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The drag coefficient of a ball plotted against
Reynold's number - a non-dimensional parameter that
takes into account both the velocity and diameter of the
ball. The drag coefficient drops suddenly when the
airflow at the surface of the sphere changes from
laminar to turbulent. The position of the discontinuity
depends on the roughness of the surface of the ball.
Footballs are relatively smooth and so need to be kicked
relatively hard to gain enough speed to move into the
turbulent phase.
First published in Physics World magazine, June 1998
pp25--27. http://physicsweb.org/article/world/11/6/8
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