This paper considers the contact between two nominally flat rough surfaces. The rough surface interaction is viewed as a multi-sphere elastic interaction which upon external application of a tangential force would exhibit partial or full slip depending on the magnitude of the applied tangential force. Constitutive relation proposed by Mindlin at small scale, governing asperity interaction, is used along with the method proposed by Greenwood and Williamson to obtain the large scale slip function through a statistical summation of asperity scale events. The slip function establishes the fraction of asperity contact in full slip. The complement of the slip parameter is a fraction of asperities in partial slip. Through slip function it is shown that it is possible to define a slip condition for the entire surface. The derivation of the slip function allows the account of transition between static friction and kinetic friction. The use of the slip function is demonstrated in the dynamic response of a block in friction contact at the rough surface interface with a flat wherein the static-kinetic friction transition is correctly addressed.

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