Suppose that the position function of two particles, $P_{1}$ and $P_{2},$ in motion along the same line are
$$s_{1}=\frac{1}{2} r^{2}-t+3 \quad \text { and } \quad s_{2}=-\frac{1}{4} t^{2}+t+1$$
respectively, for $t \geq 0$
(a) Prove that $P_{1}$ and $P_{2}$ do not collide.
(b) How close can $P_{1}$ and $P_{2}$ get to one another?
(c) During what intervals of time are they moving in opposite directions?