A bullet leaving the muzzle of a rifle barrel with avelocity v penetra

1.	A bullet leaving the muzzle of a rifle barrel with avelocity v penetrates a plank and loses one fifth of its velocity. It then strikes second plank, which it just penetrates through. Find the ratio of the thickness of the planks supposing average resistance to the penetration is same in both the cases.
Answer» VELOCITY of bullet after the FIRST plank will be, $v^{2} = u^{2} - 2a(s1)$ => Now we KNOW, => v = (4/5) * u So, => $(16/25)u^{2} = u^{2} - 2a(s1)$ $s1 = ( (<klux>9</klux>/25)u^{2} ) / 2a$ => For the SECOND plank => $0 = (16/25)u^{2} - 2a(s2)$ $s2 = ( (16/25)u^{2} ) / 2a$ => Therefore, s1 / s1 = 9 / 16