A train starting from rest first accelerates uniformly up to a speed of $80 \mathrm{~km} / \mathrm{h}$ for…
A train starting from rest first accelerates uniformly up to a speed of $80 \mathrm{~km} / \mathrm{h}$ for time $t$, then it moves with a constant speed for time $3 t$. The average speed of the train for this duration of journey will be (in $\mathrm{km} / \mathrm{h}$ ) :
40
80
30
70
Solution
$\begin{aligned} \text { Average speed } & =\frac{\text { total distance }}{\text { time taken }} \\ & =\frac{\frac{80 \times \mathrm{t}}{2}+80 \times 3 \mathrm{t}}{4 \mathrm{t}}=70 \mathrm{~km} / \mathrm{hr} .\end{aligned}$