A cricket or baseball fielder can run so as to arrive at just the righ
t place at just the right time to catch a ball. It is shown that if th
e fielder runs so that d2(tan alpha)/dt2 = 0, where alpha is the angle
of elevation of gaze from fielder to ball, then the ball will general
ly be intercepted before it hits the ground. This is true whatever the
aerodynamic drag experienced by the ball. The only exception is if th
e ball is not approaching the fielder before he starts to run.