A non-viscous liquid of constant density $500kg/m^3$  flows in a variable cross-sectional tube. The area of cross-section of the tube at two points $P$  and $Q$  at heights of $3m$  and $6m$  are $2\times {10}^{-3} m^2$  and $4\times {10}^{-3} m^2$  respectively. the work done per unit volume by the forces of gravity as the fluid from point $P$  to $Q$  is: