Question

A bus decreases its speed from 80 km/hr to 60 km/hr in 5 seconds. Find the
acceleration of the body.

Answer 1

u = 80km/h = 80*1000/60*60 = 22.22m/s

v = 60km/h = 60*1000/60*60 = 16.66m/s

Time = 5 second

a = ?

according to 1st equation of motion

v = u+at

16.66 = 22.22+5a

16.66-22.22 = 5a

-5.56 = 5a

a = -5.56/5

a = -1.112

therefore the retardation is 1.112 or acceleration is -1.112

$....$

Answer 2

▶ Solution :

✴ Given :

Initial velocity of bus = 80kmph

Final velocity of bus = 60kmph

Time interval = 5s

✴ To Find :

Acceleration of bus

✴ Concept :

Acceleration is defined as ratio of change in velocity to the time interval.

Acceleration is a vector quantity.

Acceleration can be positive, negative and zero.

✴ Conversion :

▪ 1kmph = 5/18mps

▪ 80kmph = 22.22mps

▪ 60kmph = 16.66mps

✴ Calculation :

$\mapsto\bf\:a=\dfrac{v-u}{t}\\ \\ \mapsto\sf\:a=\dfrac{16.66-22.22}{5}\\ \\ \mapsto\sf\:a=-\dfrac{5.56}{5}\\ \\ \implies\boxed{\bf{\purple{a=-1.11\:ms^{-2}}}}$

✒ Negative sign shows retardation.