Question

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

• A. 5
• B. 10
• C. 11
• D. 20

Answer 1

The correct option is C 11

Let the thickness of one plank is s
If bullet enters with velocity u then it leaves with velocity
$v=(u-\frac{u}{20})=\frac{19}{20}u$
from $v^2=u^2-2as$
$\Rightarrow (\frac{19}{20}u{)}^2=u^2-2as\Rightarrow \frac{400}{39}=\frac{u^2}{2as}$
Now if the n planks are arranged just to stop the bullet then again from $v^2=u^2-2as$

$0=u^2-2ans$
$\Rightarrow n=\frac{u^2}{2as}=\frac{400}{39}$
$\Rightarrow n=10.25$
As the planks are more than 10 so we can consider n =11