Question

A $30g$ bullet initially travelling at $120m/s$ penetrates $12cm$ 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 $120m/s(u).$ It comes to rest after travelling a distance of $(0.12m=s)$ 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, $v=0,u=120m/s,s=0.12m$
$\Rightarrow$ $a=-\frac{u^2}{2s}=\frac{{(120)}^2}{2\times 0.12}$
$\Rightarrow$ $a=-60,000m/s^2$
Resistive force $F=ma$
$\Rightarrow$ $F=0.03\times 60,000$
$F=1800N$