A bullet of mass 200g having initial kinetic energy 90J is shot...

Let us assume that the motion of the bullet is of constant retardation inside the water.

Initial kinetic energy $\dfrac{1}{2}m{v}_{0}^{2} = 90J$

$⇒ {v}_{0} = 30m{s}^{- 1}$

Let the retardation is $a$, then after $1s$

$\dfrac{1}{2}m{\left(30 - a \times 1\right)}^{2} = 40J$

$⇒ 30 - a = 20$

$⇒ a = 10m{s}^{- 2}$

Stopping distance for the constant retardation is given by,

$l = \dfrac{{u}^{2}}{2a} = \dfrac{{30}^{2}}{20} = 45m$.

Therefore, the minimum length of the pool required is $45m$.