http://webusers.npl.uiuc.edu/~a-nathan/pob/AJP-Aug2008.pdf

When I was testing the batter demo on pitcher’s duel, I observed some behaviour that seemed very odd.  The ball when hit sharply up, would seem to wander around.  Tracking it via its shadow directly below the ball, it would move back and forth over the foul line.  I thought at first this must be a bug, but the realized that is how popups behave.  The ball leaving the bat is severely undercut, and has a lot of backspin; the ball with backspin curves back toward the backstop as it rises.  When it  reaches its apex, though, it reverses direction without reversing spin.  As it falls, what was backspin is  now topspin, and pushes the ball towards the pitchers mound.  The path is thus a loop, and the shadow moves backward, then forward.  Follow the link for a much more detailed explanation.

The ball behaviour explains the way fielders, observed in MLB games, seem to wander back and forth tracking a pop fly.  They aren’t lost, they are just tracking the ball.

David

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