Q. A car moves a distance of 200 m. it covers first half of the distance at speed $60\ {kmh}^{- 1}$ and the second half at speed $v.$ If the average speed is $40\ {kmh}^{- 1},$ the value of $v$ is
A) $30\ {kmh}^{- 1}$
B) $13\ {kmh}^{- 1}$
C) $60\ {kmh}^{- 1}$
D) $40\ {kmh}^{- 1}$
Explanation
(a) Average velocity $= \frac{2v_{1}v_{2}}{v_{1} + \ v_{2}}$
Given, $v_{av} = 40\ km/h,\ v_{1} = 60\ km/h\ and\ v_{2} = ?$
$\therefore\ \ \ \ \ \ \ 40 = \frac{2 \times 60 \times v_{2}}{60 + v_{2}}$
$\ \ \ \ \ \ \ 80v_{2} = 2400$
$\ \ \ \ \ \ \ \ \ \ \ v_{2} = 30\ km/h$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.