COEFFICIENT OF FRICTION

Centuries ago, scientists noticed that frictional force resisting movement between two hard surfaces seemed to be proportional to the force (weight) pressing the surfaces together. "Coefficient of friction" is the term they used to constant ratio between the frictional force and the pressing force. It's now known that traction, or slip resistance, Is not as simple as this - particularly when water or oil separates the surfaces - but the concept of a coefficient of friction is still used.

If the two surfaces are at rest relative to each other, the static coefficient of friction applies. If they are not, dynamic coefficient of friction applies. Static coefficient is often somewhat higher than dynamic, but can be lower than or equal to dynamic. For dynamic friction, the speed is an important parameter, particularly when there's a liquid between the surfaces.

Static coefficients for wet walking surfaces usually are in the range of 0.3 - 1.0. A low coefficient of friction indicates poor traction, but a person can adjust to this and can walk on ice if aware that traction is poor. A very high coefficient of friction (as between crepe rubber soles and some carpets or sharp steel gratings) can cause stumbling. A static coefficient of friction of 0.5 or 0.6 is usually considered the minimum suitable for pedestrians, the figure chosen depending on the test method and the slider material used to represent a show soling.