A non-viscous liquid of constant density 500kg/m3  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 2×10−3 m2  and 4×10−3 m2  respectively. the work done per unit volume by the forces of gravity as the fluid from point P  to Q  is: