Spring Force

The spring force was determined through experimentation to be accurately modeled by the equation:$$|\vec{F}_{spring}| = -k_s (|\vec{L}| - L_i)\hat{L}$$$$\vec{F} = -k_s s \hat{L}$$It's easy to determine from this equation:$$s = |\vec{L}| - L_i$$
$k_s$ is the spring constant, or stiffness, of the spring.  It dictates the stretching capabilities of the spring.
$L_i$ is the length of the relaxed spring (no compression/extension).
$\vec{L}$ extends from the point of attachment of the spring to the mass at the other end (the total, final length).
The stretch of $s$ can be negative or positive (compression or extension).