Question

A 10g bullet is fired at a plank of wood with a speed of 200 m/s. After passing through the plank, which is 1.0 m thick, the speed of bullet reduces to 100 m/s. Find the average resistance offered by the plank?

Answer 1

$$\begin{gathered} \mathrm{Acceleration}\mathrm{of}\mathrm{the}\mathrm{bullet}: \\ \mathrm{a}=\frac{{\mathrm{v}}^2-{\mathrm{u}}^2}{2\mathrm{s}} \\ =\frac{{100}^2-{200}^2}{2\times 1} \\ =-1.5\times {10}^4\mathrm{m}/{\mathrm{s}}^2 \\ \mathrm{Resistance}: \\ \mathrm{F}=\mathrm{ma} \\ =\left[10\times {10}^{-3}\right]\left[-1.5\times {10}^4\right] \\ =-150\mathrm{N} \\ \mathrm{Here},-\mathrm{sign}\mathrm{represents}\mathrm{resistance}\mathrm{force}. \end{gathered}$$