  We report a phenomenon that physical perturbations sometimes can benefit the
certainty of a free-fall motion with chaotic modes, albeit, as commonly
believed, they can ruin it. We statistically compare those factors that may
lead to uncertainty, by which we find that the growth of the standard deviation
of the landing locations is directly determined by the physical perturbations.
A significant yardstick is defined in the meantime. This temporal criterion is
of big relevance to the replicability of such problems experimentally, although
they are inherently chaotic. Our hypothesis is verified by experiments from
other literature. This outcome also provides a practical strategy to evaluate
the credible prediction time by estimating the disturbances from physical
parameters as a priori.
