On a level road, a scooterist applies brakes to slow down from a speed of $36\text{km/h}$  to $18\text{km/h}$ . The mass of the scooter is 86 kg and mass of the scooterist and petrol is 64 kg. The work done by the brakes is

Mass of scooter = 86 kg

Mass of scooterist + Petrol = 86 + 64 = 150 kg

According to work energy theorem

 $K.E=\frac{1}{2}m{v}^2-\frac{1}{2}m{u}^2$

$u=36\text{km/h}$

$\text{=}\cancel{{}^2\text{36}}\text{×}\frac{5}{\cancel{18}}=10\text{m/s}$

$\text{v = 18}\text{}\text{km/h}$

$\text{=}\cancel{\text{18}}\text{×}\frac{5}{\cancel{18}}=5\text{m/s}$

$=\frac{1}{2}\left(150\right)\left({10}^2-5^2\right)=\text{75}\left(\text{1}00–\text{25}\right)$  

$=\text{75} \times \text{75}=\text{5625 J}$