Banana peel slipperiness wins IgNobel prize in physics

BOSTON — We’ve all seen the cartoons. Bugs Bunny wolfs down a banana and casually tosses the skin onto the floor. Moments later, Elmer Fudd comes racing in, steps on the banana peel and goes flying. The music plays, and Bugs Bunny wins the day again. That wascally wabbit.

No one has ever really questioned this scenario, though few of us have encountered a banana peel in such a dangerous fashion. It just makes sense that banana peels would be slippery if stepped on. But Kiyoshi Mabuchi and colleagues at Kitasato University in Minato, Japan, were not satisfied with mere legend. They decided to find out just how slick that banana peel really is.

In an awards ceremony at Harvard University on September 18, the researchers received the IgNobel prize in physics for the work. The IgNobels celebrate the truly unusual in science, technology, engineering and math. The studies honored by the prizes often make people laugh. But there’s usually serious science behind the quirky studies.

To determine exactly how slippery a banana peel is, Mabuchi and colleagues sacrificed a total of 12 Cavendish bananas, the iconic yellow banana found on grocery shelves around the world. They placed the banana skins interior-side down on samples of linoleum (the scientists stepped on five different sections of each banana peel, for a total of 60 measurements). The linoleum was placed on a force transducer, a sensor that measures the force, weight and pressure applied to an object.

Then a volunteer stepped carefully onto the banana skin and pushed his foot forward. By measuring the horizontal and vertical forces, the scientists came up the coefficient of friction that results from a banana peel sliding along the floor.

It turns out that banana skins are, indeed, very slippery. The average coefficient of friction of a banana on linoleum was 0.066, and friction was only slightly higher for a banana on hardwood flooring (0.083), the researchers reported September 30, 2012, in Tribology Online. These frictional coefficients are very low, usually reserved for substances like Teflon (with a friction coefficient of 0.04), ice (around 0.05) or other well-lubricated surfaces. By comparison, rubber sliding on concrete has a frictional coefficient of 1.02.

But of course, the friction of a banana is nothing if not compared to other fruits. The authors also examined apple peels of various thicknesses, citron peels and tangerine peels. A three-millimeter-thick section of apple peel came in a slippery second, with a coefficient of friction around 0.105. But for no-slip fruits, the tangerine peels came out on top. Their nubby skins had a coefficient of friction of 0.225, about as slippery as wood. The net result, Mabuchi says, was too much fruit. While 12 bananas made it in to the final paper, he notes that he practiced on many more. “I started eating the bananas, but there were too many. I finally distributed them to my students.”

Why is the banana peel so slick? To answer this question, the scientists crushed fruit peels and looked at them under a microscope. They observed that, when crushed, banana peels produce a gel-like substance, which might be the source of the slippery results. Mabuchi, a biomechanics researcher, was not just interested in the bananas. He would like to use these friction results and the properties of the banana skin gel to improve artificial joints. “There is a similarity between the lubrication around cartilage and banana skin,” he says. “They have similar friction.”

Greg Gbur*, a physicist at the University of North Carolina in Charlotte, says that while he has never personally slipped on a banana peel, as an ice skater he is well familiar with slippery surfaces. He says the study “is a nice straightforward piece of work” examining something that “was somewhat taken for granted but never quantified.” He also notes that while the scientists appear to have done the work just because it has never been done before, the incredibly slippery banana peel could have real world implications outside of slapstick comedy. “The gel of the banana peel is surprisingly slippery,” he says. “The coefficient of friction is not much higher than Teflon on Teflon. I wonder if, and this is a big ‘if,’ a deeper investigation of the follicular gel might lead to new forms of lubricants?”

It might be worthwhile to distinguish between the different kinds of friction present in a banana-slipping scenario, suggests Blake Stacey*, a statistical physicist at Brandeis University in Waltham, Mass. When a person steps on a banana peel on the floor, there is dynamic friction between her forward-moving shoe and the top of the banana peel. There is also static friction between the bottom of the banana peel and the floor. For the person attached to the shoe to go flying forward, Stacey says, the dynamic friction must be larger than the static friction. While this is probably the case when someone is walking or running forward, the authors do not differentiate between the two types of friction, which, Stacey notes, could result in “slightly overestimating the hazards” of the banana peel alone.

The next time you see a cartoon character go flying from a banana peel encounter, remember: It’s not just comedy, it’s real science.

*Greg Gbur and Blake Stacey are both personal friends. You can follow Gbur on twitter at @drskyskull and Stacey at @blakestacey.

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