# Force-Displacement Law

The contact-model formulation provides a force-displacement law relating the generalized internal force to the relative motion at the contact. The internal force and moment ($$\mathbf{F_c}$$ and $$\mathbf{M_c}$$) act at the contact location in an equal and opposite sense on the two pieces (see Figure 1). The formulation is expressed in terms of a set of internal parameters, most of which are associated with rheological components (see Figure 2).

Figure 1: Internal force and moment acting on the two pieces at a ball-ball contact.

Figure 2: Icons and parameters associated with rheological components.