A bullet of mass $0.02\,\text{kg}$ travelling horizontally with velocity $250\,\text{ms}^{-1}$ strikes a block of wood of mass $0.23\,\text{kg}$ which rests on a rough horizontal surface. After the impact, the block and bullet move together and come to rest after travelling a distance of $40\,\text{m}$. The coefficient of sliding friction of the rough surface is ($g = 9.8\,\text{ms}^{-2}$)