Translational Spring
====================

A translational spring/damper induces an attractive force between two points that is colinear with a vector drawn from the start to end point. The force magnitude is

    F = k (x - e) + c x'

where k is the spring stiffness, x is the distance between the two points, e is an equilibrium length of the spring, c is the damping constant, and x' is the differential speed between the two points.

**Keyword Arguments**

:param str start_point: Key for the first point.

:param str end_point: Key for the second point.

:param numeric k: Spring stiffness constant.

:param numeric c: Damping constant.

:param numeric equilibrium: Equilibrium length of the spring.

:return str: Message describing the created spring