Question

By sucking through a straw, a student can reduce the pressure in his lungs to $745\, mm$ of $Hg$ $\left(\right.$ density $\left.=13.6\, g / cm ^{3}\right)$. Using the straw, he can drink water from a glass upto a maximum depth of ________$cm$.

Answer 1

Pressure-difference between lungs and atmosphere
$=760\, mm -745\, mm$
$=15\, mm$
$=1.5\, cm$ of $Hg$
Maintaining this pressure difference, let the student drink water upto a depth of $h_{w}$.
$\therefore h _{ w } \rho_{ w } g = h _{ Hg } \rho_{ Hg } g$
$\therefore h _{ w } \times 1=1.5 \times 13.6$
$\therefore h _{ w }=20.4\, cm$
i.e., one can draw from a depth of $20.4\, cm$ of water