Credit: J. Wang and U. Pesavento/Cornell University

Exactly what governs the motions of falling paper?

While college students suspect the answer is known to lazy professors – the ones who allegedly grade essays by throwing them down stairwells to see which sails the farthest – the so-called falling paper problem has long intrigued scientists.

Now an enterprising professor and her graduate student at Cornell University have solved the falling paper problem -- in part by calculating the motions of a scientific journal page in flight – and their report must have made the grade: The journal Physical Review Letters (Vol. 93, No. 14, "Falling Paper: Navier-Stokes Solutions, Model of Fluid Forces, and Center of Mass Elevation") article by Z. Jane Wang, associate professor of theoretical and applied mechanics, and Umberto Pesavento, a Ph.D. candidate in physics, explains it all.

The same falling-paper principles apply, the physicists believe, to naturally flat things like leaves. If they are right, Wang and Pesavento may have finally solved the mystery of why autumn leaves depart from a neighbor's tree on a windless day . . .

. . . rise into the air . . .                    . . . rise again . . .
                              . . . glide along . . .

. . . and have to be raked from yards that don't contain a single tree.

As Wang explains, "Leaves and paper fall and rise in a seeming chaotic manner. As they fall, air swirls up around their edges, which makes them flutter and tumble. Because the flow changes dramatically around the sharp edges of leaves and paper, known as flow singularity, it makes the prediction of the falling trajectory a challenge."

Among the first scientists to be intrigued by the behavior of falling paper was Scottish physicist James C. Maxwell, who pondered the tumbling motions of playing cards in 1853. But while Maxwell was a brilliant mathematician, he lacked the today's computer-modeling techniques, not to mention access to fast, powerful computers. Wang and Pesavento put those advanced tools to good use to show why the falling trajectory of thin flat things – and the behavior of airflow and other forces – is not predicted by the classical aerodynamic theory.

"There were a few surprises," Wang notes. "We found the flat paper rises on its own as it falls, which would not happen if the force due to air is similar to that on an airfoil. Instead, the force depends strongly on the coupling between the rotating and translational motions of the object."

Wang and Pesavento also showed that the falling-paper effect is almost twice as effective for slowing an object's descent, compared with the parachute effect (that is, if an object falls straight down). And that evidently benefits trees and other plants that need to disperse seeds some distance from the point of origin. Plants with flattened seedpods also take advantage of the falling-paper effect.

The research was funded by National Science Foundation, the U.S. Air Force Office of Scientific Research and the Packard Foundation.

Says the professor who does not use the falling-paper effect to grade student essays and forecast their future: "What is predictable is that as the autumn leaves tumble down, they drift in particular directions, depending on the way they turn. This may explain, Wang adds, "why you are getting the leaves from your neighbor."