A cylindrical block of area of cross-section $A$ and of material of density $\rho$ is placed in a liquid of density one-third of density of block. The block compresses a spring and compression in the spring is one-third of the length of the block. If acceleration due to gravity is $g$, the spring constant of the spring is:
Diffcult
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since the block is at rest, net force on the cylindrical block is zero, l.e.

net upward $=$ net downward force

$F_{s}+F_{b}=W$

where

$F_{s}=$ force due to compression in the spring

$F_{b}=$ buoyant force

$W=$ weight of the cylindrical block

$\Rightarrow k x+A L \frac{\rho}{3} g=A L \rho g$

$\Rightarrow k \frac{L}{3}+A L \frac{\rho}{3} g=A L \rho g$

$\Rightarrow k \frac{L}{3}=\frac{2 A L \rho g}{3}$

$\Rightarrow k=2 \rho A g$

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