Question

A rifle bullet loses 1/20th of its velocity in passing through a plank. Assuming constant resistive force, the least number of such planks required just to stop the bullet is :

• A. 15
• B. 10
• C. 11
• D. 20

Answer 1

The correct option is C 11

Let v the initial velocity of the bullet, then
$\frac{1}{2}{mv}^2-\frac{1}{2}{m\left(\frac{19v}{20}\right)}^2=F.x$.........(i)
F= average forces, x = thickness of plank
$\frac{1}{2}{mv}^2=Fx.n$...........(ii)
because of work-K.E. theorem
n=number of blocks
Dividing (ii) by (i), we get n=10.25
n=11 (in round numbers)