When the ball is kicked left of center the ball spins clockwise and the force acts right, causing the ball to curve right. This can result in a ball deviating as much as several feet from the original trajectory by the time it reaches the net. This is no doubt a useful strategy when attempting to make a goal, since it makes the path of the ball less predictable to the goalie as he's preparing to block the shot.
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