sentences:  In contact between a "random rough" surface and an elastic half-space, the true contact area is related to the normal force formula_4 by
  with formula_95 equal to the root mean square (also known as the quadratic mean) of the surface slope and formula_96 .The median pressure in the true contact surface
  can be reasonably estimated as half of the effective elastic modulus formula_98 multiplied with the root mean square of the surface slope formula_95 .Greenwood and Williamson in 1966 (GW) proposed a theory of elastic contact mechanics of rough surfaces which is today the foundation of many theories in tribology (friction, adhesion, thermal and electrical conductance, wear, etc.