Given, velocity of the bullet v=100ms−1 Let, thickness of one plank, t=s According to third equation of motion, we get v2−u2=2as (0)−(100)2=2×a×(2s) ( ∵ two planks are penetrated) ⇒−1000=4 as . . . (i) When velocity of bullet is doubled, v=200ms−1 Applying third equation of motion again, we get ⇒0−(200)2=2×a×(ns) (where, n is number of planks) −40000=2ans . . . (ii) From Eqs. (i) and (ii), we get
−40000
−10000
=
2ans
4as
⇒n=8 Hence, 8 planks can be penetrated by the same bullet if speed is doubled.