9th Class Science Motion Question Bank Motion

  • question_answer A boy runs for 10 minutes at a uniform speed of \[\text{9 km }{{\text{h}}^{\text{-1}}}\]. At what speed should he run for the next 20 minutes so that the average speed comes to\[\text{12 km }{{\text{h}}^{\text{-1}}}\]?

    A) \[\text{13}\text{.5 km }{{\text{h}}^{\text{-1}}}\]  

    B) \[\text{10}\text{.2 km }{{\text{h}}^{\text{-1}}}\]

    C)                \[\text{8}\text{.2 km }{{\text{h}}^{\text{-1}}}\]    

    D)        \[\text{7}\text{.72 km }{{\text{h}}^{\text{-1}}}\]  

    Correct Answer: A

    Solution :

    Let, the boy runs with speed v for next 20 min. Given that, Average speed\[\text{= 12 km }{{\text{h}}^{\text{-1}}}\] As, average speed\[\text{=}\frac{\text{Total distance covered }}{\text{Total time taken}}\] \[\therefore \text{12=}\frac{\text{Total distance covered}}{\frac{(10+20)}{60}h}\] \[\Rightarrow \]Total distance covered \[=12\times \frac{30}{60}=12\times \frac{1}{2}=6km\] Distance covered in 10 min \[=9\times \frac{10}{60}=1.5km\] Remaining distance \[=6-1.5=4.5km\] For next 20 min, he should run with speed \[v=\frac{4.5}{20/60}=4.5\times 3=13.5\,\,km\,\,{{h}^{-1}}\]

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