$\"\"$

\

Volume of the disk is $\"\"$ .

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Weight of the disk is $\"\"$ .

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Distance the disk of the water moved is $\"\"$ .

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Find the value of $\"\"$ in terms of $\"\"$ .

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$\"\"$ .

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Now write the force increment as $\"\"$ .

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Note that distance $\"\"$ is moved from the bottom of the tank must be moved to $\"\"$ .

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Therefore the increment of the work is $\"\"$ .

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Substiute the value of $\"\"$ and $\"\"$ in the equation $\"\"$ .

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$\"\"$

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$\"\"$

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Because the tank is half full $\"\"$ ranges from $\"\"$ to $\"\"$ .

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Now calculate the work required to empty the tank by Substiuting the equation $\"\"$ \ \ in equation $\"\"$ .

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$\"\"$

\

Apply integration :

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$\"\"$

\

$\"\"$

\

The work required to empty the tank is $\"\"$ or $\"\"$ .