Engineering
Physics
Liquid Pressure
Question

For the system shown in the figure, the cylinder on left has a mass (M) of 25 kg, cross-sectional area 20 cm2 and thickness 5 cm. It is connected to a spring of spring constant 1880 N/m. The piston on the right has mass m(= 5 kg) and cross-sectional area 4 cm2. Initially heights of water column in both cylinders are equal and pistons are in equilibrium. The minimum mass m (in kg) to be kept on m so that water just spills out from the left is (g = 10 m/s2)

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Solution

Kx + P0 A' + Mg = P1 A'

P0+KxA'+MgA'=P1         ...(i)

P0 A + mg = P1 A

P0+mgA=P1                  ...(ii)

(i) – (ii)

KxA'+MgA'-mgA=0

Solving, x = 0
So, spring is in natural length initially.  
Let mass put on 5 kg be m
For water to just spill out, y = 5cm

as volume is conserved, y × A' = x × A
                y × 20 = x × 4
                x = 5y = 25 cm
h = x + y = 30 cm = level difference between two masses 
For equilibirum 

KxA'+ρgh=mgA

Solving, m = 2