Question

A motor car running at the rate of 7m/s can be stopped by applying brakes in 10 m. Show that the total resistance to the motion when brakes are on ,is one fourth of the weight of the car.

Answer 1

$$\begin{gathered} \mathrm{Answer} \\ \mathrm{u}=7\mathrm{m}/\mathrm{s} \\ \mathrm{v}=0 \\ \mathrm{S}=10\mathrm{m} \\ \mathrm{we}\mathrm{know} \\ {\mathrm{v}}^2-{\mathrm{u}}^2=2\mathrm{aS} \\ \mathrm{a}=\frac{{\mathrm{v}}^2-{\mathrm{u}}^2}{2\mathrm{S}}=\frac{0-7^2}{2\times 10} \\ \mathrm{a}=-2.45\mathrm{m}/{\mathrm{s}}^2 \\ \mathrm{if}\mathrm{we}\mathrm{take}\mathrm{g}=9.8\mathrm{m}/{\mathrm{s}}^2 \\ \mathrm{we}\mathrm{get}\mathrm{a}=\frac{-9.8}{4}=-\frac{\mathrm{g}}{4} \\ \mathrm{total}\mathrm{resistance}\mathrm{of}\mathrm{motion} \\ -\mathrm{F}=-\mathrm{ma}=\frac{\mathrm{mg}}{4} \\ \mathrm{resistance}=\frac{\mathrm{weight}\mathrm{of}\mathrm{the}\mathrm{body}}{4} \end{gathered}$$