A bullet loses 120 of its velocity in passing through a plank. The least number of planks required to stop the bullet is
Let length of plank be ‘L’
(19u20)2−u2=2aL ⇒2aL=−39400u2 (i)
For stopping the bullet, the final velocity of the bullet will be zero
From eq. (i), substitute the value of acceleration here, we'll get
which means that we will need more than 10 planks to stop the bullet and as the answer should be in whole number, we need 11 of them.