# Physicists figure out why paper falls the way it does.

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.

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## 8 thoughts on “Physicists figure out why paper falls the way it does.”

1. See!! What was first thought of as random chaos by those demonic atheist scientists is actually a manifestation of intelligent design theory!!!  God had a reason for leaves and paper to flow in this pattern, every time!!!!
(Please note the sarcasm and insane use of multiple exclamation points.  The only thing missing is putting it in ALL CAPS.)

– Matt

2. I sense next year’s recipient of the Ig-Nobel prize.  This research will fit right in with farting geeseand learning why cookies crumble.

3. As far as falling things go, I’m more interested in the scientific theory behind the “two second” law.

As in, if you drop a cookie, but it’s on the floor for less than two seconds, it’s still OK to pick it up and eat it.

Mind you, I’m not crazy—I don’t dispute the fact that the “two second” law works, or can be observed in action (unless the dog gets to the cookie first), I’m just saying that no one has provided a sufficiently compelling scientific explanation of the principles underlying the “two second” law.

4. Actually, they’ve already covered that one. It’s bullshit. Soon as that puppy hits the ground it’s contaminated.

I’ll see if I can dig up a link on it.

5. I have to disagree with Iolite, sometime the seeming inane can lead to some unintended application or inspire “outside-the-box” ideas.

IANAE but, it would be interesting to see if an application for safely landing spacecraft could be derived from this, so as to avoid fiascos like the Genesis crash.

6. Jonesy

I for one believe that it would be possible to increase an objects rate of deceleration whil in sub-orbital flight using this research sometime in the forseeable future, but its still a long way off.

7. I’m not quite sure why this pisses me off (a lot less than plenty of other things, to be sure), but it does! I can’t see wtf they figured out!?!?! (I guess I’m agreeing with what I take to be Matt’s meaning).

I suppose they managed a computer model of falling paper, and figured out that this translational/rotational coupling plays a role in the flight path? BFD!

When I was getting my pilot’s license, I learned a few things. 1) physicists don’t explain lift very consistently. The FAA uses Bernoulli’s principle. But there is also dynamic lift (check out a balsa wood airplane, flat wings, no airfoil camber. That stuff contributes to lift too – maybe less, maybe more. Newton’s third law). Now, I’m sure someone know all about it, but even THAT ain’t simple.

2) When you stall a stable airplane (that is, angle of attack of the airfoil exceeds a critical point, and lift is spoiled) the nose drops, and, if coordinated, you dive until lift is re-established, then level off, stall, dive, level, etc. Called a “falling-leaf stall.” Not exactly falling paper, but a lot is known about this, and it is studied and modeled tirelessly.

But, okay, so these guys have modeled paper falling. I’m down with that. That’s good science, from the modeling camps. But THEN they gotta make a teleological argument? Or at least an evolutionary one? Leaf falling confers an advantage? Well, I’m sure it does, or else it wouldn’t be there, but leaves look the way they do for a whole lotta other reasons too! Overdetermined. Like mucho surface area for photosynthesis. I mean, if they wanted the parachute effect instead of the paper effect, then they’d be, well, acorns. And animals would have to eat ‘em and shit ‘em out somewhere else. Or apples. Hey, what about pine cones? Seen any o’ them wafting delicately through the air recently? Eaten any?

What about a samara? Like a maple’s? They fall in a flat spin, like certain airplanes in an uncoordinated, aggressive stall.

Hmmm, guess it’s been a long day. But, I’m hypo-impressed with the overblown conclusions (I know, they said “may” and “might” – I think scientists got that shit from Watson and Crick – “This structure has novel features which are of considerable biological interest. ” – but they weren’t just fucking around, those guys …)

Ahh … what the hell do I know … maybe I’m pissed off at the dog, or the wife or sumpthin …

Well, I can think of SOME applications for this great discovery, but hey Walmart beat ‘em to it.

I leave you with this warning on wunna their Halloween Superman costumes:

“Caution: Cape does not enable user to fly.”

And who said there were no rocket scientists over there at Walmart …

8. It seems interesting to me, that the model shows results that were unexpected, judging from the small segment posted here.  This is encouraging, because if predictive algorithms can be determined from this, what else might they be applied to?  What insights can be gained into chaos theory from this?

If there is a way around “standard” aerodynamic physics that can be determined from such experiments, such that more efficient, safer, better aircraft can be developed, isn’t that worthwhile?

Now granted, I assume some portion of taxpayer’s dollars went to fund this project, but this certainly seems to be a better expense than why peanut butter makes a dog’s tongue stick to the roof of its mouth.

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