A bullet moving at  a speed of 20m/sec  strikes a wooden plank and penetrates 4cm before coming to stop. The time taken to stop is 

  $$\begin{gathered} given initial velocity of bullet=u=20m/\sec \\ displacement=s=4\times {10}^{-2}m \\ Final velocity zero v=0 \end{gathered}$$

$\begin{array}{l}equation of motion is v^2-u^2=2as\\ acceleration=a=\frac{0^2-u^2}{2s}\\ a=\frac{-20\times 20}{2\times 4\times {10}^{-2}}\\ a=5000m/s^2\end{array}$ 
equation of motion    $v=u+at$
$$\begin{gathered} 0=u+at \\ \Rightarrow t=\frac{-u}{a} \\ t=\frac{-20}{-5000} \\ t=0.004\sec =time taken to stop bullet \end{gathered}$$