BCECE Engineering BCECE Engineering Solved Paper-2015

  • question_answer
    A tank is filled with a liquid up to a height H. A small hole is made at the bottom of this tank. Consider \[{{t}_{1}}\] be the time taken to empty first half of the tank and \[{{t}_{2}}\] be the time taken to empty rest half of the tank. Then, determine the ratio \[\frac{{{t}_{1}}}{{{t}_{2}}}.\]

    A) \[~\text{1}.\text{33}\]                 

    B) \[\text{1}.\text{5}\]

    C) \[\text{2}\]                                        

    D) \[0.\text{414}\]

    Correct Answer: D

    Solution :

    The volume of liquid in time dt \[dV=av\times dt=a\sqrt{2gH}.dt\] (\[\because dH\]level falls in dt time ) The volume of liquid coming out of hole \[\text{dV}=\text{A}\left( -\text{dH} \right)\] \[A(-dH)=a\sqrt{2gH}.dt\] since-    \[\int_{0}^{t}{dt=\frac{A}{a\sqrt{2g}}\int_{H}^{0}{{{y}^{-1/2}}dy}}\]                       ? (i) Now, substituting the proper limits in Eq. (i), derived in the theory, we have \[\int_{0}^{{{t}_{1}}}{dt=\frac{A}{a\sqrt{2g}}\int_{H}^{H/2}{{{y}^{-1/2}}dy}}\] \[\Rightarrow \]                               \[{{t}_{1}}=\frac{2A}{a\sqrt{2g}}[\sqrt{y}]_{H/2}^{H}\] \[\Rightarrow \]                               \[{{t}_{1}}=\frac{2A}{a\sqrt{2g}}\left[ \sqrt{H}-\sqrt{\frac{H}{2}} \right]\] \[\Rightarrow \]                               \[{{t}_{1}}=\frac{A}{a}\frac{\sqrt{H}}{g}(\sqrt{2}-1)\]      ? (ii) Similarly, \[\,\int_{0}^{{{t}_{2}}}{dt}=-\frac{A}{a\sqrt{2g}}\int_{H/2}^{0}{{{y}^{-1/2}}dy}\] \[\Rightarrow \]               \[{{t}_{2}}=\frac{A}{a}\sqrt{\frac{H}{g}}\]                                             ? (iii) From Eqs.(ii) and (iii),weget \[\frac{{{t}_{1}}}{{{t}_{2}}}=\sqrt{2}-1\Rightarrow \frac{{{t}_{1}}}{{{t}_{2}}}=0.414\]


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