Question

A rifle bullet loses $\frac{1}{20^{t h}}$ 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, $\left(\frac{19 v}{20}\right)^{2}-v^{2}=2 a x$
and $o^{2}-v^{2}=2 a n x$
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 $11^{t h}$ plank.