A cylindrical vessel 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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Answer

A

Explanation

Let the pressure of the liquid just below the piston be p. The force acting on the piston are:
(a) its weight, mg(downwards)
(b) force due to the air above it, `p_(0)A`(downwards)
(c ) Force due to the liquid below it, `p_(A)`(upwards)
It the piston is in equilibrium,
`p_(A)=p_(0)A+mg`
or, `p=p_(0)+(mg)/(A)`.