= Contact mechanics
<Contact mechanics> studies how touching bodies deform, transmit <forces> and separate. Even a ball that appears to touch a flat surface at one point develops a finite contact patch: local <elastic deformation> spreads the load into a nonuniform <pressure> distribution. <Hertzian contact> is a useful ideal model when the surfaces are smooth, the contact patch is small and <adhesion> and <friction> can be neglected.
The contact patch is part of the solution, rather than a prescribed loaded area. In nonadhesive contact the <nonpenetration constraint> prevents overlap, and the <normal contact force> can push but cannot pull; <unilateral contact complementarity> joins those requirements. Pressing a spherical probe into a specimen while recording <force> and indentation tests the contact law and can determine a <reduced elastic modulus>. Surface roughness, <plastic deformation> and <adhesion> require additional modeling, especially when interpreting a measured <pull-off force>.
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