Among the more bizarre incidents that can occur in baseball is when a practised fielder struggles to catch an apparently simple "pop-up" - a ball that flies almost vertically off the bat. Now a picture of how the ball travels through the air explains why it can be so tricky.
By modelling forces such as the strike of the bat and air resistance, a team led by Alan Nathan at the University of Illinois, Urbana-Champaign, and Terry Bahill at the University of Arizona, Tucson, calculated the various trajectories that a pop-up can take.
A normally struck baseball follows a parabolic path. But a pop-up's path is influenced by the huge backspin it acquires by hitting the top of the bat. This generates a rotating sheath of air around the ball, which makes its trajectory curve.
In the worst case for the fielder, a backspinning ball begins by flying forwards at ...
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