Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (3rd Edition)

Published by Pearson
ISBN 10: 0321740904
ISBN 13: 978-0-32174-090-8

Chapter 15 - Fluids and Elasticity - Exercises and Problems - Page 438: 61

Answer

$28.04\;\rm cm$

Work Step by Step

As we see in the figure below, the water flows from point 1 to point 2. We can use Bernoulli’s equation to find the pressure at each point. So, $$\color{red}{\bf\not}P_1+\frac{1}{2}\rho v_1^2+\rho g y_1=\color{red}{\bf\not}P_2+\frac{1}{2}\rho v_2^2+\rho g y_2$$ We can see that both $P_1$ and $P_2$ are atmospheric pressure. $$ \rho g y_1-\rho g y_2= \frac{1}{2}\rho v_2^2- \frac{1}{2}\rho v_1^2$$ Noting that $y_1-t_2=h$, if we chose $y_2=0$ $$ \color{red}{\bf\not}\rho g h= \frac{1}{2}\color{red}{\bf\not}\rho (v_2^2- v_1^2)$$ $$ g h= \frac{1}{2} (v_2^2- v_1^2)\tag1 $$ Now we need to find $v_1$ and $v_2$. We can find them by the given rate of flow and the continuity equation, where $$Q=A_1v_1=A_2v_2$$ Hence, $$v_1=\dfrac{Q}{A_1}=\dfrac{Q}{\pi r_1^2}=\dfrac{Q}{\pi\left[\frac{d_1}{2}\right]^2}$$ And by the same approach, $$v_2=\dfrac{Q}{A_2}=\dfrac{Q}{\pi r_2^2}=\dfrac{Q}{\pi\left[\frac{d_2}{2}\right]^2}$$ Plugging these two velocitites into (2); $$ g h= \frac{1}{2}\left[\left(\dfrac{Q}{\pi\left[\frac{d_2}{2}\right]^2}\right)^2- \left(\dfrac{Q}{\pi\left[\frac{d_1}{2}\right]^2}\right)^2\right]$$ $$ h= \dfrac{1}{2g }\left[\dfrac{16Q^2}{\pi^2 d_2^4 }- \dfrac{16Q^2}{\pi^2 d_1^4 }\right]= \dfrac{16Q^2}{2\pi^2g }\left[\dfrac{1}{ d_2^4 }- \dfrac{1}{ d_1^4 }\right]$$ $$ h = \dfrac{8Q^2}{ \pi^2g }\left[\dfrac{1}{ d_2^4 }- \dfrac{1}{ d_1^4 }\right]$$ Plugging the known and remember to convert the units to SI units. $$ h = \dfrac{8(0.2\times 10^{-3})^2}{ \pi^2(9.8) }\left[\dfrac{1}{ (10\times 10^{-3})^4 }- \dfrac{1}{ (16\times 10^{-3})^4 }\right]$$ $$h=0.2804\;\rm m=\color{red}{\bf 28.04}\;\rm cm$$
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