A bullet travelling horizontally looses `1//20^(th)` of its velocity while piercing a wooden plank. Then the number of such planks required to stop the bullet is

Options

A. 6
B. 9
C. 11
D. 13

Explanation

Final velocity after passing through a plank
`v=(19u)/(20)`
So, `v^(2)=u^(2)+2as,2as=-(39)/(400)u^(2)`
if bullet passes through n plands and stoopped.
`0=u^(2)+2as,u^(2)+2a(ns).`
`n=-(u^(2))/(2as)=(400)/(39)=10.3` `"So,"(n=1)`