Question

a car travels at 54 km per hour for first 20 seconds and 36 km per hour for next 30 second and finally 18 km per hour for next 10 seconds find average speed

Answer 1

GIVEN, S1= 54KM/H S2= 36KM/H S3= 18KM/H
T1= 20 S T2=30 S T3=10 S
TO FIND, AVERAGE SPEED
SOLUTION,

S1 = D1/T1 = 54 = D1/20 = D1 = 54 ×20 = 1080m
S2 = D2/T2 = 36 = D2/30 = D2 = 36 ×30 = 1080m
S3 = D3/T3 = 18 = D3/10 = D3 = 18 ×10 = 1080m



NOW, AVERAGE SPEED = TOTAL DISTANCE/
TOTAL TIME
= 1080+1080+1080/
20+30+10
= 54 KM/H

Answer 2

We know that :

⇒ $Speed = \frac{Distance}{Time}$

⇒ Distance = Speed × Time

⇒ Speed in 1st 20 seconds = 54km/hr = 54000/3600 = 15m/sec

Distance Travelled by the Car in 1st 20 seconds = 15 × 20 = 300m

⇒ Speed in Next 30 seconds = 36km/hr = 36000/3600 = 10m/sec

Distance Traveled by the Car in next 30 seconds = 10 × 30 = 300m

⇒ Speed in Next 10 seconds = 18km/hr = 18000/3600 = 5m/sec

Distance Traveled by the Car in next 10 seconds = 5 × 10 = 50m

We know that Average Speed is the Ratio of Total Distance Traveled to that of Total time taken.

Total Distance Travelled by the Car = 300 + 300 + 5 = 665m

Total Time Taken by the Car = 20 + 30 + 10 = 60 seconds

⇒ Average Speed of the Car = 665/60 = 11.08 m/sec