# Friction coefficients

 Material Static Dynamic or rolling Steel on steel 0.74 0.57 Steel on steel lubricated 0.11 0.05 Aluminum on steel 0.61 0.47 Copper on steel 0.53 0.36 Brass on steel 0.51 0.44 Glass on glass 0.94 0.40 Copper on glass 0.68 0.53 Teflon on teflon 0.04 0.04 Teflon on steel 0.04 0.04 Steel on air 0.001 0.001 Steel on ice 0.027 0.014 Wood on stone 0.7 0.3 Rubber on concrete (dry) 0.65 0.5 Rubber on concrete (wet) 0.4 0.35 Rubber on ice (dry) 0.2 0.15 Rubber on ice (wet) 0.1 0.08 Grafite on grafite 0.1 Rubber on asphalt 0.97

## Calculate the friction force

The friction force Fa opposes the relative movement between tow parts in contact, pushed one against the other one with a force Fn normal to the contact surface. The friction coefficient is mu.

Fa=mu * Fn

The size of the contact area does not matter, only the force matter. If you are dimensioning a brake, you calculate the surface so that the pressure (Force/area) is within the maximum stress of the weakest material (for example, the pads of a disk brake are quite soft compared to the disk!).

The “static” friction is the friction between two parts when they have no relative movement. Once the parts “slide” one with the other, the friction diminish and is called “dynamic”.

If instead of sliding, the two parts “revolve” (for example a car wheel on the asphalt), the friction is “rolling”.