ContactForceT.html ContactForceT> class for
% the implementation of the *Simple Spring* tangent contact force model.
%
% This model assumes that the tangent contact force has only an elastic
% component $F_{t}^{e}$, provided by a linear spring.
%
% $$\left \{ F_{t} \right \} = -F_{t}^{e} \{-\hat{t}\}$$
%
% $$F_{t}^{e} = K_{t} \delta_{t}$$
%
% The tangent stiffness coefficient $K_{t}$ can be computed as a function
% of the normal stiffness coefficient $K_{n}$ and the effective Poisson
% ratio $\nu_{eff}$:
%
