Direct connections between two solids (S1) and (S2) in geometric contact

We have seen that, in order to use the general theorems of dynamics, we need to know what mechanical actions act on systems. Since solids are assembled together to form systems, it is clear that in every problem we will be confronted with the study of contact between solids. That's why we're going to establish an initial model of the laws governing this contact.

Contact takes place over a deformation zone surrounding the geometric contact point I (figure 21 ). Let's denote the deformation zone by ( Σ ). On each dσ element surrounding a point M of the deformation zone there is an elementary action of (S ₁ ) on (S ₂ ). Let's denote it by