Question

a cyclist driving at 90 km/h is slowed down to 36 km/h in 5 sec. calculate the retardation.

Answer 1

acceleration = change in velocity by time
90km/h = 25 m/s
36 km/h = 10 m/s

so, a = 25-10 /5 = 3
now, as it is retardation so -3 m/s

Answer 2

v=36=10ms-1 u=90=25ms-1 t=5sec

a=(v-u)/t=(10-25)/5=-3=|-3|=3ms-2