Question

A bullet loses $\frac{1}{20}$ of its velocity in passing through a plank. The least number of planks required to stop the bullet is

• A. 10
• B. 11
• C. 12
• D. 13

Answer 1

The correct option is B

11

Let length of plank be ‘L’

Now, $v^2–u^2=2as$

${\left(\frac{19u}{20}\right)}^2-u^2=2aL\Rightarrow 2aL=\frac{-39}{400}{u}^2$ (i)

For stopping the bullet, the final velocity of the bullet will be zero

$0^2-u^2=2anL$

From eq. (i), substitute the value of acceleration here, we'll get

$n=10.26$

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.