Question

A car of mass 1000 kg moving at a speed of 60 km/hr is stopped in oneminute by applying brakes. Calculate the average power applied by brakes.

Answer 1

$$\begin{gathered} \mathrm{Initial}\mathrm{velocity}: \\ \mathrm{u}=60\mathrm{km}/\mathrm{h} \\ =60\times \frac{5}{18} \\ =\frac{300}{\text{'}18}\mathrm{m}/\mathrm{s} \\ =16.7\mathrm{m}/\mathrm{s} \\ \mathrm{Power}: \\ \mathrm{P}=\frac{\mathrm{Work}}{\mathrm{time}} \\ =\frac{{\displaystyle \frac{1}{2}}\mathrm{m}\left({\mathrm{u}}^2-{\mathrm{v}}^2\right)}{\mathrm{t}} \\ =\frac{{\displaystyle \frac{1}{2}\times 1000\times \left(16.7^2-0\right)}}{60} \\ =2314.8\mathrm{W} \end{gathered}$$