Question

A car travels at 36 km/h for first 20s, 54km/h for next 30s and finally 72km/h for next 10s. Find its average speed. (A) 10.8 m/s (B) 14.2 m/s (C) 13.6 m/s (D) 18.3 m/s

Answer 1

Answer:

B) 14.2 m/s

Explanation:

1st distance traveled by the car - [36×(5/18) ×20] meter = 200 meter

2nd distance traveled by the car - [54×(5/18)×30 ]meter = 450 meter

3rd distance traveled by the car - [72×(5/18)×10]meter = 200 meter

now, total distance traveled by the car =200+450+200=850meter

& total time taken =20+30+10=60 seconds

hence average speed = total distance /total time

= 850/60 =14.16m/s or 14.2 m/s (approximately)

Answer 2

In first case,

Speed = 36km/hr = 10m/s

Time = 20sec

distance travelled = speed x time = 20x10 = 200m-----(1)

In second case,

speed = 54km/hr = 15m/s

time = 30sec

distance = speed x time = 15x30 = 450m----------(2)

In third case,

speed = 72km/hr = 20m/s

time = 10sec

distance = speed x time = 20x10 = 200m----------(3)

Average speed = total distance / total time

=> (200+450+200) / (20+30+10)

=> 850/60 = 14.16m/s = 14.2m/s