A train starting from rest first accelerates uniformly up to a speed of 80 km/h for time t, then it moves with a constant speed for time 3t. The average speed of the train for this duration of journey will be (in km/h) :
(1) 70
[figure]
Distance = area under the v–t graph
d1 = 12 80 t = 40t, d2 = 80 3t = 240t
vav = 40t + 240tt + 3t = 280t4t = 70 km/h
Answer: (1)
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