This information is part of the Modelica Standard Library maintained by the Modelica Association.

All massless line force elements should be based on this base model. This model defines frame_a and frame_b, computes the (guarded) relative distance s and provides the force and torque balance of the cut-forces and cut-torques at frame_a and frame_b, respectively. In sub-models, only the line force f, acting at frame_b on the line from frame_a to frame_b, as a function of the relative distance s and its derivative der(s) has to be defined. Example:

model Spring
   parameter Real c "spring constant",
   parameter Real s_unstretched "unstretched spring length";
   extends Modelica.Mechanics.MultiBody.Interfaces.PartialLineForce;
equation
   f = c*(s-s_unstretched);
end Spring;