A bullet moving with a speed of `100 ms^(-1)` can just penetrate into two planks of equal thickness. Then the number of such planks, if speed is doubled will be .

Options

A. 4
B. 8
C. 6
D. 10

Answer

C

Explanation

Let the thickness of eachof plank b s, If the initial speed of bullet is `100 ms^(-1)` then it stops by coverting a distance 2s. By applying `v^(2) = u^(2) - 2as`
`rArr 0 = u^(2)-2as`
` s = (u^(2))/(2a)` or `s alpha 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) rArr s_(2) = 8s`
So, number of planks requried = 8`