Find the average frictional force needed to stop a car weighing $800\mathrm{ }\mathrm{k}\mathrm{g}$ running with an initial speed of $54\mathrm{ }\mathrm{k}\mathrm{m}/\mathrm{h}$ with a distance of $25\mathrm{ }\mathrm{m}$.

Calculate the frictional force needed to stop a car by $F=ma$

Given:

Mass of the car is, m $=800kg$

Initial speed of the car is, $u=54km/hr=54\times \frac{5}{18}m/s=15m/s$ .

Distance covered is, S = 25m

To find the frictional force needed to stop a car, we find the acceleration of the car by the formula:

$v^2-u^2=2aS$

 $$\begin{gathered} 0-{15}^2=50a \\ a=4.5m{s}^{-2} \end{gathered}$$

The frictional force is calculated by,

$F=ma$

 $$\begin{gathered} F=800\times 4.5 \\ =3600N \end{gathered}$$

Therefore, the average frictional force needed to stop the car is $3600N$ .

Hence option $2$ is correct.