Coefficient of friction

From Combat Robot

The coefficient of friction (COF) measure of the effort required to slide a material over a surface, expressed as the force required to slide the material divided by the force pressing the material to the surface. 'Sticky' materials like common tire rubber on concrete have a high COF (about 0.8), while 'slick' materials like Teflon on steel have a low COF (0.04).

While it is often stated that the COF is a material property, it is better categorized as a "system property." Unlike true material properties (such as conductivity, dielectric constant, yield strength), the COF for any two materials depends on system variables like temperature, speed, atmosphere, contact geometry and others.