A hailstone is released from rest and falls vertically. While it falls, its acceleration in $\mathrm{m\,s^{-2}}$ is $a = 10\left(1 - (kv)^{2}\right)$, where $v$ $\mathrm{m\,s^{-1}}$ is its velocity and $k = \dfrac{1}{40}$.
(a) · 4 marks
Find the exact velocity of the hailstone after $4$ seconds.