A $95\mathrm{.}0-$kg bungee jumper steps off a bridge with a light bungee cord tied to her and to the bridge. The unstretched length of the cord is $12\mathrm{.}0$ m$.$ The jumper reaches reaches the bottom of her motion $36\mathrm{.}0$ m below the bridge before bouncing back. We wish to find the time interval between her leaving the bridge and her arriving at the bottom of her motion. Her overall motion can be separated into an $12\mathrm{.}0-$m free$-$fall and a $24\mathrm{.}0-$m section of simple harmonic oscillation. For the free$-$fall part, what is the appropriate analysis model to describe her motion.