A bullet of mass $20\times {10}^{-3}kg$  travelling with a velocity of $\text{750}\text{m/s}$  travels 0.5 m thick plank and emerges with a velocity of $\text{250}{\text{ms}}^{-1}$ . What is the resistance offered by the plank.

Bullet mass (m) $=20\times {10}^{-3}\text{kg}$

Initial velocity (u) $=\text{750m/s}$

Distance (s) = 0.5 m

Final velocity $\left(\text{u}\right)=\text{250m/s}$

Force (f) = ?

$\text{F}=\text{ma}$

${\text{v}}^{\text{2}}-{\text{u}}^{\text{2}}=2\text{as}$

$\text{a = }\frac{{\text{v}}^{\text{2}}-{\text{u}}^{\text{2}}}{2\text{s}}$

$=\frac{{\left(250\right)}^2-{\left(750\right)}^2}{2\times 0.5}$

$=\frac{62500-562500}{1.0}$

$\text{a}=-500000=-5\times {10}^5{\text{m/s}}^{\text{2}}$

$\text{a}=5\times {10}^5{\text{m/s}}^{\text{2}}$

$\text{F}=20\times {10}^{-3}\times 5\times {10}^5={10}^2\times {10}^{-3}\times {10}^5={10}^7\times {10}^{-3}$

$\text{F}={10}^4\text{N}$