A cylidrical vesel containing a liquid is closed by a smooth piston of mass m as shown in the figure. The area of cross section of the piston is A. If the atmospheric pressure is `P_0`, find the pressure of the liquid just below the piston.
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Explanation

Let the pressure of the liquid just below the piston be P. The forces acting on the piston are
a. its weight mg (downward)
b. force due to the air above it. `P_0` A (downward)
c. Force due to the liquid below it PA (upward). If the piston is in equilibrium
`PA=P_0+mg`
or P=P_0+(mg)/A`