A bullet, incident normally on a wooden plank, loses one-tenth of its speed in passing through the plank. The least number of such planks required to stop the bullet is

Let v be the speed of the bullet incident on the first plank. Its speed after it passes the plank =9v10. If x is the thickness of the plank, the deceleration a due to the resistance of the plank is given by              2ax=v2−9v102=19v2100            (1)Suppose the bullet is stopped after passing through n such planks. Then the distance covered by the bullet is s = nx. Thus, we have               v2−0=2as=2anxor    n=v22ax=v2×10019v2 [use Eq. (i) above]                        =10019=5.26Thus the minimum number of planks required is 6. Hence the correct choice is (2).