Kinematics

JEE Main level, single correct, medium

9 of 308

Question 9 of 308

A train starting from rest first accelerates uniformly up to a speed of 80 km/h80 \mathrm{~km} / \mathrm{h} for time tt, then it moves with a constant speed for time 3t3 t. The average speed of the train for this duration of journey will be (in km/h\mathrm{km} / \mathrm{h} ) :

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Answer: (D) 70

 Average speed = total distance  time taken =80×t2+80×3t4t=70 km/hr.\begin{aligned} \text { Average speed } & =\frac{\text { total distance }}{\text { time taken }} \\ & =\frac{\frac{80 \times \mathrm{t}}{2}+80 \times 3 \mathrm{t}}{4 \mathrm{t}}=70 \mathrm{~km} / \mathrm{hr} .\end{aligned}

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