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

B

Explanation

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 rArr 0 = u^(2)-2as`
`s=(u)/(2a)s prop 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 required = 8