Human Tribology
The Slippery Banana Skin
Just how slippery is a banana skin? If you step on one in the street will you slip over and hurt yourself, or is it simply a cinematic myth?
As a public service, researchers at the Leonardo Centre have performed a series of experiments to answer this question. The test apparatus is known as a pendulum tester (shown in the picture below). This is the standard device used by the UK Health and Safety Laboratory to measure pedestrian slip resistance (and documented in the British Standard BS7976: Parts 1-3).
The idea is that the pendulum arm, shown clipped up on the right side of the machine, has a section of rubber similar to that used in shoes soles. When the pendulum is released it swings down and strikes the floor surface, and then swings up. If the friction between the rubber and the floor specimen is high, then energy is lost from the pendulum and it does not swing up so far. If the friction is low then the pendulum swings further up along the scale.
We used a polished granite surface as our floor base. And then applied banana skins in various orientations; the yellow outside up and then the white inside up. We then repeated the tests with both wet and dry cases. As a base line we tried both wet and dry granite on its own. The bar chart below shows the results for the dry case.
The results are given in terms of a friction coefficient (the force of friction divided by the normal load). Basically, the lower the number the lower the friction and the more likely you are to slip. The standard for floor slipperiness says that when the friction coefficient (u) is less than 0.24 this is the danger area. When it is greater than 0.36 this is safe. In between 0.24 and 0.36 is a marginal risk.
The first thing we noticed is that polished granite varies enormously between when it is wet and when it is dry. A wet polished surface is very slippery. This is why so many people found the Princess Diana memorial in Kensington Gardens so slippery. But when it is dry there is good grip between the shoe rubber and the surface.
The effect of a fresh banana skin was to reduce the friction to 0.48. Therefore, we are still in the low slip risk region. So really not that bad. Incidentally it did not matter which way up the banana was. The white inside was more slippery but if this was facing up then slip occurred between the white part and the shoe. But if it was facing down then slip occurred again between the white part and the floor. In both cases the measured friction coefficient was about the same.
If the banana skin was a bit old, soggy, and brown however, then the friction fell well into the danger zone. Worse still was a paste of banana skin mashed up with water.
The banana skin is an important prop in slapstick comedy. However, there is a serious side to this. About 1 in 3 of all work place accidents is caused by slips and trips. There are many serious injuries caused by this and 1,110,000 working days lost through these kinds of accidents. Most can be avoided by good design of shoe and floor surfaces. Testing like that performed in these simple experiments also plays an important role.
So to the conclusions; first a dry banana skin is probably ok, definitely don't step on any wet banana skins. But perhaps best of all deposit you banana skins in the rubbish bin.