Linear spring and linear damper in parallel acting as line force between frame_a and frame_b. A force f is exerted on the origin of frame_b and with opposite sign on the origin of frame_a along the line from the origin of frame_a to the origin of frame_b according to the equation:
f = c*(s - s_unstretched) + d*der(s);
where "c", "s_unstretched" and "d" are parameters, "s" is the distance between the origin of frame_a and the origin of frame_b and der(s) is the time derivative of s.