Question

1) A motorcycle travels a distance with the speed of 30 km/hr.And covers the same distance back by the speed of 20 km/hr . find average speed.

2)A car moves with a speed of 30 km/hr for 30min and then 25km/hr for 1hr.and then 40 km/hr for 2 hr.find average speed

3)a body covers half the distance with 20m/s and other half 30m/s.find average speed

Answer 1

1.

Let the distance covered be d

Time taken to reach at a speed 30 kmph t₁ = d/30

Time taken to return at a speed 30 kmph t ₂ = d/20

Avg. speed = Total distance / total time taken

= 2d/(t₁+t₂)

= 2d/(d/30 + d/20)

= 24 kmph.

2.

Total distance = ½ ×30 + 1×25 + 2×40 = 120

Total time = ½ + 1 + 2 = 3.5 hr

Avg. speed = 120/3.5 = 38.3 kmph

3.

Let the total distance be d

Time taken to cover distance d/2 with speed 20ms^-1 = (d/2)/20 = d/40 s

Time taken to cover the next d/2 distance with speed 30ms^-1 = (d/2)/30 = d/60 s

Avg. speed = (d/2 + d/2 )/ (d/40 + d/60)

= 24 ms^-1