Question

Find the average frictional force needed to stop a car weighing 500 kg at a distance of 25 m if the initial speed is 72 km/h.

Answer 1

$$\begin{gathered} \text{Given}: \\ \mathrm{Mass}\mathrm{of}\mathrm{the}\mathrm{car},m=500\mathrm{kg} \\ \mathrm{Distance}\mathrm{covered}\mathrm{by}\mathrm{the}\mathrm{car},s=25\mathrm{m} \\ \mathrm{Initial}\mathrm{speed}\mathrm{of}\mathrm{the}\mathrm{car},u=72\mathrm{km}/\mathrm{h}=20m/s \\ \mathrm{Final}\mathrm{speed}\mathrm{of}\mathrm{the}\mathrm{car},\nu =0m/s \end{gathered}$$
Retardation of the car,
$$\begin{gathered} a=\frac{\left({\nu }^2-u^2\right)}{2s} \\ \Rightarrow a=-\frac{400}{50}=-8m/s^2 \\ \mathrm{Frictional}\mathrm{force},F=ma=500\times 8=4000\mathrm{N} \end{gathered}$$