Continuing with the science theme I seem to have hit on today, we come to a news item about a bunch of physicists who have figured out the falling paper problem. What is the falling paper problem? Well, pick up any flat sheet of paper and drop it and you’ll see. Instead of falling straight to the ground the paper will twist and turn and float and spin and, if there are any thermal updrafts in the area, might even rise up a bit before continuing its dance to the floor. Apparently a number of physicists have been asking themselves “why the hell does it do that” since at least as far back as 1853. Now a Cornell University professor and her graduate student think they may have figured it out.
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 Pensavento 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.
In the grand scheme of things this probably isn’t as big a deal as figuring out the evolutionary origins of the human eyeball, but it’s still an interesting bit of work on a problem that has puzzled scientists for quite a long time. I’d imagine there’s probably some practical benefit in this knowledge waiting to be tapped as well even if I can’t quite imagine what it might be at the moment.