Question

if the speed of car is reduced from 108 km by hour to 36 km by hour in 4 second calculate acceleration.

Answer 1

Initial velocity (u) = 108 km/h = 108×1000/60×60 m/s = 30m/s

Final velocity (v) = 36 km/h = 36×1000/60×60
m/s = 10m/s

Acceleration (a)= ?
Time (t) =4s
a=v-u/t
a=10-30/4
a=-20/4
a= -5 m/sec square

Answer 2

Given,

Initial Velocity (u) = 108 km/hr. = 108 × 5/18 = 30 m/s

Final Velocity (v) = 36 km/hr = 36 × 5/18 = 10 m/s

Time  = 4 second

Using  First  Equation of  motion,

v =  u + at  where a is  acceleration of  car

10 = 30 +4a

-20  = 4a

a = -5 m/s²

Note:- 1. Here  negative  sing shows retardation

2. Acceleration is  known as  rate  of  change  in velocity with respect  to time