Question

A $30\,g$ bullet initially travelling at $120 \,m/s$ penetrates $12\, cm$ into wooden block. The average resistance exerted by the wooden block is:

• A. 1800 N
• B. 2000 N
• C. 2200 N
• D. 2850 N

Answer 1

Answer: (A)

The bullet comes to rest inside the block, hence final velocity is zero.
Let $ m $ be mass of bullet, travelling with initial velocity of $ 120\,m/s(u). $ It comes to rest after travelling a distance of $ (0.12\,m=s) $ inside the block. Since, it comes to rest the final velocity $ (v) $ is zero. From equation of motion, we have

$ {{v}^{2}}={{u}^{2}}+2as $
Here, $ v=0,u=120\,m/s,\,s=0.12\,m $
$ \Rightarrow $ $ a=-\frac{{{u}^{2}}}{2s}=\frac{{{(120)}^{2}}}{2\times 0.12} $
$ \Rightarrow $ $ a=-60,000\,m/{{s}^{2}} $
Resistive force $ F=ma $
$ \Rightarrow $ $ F=0.03\times 60,000 $
$ F=1800\,N $