Explanation

(c) Let the thickness of one plank be $s$
[image:6_image51.png]
If bullet enters with velocity $u$ then it leaves with velocity
$v = \left( u - \frac{u}{20} \right) = \frac{19}{20}u$
From $v^{2} = u^{2} - 2as$
$\Rightarrow \left( \frac{19}{20}u \right)^{2} = u^{2} - 2as \Rightarrow \frac{400}{39} = \frac{u^{2}}{2as}$
Now if the $n$ planks are arranged just to stop the bullet then again from
$v^{2} = u^{2} - 2as$
[image:6_image52.png]
$0 = u^{2} - 2ans$
$\Rightarrow n = \frac{u^{2}}{2as} = \frac{400}{39}$
$\Rightarrow n = 10.25$
As the planks are more than 10 so we can consider $n = 11$