Rubbers and plastics in shoes and flooring: the importance of kinetic friction

Abstract

Traditionally it has been assumed that static friction and dynamic friction are distinct with static always greater than dynamic. Experimental evidence indicates that rubbers and plastics show a continuous change of friction with velocity. Walking is safe if friction (slip resistance) increases rapidly as velocity increases, or alternatively if, under all conditions, the coefficient of friction is greater than the maximum H/V value observed in a force plate experiment. If, however, friction decreases as velocity increases then the situation is potentially dangerous since the requirements for stability increase as the stride is inadvertently lengthened. Sometimes a decrease in temperature can change an increasing friction coefficient to a decreasing one and instability in walking may result. Contaminants drastically alter the friction of all materials and mud or other wet slurries are extremely dangerous. Because walking stick ferrules need to have a much higher friction than shoe soles or heels, a level of dry dust which is not dangerous under normal conditions may be hazardous to those needing a walking aid. Laboratory assessment of flooring materials should always take account of contaminants and cleaning methods.