A bullet moving with a speed of `100 ms^1` can just penetrate two planks of equal thickness. Then, the number of such planks penetrated by the same bullet when the speed is doubled will be

Options

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

Answer

D

Explanation

From equation of motion `v^(2)=u^(2)+2as`Let thickness of one plank is x, when bullet speed is 100 ms - then it is able to penetrate two planks. Thus, `0=100^(2)+2axx2xrArr2a=-(100)^(2)/(2x)`
Let it be able to penetrate r planks when bullet speed is doubled. 0=`(200)^(2)=(100)^(2)/(2x)xxpixrArr(n)/(2)=4rArrn=8`