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) :
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Question

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) :

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G1 Physics · Accepted Answer

During uniform acceleration from 0 to 80 km/h, average speed is: (0 + 80)/2 = 40 km/h Distance in first interval: s₁ = 40t Distance during constant-speed interval: s₂ = 80(3t) = 240t Total distance: s = 280t Total time: 4t Average speed: v(avg) = 280t/(4t) = 70 km/h