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27. Falling Paper, Falling Leaves

Z. Jane Wang, Theoretical and Applied Mechanics, and Ph.D student Umberto Pesavento solved the long-intriguing mystery of the rise and glide of falling paper and of other thin, flat things such as autumn leaves on a windless day. Using advanced computer technology, the researchers showed why the falling trajectory of thin flat things—and the behavior of airflow and other forces—is not predicted by the classical aerodynamic theory. They explained that flat paper rises on its own as it falls, which would not happen if the force due to air were similar to that on an airfoil. The force, instead, depends on the coupling between the rotating and translational motions of the object. The researchers also showed that the falling-paper effect is almost twice as effective for slowing an object’s descent, compared to the straight down parachute effect. The falling-paper effect is beneficial to trees and other plants that need to disperse seeds some distance from the point of origin. Plants with flattened seedpods also exploit this effect.

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