Sidewalk Ice Dancing

We all know how slippery the sidewalks are in the winter. Falling on icy surfaces are "same-level" falls and two types of slips usually occur. One type of slip is where the heel of the forward foot contacts the walking surface, the front foot slips forward and the person falls backward. Another type of slip is when the rear foot slips backwards, the forward force is on the sole of the rear foot and as the rear heel is lifted the force moves to the front of the sole and the person falls.

Traction is the force that allows us to walk without slipping and traction is measured as the "coefficient of friction (COF)." To prevent slips and falls a high COF is needed between shoes and the walking surface. Shoes with soft rubber soles and heels with rubber cleats provide a high COF (0.40-0.50). On very slippery surfaces, falling down may be unavoidable, especially when you're in a rush, and no type of footwear may provide a safe COF.

On a positive note, think of all the cool new dance moves that are created when people have close calls. For instance, there's the Jazz Hands, the Disco Man, the Bear Crawl, the Pretty Princess, the Matrix Man, and the MJ. Maybe you have some of your own moves to add to the list.

Source: University of Florida IFAS Extension

Distracted Walking

Bob, distracted by a big exam he was about to take, walked out of his residence hall on a mission to make it to class on time. He wanted to get an A on this test. He was listening to his iPod to pump himself up. A friend texted him "good luck." While texting back he failed to notice the ice patch on the sidewalk ahead of him. He hit the ice and his feet flew out in front of him. Not only did he fall, he slid into a group of people and knocked them over.

Why is ice slippery? There's a lot of science behind walking on a warm sunny day, but add some winter into the mix and we have some difficulties. Ice is the solid state of water. Many solids retain their shape, but with ice it's not quite like that. The top layer isn't "packed in" like the rest of the layers, so it can move a bit. This is called a quasi-liquid layer and this thin layer of water causes us to slip. Slips and falls on low-friction surfaces cause many injuries a year and tribologists continue to study sliding surfaces and the role of friction.

What could Bob have done to prevent a fall?

1. Paid closer attention to his surroundings.
2. Taken smaller steps to maintain his balance when he encountered the ice patch.
3. Chosen appropriate footwear for the weather conditions.

Sources:
www.purdue.edu/newsroom/research/2009/091207KneppSnowflake.html
www.newscientist.com/article/mg20427392.000