In a hydraulic lift, the surface area of the input piston is $6\;{\rm{c}}{{\rm{m}}^2}$ and that of the output piston is $1500\;{\rm{c}}{{\rm{m}}^2}$. If 100 N force is applied to the input piston to raise the output piston by 20 cm , then the work done is __________ kJ .