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Let, diameter of cylinder hole of height 30m and diameter 2m filled with water. In a book, when author tried to find out the power for lifting all of the water then he used average height. why is that. I mean he wrote average height $ h = \frac{0+30}{2}$. why is that?

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I don't know how to answer without more details from the problem. He might be treating the water as a single object. Then the work done in lifting it out of the container will be equal to the work done in lifting a single particle with mass equal to the water, concentrated at the water's center of mass. The water's center of mass is initially at $y=15$ m. – jld Feb 23 '13 at 9:10
    
@elfmotat: Sounds good to me. You should make a real answer out of that. – Rafael Reiter Feb 23 '13 at 9:14
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But what is the problem, If I do not consider as a single object, so i will use the formua $p= \frac{mgh}{t}$ but they are using $p= \frac{mgh}{2t}$ because of center of mass. – ghost Feb 23 '13 at 11:56

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