Science of Snow Sliding: Gravity, Friction, and Other Forces

By Coggin Heeringa, Interpretive Naturalist, Crossroads at Big Creek, Inc.

Finally! Crossroads at Big Creek has morphed into an enchanting snow-covered winter landscape, and while we continue with our education and restoration efforts, our attention now turns to sliding. Sliding is defined as “moving smoothly along a surface.” And while we fervently hope nobody slides off a snowy road or slips on the ice, when conditions are adequate, we will be grooming selected trails so our visitors can move smoothly on our trails using skis, sleds and kicksleds.

Months ago, the topic “Sliding” was chosen for this week’s Saturday Science family program.  Geared for elementary students but open to learners of all ages,  the program is an introduction to the  basic physics which enable winter sports. Participants also will  learn a bit about the otters that live in The Cove Estuary and through videos, demonstrations and (conditions permitting) through outdoor experiences, learn about how gravity, force and friction influences smooth movement.

But wait. There’s more.

Scientists are beginning to understand that “sliding” involves more than just force and friction. Obviously, it is difficult  to slide on a bumpy or soft, fluffy surface. A groomed trail is solid and a ski is solid, and though both are essentially smooth, one would expect there to be friction. But skis slide.

So what really makes snow and ice slippery? The answer: water. 

Apparently, there almost always is a very thin film of water, sometimes called a “quasi-liquid layer,” on the surface of snow and ice. Water can be a solid or a liquid or a gas, but no matter what form, the substance is the same. Water is made up molecules, each of which consists of one oxygen atom and two hydrogen atoms (H2O).

The different states of matter are due to the amount of energy each molecule has. In the liquid state, water molecules vibrate more than they do in solid ice. On snow – fluffy or groomed – an extremely thin layer of water exists because the water molecules at the surface, for a variety of reasons, vibrate and move more  than those deeper in the snow pack or in solid ice. The water molecules at the surface tend to be liquid even when the temperature is below the official melting point. And water (even a microscopic film) is slick.  

This phenomenon explains how we offer can environmentally friendly winter sports. It also enables our river otters to “toboggan” over the snow and to create slides into The Cove Estuary. So exactly what is an estuary? That will be explained in a lecture offered on Monday, February 24 at 1:30. The talk will cover geological and human history of The Cove and discuss the wildlife that use an estuary for breeding and migration.