Twenty identical cubical blocks each of mass 100gm and of side 10cm are lying on a horizontal surface work done in piling them one above the other is $\left(\mathrm{g}=10\mathrm{m}/{\mathrm{s}}^2\right)$ (in J)

Work done will be equal to the change in gravitational potential energy

Initially the center of masses of the blocks will be at a height of $\frac{\mathrm{l}}{2}$.

Now the position of the center of masses of the blocks will be $\frac{3\mathrm{l}}{2},\frac{5\mathrm{l}}{2},\frac{7\mathrm{l}}{2}$

$\Rightarrow {\mathrm{d}}_1=\mathrm{l}, {\mathrm{d}}_2=2\mathrm{l}, {\mathrm{d}}_3=3\mathrm{l}, .........{\mathrm{d}}_{19}=19\mathrm{l}$

$\therefore \mathrm{W}={mgd}_1+{mgd}_2+\dots \dots +{mgd}_{19}$

$=\mathrm{mgl}(1+2+3+\dots .19)$

$\begin{array}{l}\mathrm{W}=\frac{0.1\times 10\times 10\times {10}^{-2}\times 20\times 19}{2}\\ \Rightarrow \mathrm{W}=19\mathrm{J}\end{array}$