A force element exerts a force and torque on frame_a and with opposite direction on frame_b. In subclasses of this class the cut-forces/torques at frame_a, have to be given as function of the relative quantities computed in class Interact2 (S_rel, r_rela, v_rela, w_rela).
In class Force, the cut-forces/torques at frame_b are computed from the cut-forces/torques at frame_a.
Note:
Both frames of a force element have always to be connected
at frame_a of a joint, of a body or of the inertial system.
It is not possible to, e.g., connect two force elements in series.