5. Given $\overrightarrow{O P}=(2,1), \overrightarrow{O A}=(1,7), \overrightarrow{O B}=(5,1)$, let $C$ be a point on the line $O P$ (where $O$ is the origin). When $\overrightarrow{C A} \cdot \overrightarrow{C B}$ reaches its minimum value, the coordinates of $\overrightarrow{O C}$ are $\qquad$ .
Let $\overrightarrow{O C}=(2 t, t)$, then $\overrightarrow{C A} \cdot \overrightarrow{C B}=(2 t-1)(2 t-5)+(t-7)(t-1)=5 t^{2}-20 t+12=5(t-2)^{2}-8$, so $\overrightarrow{C A} \cdot \overrightarrow{C B}$ reaches its minimum value when $\overrightarrow{O C}=(4,2)$.