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  4. A car moves for half of its time at 80 kmh-1 and for remaining half at 40 kmh -1 If total distance is 60 km, the average speed of the car is

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Q. A car moves for half of its time at $80\, kmh^{-1}$ and for remaining half at $40\, kmh ^{-1}$ If total distance is $60 \,km$, the average speed of the car is

Motion in a Straight Line

Solution:

$\upsilon_{av} = \frac{\upsilon_1 t_1 + \upsilon_2 t_2}{t_1 + t_2}$
= $\frac{80 \times \frac{t}{2} + 40 \times \frac{1}{2}}{\frac{t}{2} + \frac{t}{2}}$
i.e. $\upsilon_{av} = \frac{\frac{t}{2} (80 + 40)}{ \frac{t}{2}(2)} = \frac{120}{2}$
= $60 \, km \, hr^{-1}$