
Initially, the centre of mass of the right column will be at height of 75cm and the centre of mass of the left column will be at 50cm. After opening of the valve both columns will have height of 125cm and their centre of mass will be at height 2125cm. Therefore,
W=−ΔU=−[(m1+m2)g4h1+h2−m1g2h1−m2g2h2]
Now, m1=ρAh1 and m2=ρAh2
Putting ρ=1000kgm−3,g=10ms−2,h1=100cm,h2=150cm and A=16×10−4m2, we get,
W=1J