sentences:  Further important insights along these lines were provided by Greenwood and Williamson (1966), Bush (1975), and Persson (2002).The main findings of these works were that the true contact surface in rough materials is generally proportional to the normal force, while the parameters of individual micro-contacts (i.e., pressure, size of the micro-contact) are only weakly dependent upon the load.The theory of contact between elastic bodies can be used to find contact areas and indentation depths for simple geometries.