• # question_answer 5) The average force necessary to stop a bullet of mass $20g$ and $250\text{ }m/s$ as it penetrates wood to a distance of $12cm,$ is: A)  $5.2\times {{10}^{3}}N$        B)  $4.2\times {{10}^{3}}N$C)  $3.2\times {{10}^{3}}N$              D)  $2.2\times {{10}^{3}}N$

Here mass of the bullet $=20g=0.02\,kg$ Speed $\upsilon =250\,m/s$. Distance $s=12\,cm=0.12\,m$ $\therefore$ Loss of kinetic energy                                          $=\frac{1}{2}\,m{{\upsilon }^{2}}$                 $=\frac{1}{2}\times 0.02\times {{(250)}^{2}}$                 $=625\,J$ Now from energy theorem, we have             $F\times 0.12=625$ $\Rightarrow$               $F=\frac{625}{0.12}$ $\Rightarrow$               $F=5.2\times {{10}^{3}}N$