Question

by seeing a child on a railway track the driver applies brakes the speed of train is reduced from 90 km/h to 36km/h in 5s then train retards by

Answer 1

The retardation is

$$a=\frac{v_f-v_i}{t}=\frac{36-90}{5}=-10.8\text{ km/h/s}.$$

If we convers the speed to m/s, we have

$$a=-10.8·1000/3600=-3\text{ m/s}^2.$$