Question

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

• A. $5$
• B. $10$
• C. $11$
• D. $20$

Answer 1

Answer: (C)

Given, ${\left(\frac{19v}{20}\right)}^2-v^2=2ax$
and $o^2-v^2=2anx$
Dividing, $n=\frac{-v^2}{{\left(\frac{19}{20}v\right)}^2-v^2}=\frac{1}{1-{\left(\frac{19}{20}\right)}^2}$
$=\frac{20\times 20}{(20+19)(20-19)}=\frac{400}{39}=10.3$
So, the bullet shall stop in $11^{th}$ plank.