First slide

Rectilinear Motion

Question

A car moving on a straight road covers one third of the total distance with a speed of 20 km/hr and the rest with a speed of 60 km/hr. The average speed is

Moderate

A

40 km/hr
B

80 km/hr
C

36 km/hr
D

46 $\frac{2}{3}$ km/hr

Solution

$v_{a v e} = \frac{s}{t_{1} + t_{2}} w h e r e t_{1} = \frac{s / 3}{v_{1}} a n d t_{2} = \frac{2 s / 3}{v_{2}} = \frac{3 v_{1} v_{2}}{2 v_{1} + v_{2}} = \frac{3 \times 20 \times 60}{2 \times 20 + 60} = 36 k m p h$

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