The genesis of solid friction

The laws of friction

Friction is the resistance encountered when one body moves tangentially over another with which it is in contact. The work expended against friction is often redundant, that is, it makes no useful contribution to the overall operation of the device of which the bodies are part, and ultimately must be dissipated as waste heat. Consequently, in most tribological designs our aim is to keep these frictional forces as small as possible. Of course there are exceptions to this general rule, occasions when sufficient friction is essential to continued progress and there are many practical devices which rely on the frictional transmission of power: automobile tyres on a roadway, vehicle brakes and clutches, as well as several of the variable-speed transmission systems now finding wider application. When two objects are to be held together, the only alternative to methods which rely on friction is the formation of some sort of chemical or metallurgical bond between them. The development of this sort of technique–adhesives and ‘superglues’, and even welding and brazing–are relatively recent; ‘traditional’ forms of fixing rely almost exclusively on friction. A nail hammered into a piece of wood is held in place by frictional effects along its length; if the frictional interaction were substantially reduced, the nail would be squeezed out. Similarly, the grip between a nut and a bolt depends on adequate friction between them.