|MadSci Network: Physics|
The question is, basically, why is a sidewalk (for instance) slippery with water or ice on it?
The reasons are two.
First, friction is less between the bottom of the shoe and the sidewalk with a layer of water than it would be between the shoe and a dry sidewalk.
Second, ice is an interesting substance which has a raised freezing temperature under increased pressure, so when you walk on the ice and the pressure on the ice is increased a thin layer of ice melts and becomes liquid water, so, again, the friction is reduced. When you are on ice the friction is even less than on just a wet sidewalk because the still-frozen surface of the ice is very smooth.
You can find more information about this by looking at books that describe the "triple point" of water or the characteristics of ice, and also books that discuss friction. Try looking at some of the sources available at the MadSci network, but don't stop there!
MadSci Scientist and Administrator
Try the links in the MadSci Library for more information on Physics.