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, (g = 10ms–2)

d1 = Displacement of C.G. of block -1 = l, where l = length of side. d2 = 2l, d3 = 3l,....d19 = 19l W = mgd1 + mgd2 + ......+mgd19        = mgl (1+2+3+....19)        = 190 x 0.1 x 10 x 0.1J= 19J