A bullet moving with a velocity of ${\text{10 ms}}^{-\text{1}}$ is brought to rest after penetrating a wooden plank of 4 cm thickness. The retardation of the bullet is 

$\text{s}=4\times {10}^{-\text{2}}\text{m}$                                      $-100=8\times {10}^{-2}\text{a}$

${\text{v}}^2-{\text{u}}^{\text{2}}=2\text{as}$                                          $\text{a}=\frac{-100\times {10}^2}{8}$

$\text{0}-{\left(10\right)}^2=2\left(\text{a}\right)(4\times {10}^{-2})$                  $\text{a}=\frac{-10000}{8}{\text{m/s}}^{\text{2}}$

$\text{acceleration}\left(\text{a}\right)=-1250{\text{ms}}^{-2}$       $\text{a}=-1250{\text{ms}}^{-2}$

$\text{reterdation}=125{\text{0ms}}^{-2}$