From Mechanics

In this case there is resistance to the motion.

Energy at start = Energy lost + Energy and finish.

\displaystyle \begin{align} & 40\times 9 \textrm{.}8\times 12=\text{Energy Lost }+\frac{1}{2}\times 40\times {{8}^{2}} \\ & \text{Energy Lost }=\text{ 3424 J} \\ \end{align}