Question

a bus decreases it's speed from 30m/s to 10m/s in 5 sec find its acceleration

Answer 1

By first equation of motion

v = u + a * t

v = 10 m/s
u = 30 m/s
t = 5 second

So
10 = 30 + a * 5
=> -20 =a * 5
=> a = -4 m/s2

So acceleration of bus is -4 m/ s2 i.e. bus's velocity is decreasing by 4 m/s per second.