The ratio of the force required to move an object across a surface to the weight of the object.  Hard Polyurethanes with Coefficients of Friction as low as 0.2 are possible, whereas soft Polyurethanes can range from 2 to 3.  As with Abrasion Resistance tests, values of friction obtained in laboratory tests may be misleading in actual service conditions, where such factors as abrasion debris, surface cleanliness, contaminating lubricants, heat etc can change results dramatically.