The current position of the source and destination points, xs and xd, can be written as

where Xs and Xd are the original positions of the two points, and us and ud are their respective displacements. For the special case where the source point is selected as a fixed point,

The initial spring length, l0, is

The current spring length, l, is

In addition to the initial geometrical distance between the two points you can specify an initial spring extension Δl0, so that the free length of the spring is

The spring extension Δl is computed as the difference between the current spring length and the free length,