Walking in A Winter Wonderland

The snow that was pelting Atlanta earlier has finally made it up here. It’s still at the “ooh, isn’t that pretty!” stage, rather than the “oh, no, not more goddamn snow” stage, so I went for a walk in it.

Yes, the world shrouded in snow and silence was beautiful. But what struck me is that I unconsciously started walking differently.

I usually walk differently in the snow; that part isn’t surprising. What I found surprising is that apparently I’ve internalized my snow gait to the point where I didn’t need to consciously turn it on. Huh.

Normally, when I walk on dry ground, my heel hits the ground first, followed by the ball. My legs don’t bend much.

But when I’m walking on snow, my whole foot hits the ground at the same time. I also bend my knees so that my foot comes down straight, rather than at an angle. It’s a bit like an elephant or a Star Wars AT-AT, where the thigh moves the most, and the shin hangs vertically down from there. As you can imagine, my steps are shorter and I don’t walk as quickly.

It all has to do with coefficients of friction. Plural. For those who have forgotten High School physics, there are two coefficients of friction between two given surfaces: the static one, and the dynamic one. Dynamic friction refers to how hard it is to rub the two surfaces against each other. Static friction refers to when the surfaces are motionless, and how hard it is to get one of them moving.

If you’ve ever received a box of equipment (say, a new computer) and tried pushing it across the floor, you may have noticed that pushing it along the floor is easier than getting it to move in the first place. That’s because when it’s just lying there, you have to overcome static friction to get it to move. When it’s already moving, and you want it to keep moving, you have to overcome dynamic friction. And the dynamic friction coefficient is typically lower than the static one.

Normally, when you’re walking, this doesn’t matter a lot: when your sole hits dry pavement, both the static and dynamic coefficients of friction are high enough that you can trust them to hold your foot in place and allow you to push against the ground for the next step. But on snow or ice, there’s a very real danger of slipping, falling into traffic, and having your head squished by a passing car.

In this light, I think my snow gait makes sense: hitting the ground with my entire foot at once means there’s more surface to take advantage of what little friction there is (though on the other hand (foot?), my body weight is distributed over a larger area, so there’s less friction per square centimeter; I don’t know how this affects things).

Putting my foot straight down means that it’s not moving with respect to the ground when it hits, which in turn means that I’m taking advantage of the static friction coefficient, which is higher than the dynamic one, to keep from slipping. And the “elephant walk” bit is just so that I can put my foot down vertically.

[youtube http://www.youtube.com/watch?v=4B_8DY30xDg&fs=1&hl=en_US]

So now that you know how to safely walk on slippery surfaces, go out there and enjoy the snow.