Consider a horse on a merry-go-round moving along a circular path in the vertical plane. The position is given by $\vec{r}=(5.0 \mathrm{~m}) \cos [(2.0 \mathrm{rad} / \mathrm{s}) t] \hat{i}+(5.0 \mathrm{~m}) \sin [(2.0 \mathrm{rad} / \mathrm{s}) t] \hat{j}$, where r is in meters, and t is in seconds. Determine the horse's acceleration vector.