The fresh water behind a reservoir dam is `15 m` deep. A horizontal pipe `4.0 cm` in diameter passes through the dam `6.0 m` below the water surface as shown in the figure. A plug secures the pipe opening :
(a) Find the friction force between the plug and the pipe wall.
(b) The plug is removed. What volume of water flows out of the pipe in `3.0` hour ? Assume that reservoir is large.
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Explanation

`P_0` : atmospheric pressure
Force of friction `F = (P_0 + rho gh - P_0) A`
=`A rho gh = (pi(4 xx 10^-2))/(4) xx 1000 xx 10 xx 6`
`~= 75 N`
(b) Velocity of efflux `v = sqrt(2 gh) = sqrt(2 xx 10 xx 6) ~= 11 m//s`
Since reservoir is large, height of liquid remains same
`V = a vt = (pi(4 xx 10^-2)^2)/(4) xx 11 xx 33600`
=`149 m^3`.
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