$[$Variate$]$ PIPE FLOW $-$ A smooth vertical pipe discharges water downwards as a free jet. There is a tiny hole on the side of the pipe above the outlet. For large flow rates, water squirts out of the hole, whereas for small flow rates, air rushes into the hole. Use the density of the water.

$4\mathrm{.}1.$ Explain how this "trick" works, using the pipe flow energy equation to back up your explanation.

$4\mathrm{.}2.$ Find the critical flow rate above which water will squirt out of the hole if $\backslash (\backslash$mathrm$\{$D$\}=30\backslash$mathrm$\{$~mm$\}\backslash )$ and $\backslash (\backslash$mathrm$\{$T$\}=20^\{\backslash$circ$\}\backslash$mathrm$\{$C$\}\backslash ).$

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