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Consider a vertical relatively long cylinder of constant radius open at both ends. We fill this cylinder with water and prevent water from falling down by a certain sheet as seen in the figure.

Now suppose we remove the sheet suddenly. Let v1 be the speed of the upper surface of water, and v2 be the speed of the water at the bottom of the cylinder.

According to the equation of continuity A

_{1}v

_{1}= A

_{2}v

_{2}, since the cylinder has constant radius then A

_{1}=A

_{2}and so v

_{1}= v

_{2}.

According to Bernoulli's equation: P

_{1}+ 0.5pv

_{1}

^{2}+ pgh

_{1}= P

_{2}+ 0.5pv

_{2}

^{2}+ pgh

_{2}

Setting P

_{1}= P

_{2}= P

_{0}(of atmosphere), h

_{1}- h

_{2}= H, we get v

_{2}

^{2}= v

_{1}

^{2}+ pgH which states clearly that v

_{2}> v

_{1}

Question: Which one is correct and why?