A non viscous liquid of constant density 500 kg/m3 flows in a variable cross-sectional tube [Fig. (6)]. The area of cross section of the tube at the two points P and Q at heights of 3 m and 6 m are 2x10-3 m3 and 4x10-3 m3 respectively. The work done per unitvolume by the forces of gravity as fluid flows from point P to Q is