Kinematics

JEE Main level, single correct, medium

23 of 308

Question 23 of 308

A vehicle travels 4km4km with speed of 3kmh−13km{h}^{-1} and another 4km4km with speed of 5kmh−15km{h}^{-1}, then its average speed is :

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Answer: (D) 3.75kmh−13.75km{h}^{-1}

Average speed in this case is given by, 2vav=1v1+1v2\dfrac{2}{{v}_{av}}=\dfrac{1}{{v}_{1}}+\dfrac{1}{{v}_{2}}
⇒2vav=13+15\Rightarrow \dfrac{2}{{v}_{av}}=\dfrac{1}{3}+\dfrac{1}{5}
⇒2vav=815\Rightarrow \dfrac{2}{{v}_{av}}=\dfrac{8}{15}
⇒vav=154=3.75kmh−1\Rightarrow {v}_{av}=\dfrac{15}{4}=3.75km{h}^{-1}

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