100 m/s ના વેગથી જતી ગોળી સમાન જાડાઇ ધરાવત

Let the thickness of each plank is $s$. If the initial speed of bullet is $100 m/s$ then it stops by covering a distance $2s$ By applying ${v^2} = {u^2} - 2as$

$⇒$ $0 = {u^2} - 2as$ $s = \frac{{{u^2}}}{{2a}}$

$s \propto {u^2}$ [If retardation is constant]

If the speed of the bullet is double then bullet will cover four times distance before coming to rest

i.e. ${s_2} = 4({s_1}) = 4(2s)$

$⇒$ ${s_2} = 8s$

So number of planks required $= 8$