Kinematics of motion in a straight line — practice question

A particle travels along a straight line from a point $O$. Its velocity $v\text{ m s}^{-1}$, $t\text{ s}$ after it leaves $O$, is $v = t^3 - \frac{9}{2}t^2 + 1$ for $0 \leq t \leq 4$. You may take the velocity to be positive when $t < \frac{1}{2}$, to be zero when $t = \frac{1}{2}$ and to be negative when $t > \frac{1}{2}$.