A block of mass 100kg slides over a distance of 10m on a horizontal...

Given $m = 100kg$

$s = 10m$

$\mu = 0 . 4$

The frictional force is given by

 $\begin{cases}f & = & \mumg \\ & = & 0 . 4 \times 100 \times 10 \\ & = & 400N\end{cases}$

So, the work done by the force can be calculated as follows:

$\begin{cases}W & = & f \times s \\ & = & 400 \times 10 \\ & = & 4000J\end{cases}$