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9th Class Science Gravitation and Floatation Question Bank Gravitation IIT JEE Objective Problems

  • question_answer
    A person standing on the edge of a well throws a stone vertically upwards with an initial velocity \[5\,\,m{{s}^{-1}}.\]The stone gone up, comes down and falls in the well making a sound. If the person hears the sound 3 second after throwing, then the depth of water (neglect time travel for the sound and take \[g=10m{{s}^{-2}}\])

    A)  1.25 m                 

    B)  21.25 m  

    C)  30 m    

    D)  32.5 m

    Correct Answer: C

    Solution :

      \[u=5\text{ }m/s\] at max. height\[v=0\] From the equation \[v=ugt\]                 \[0=510\times t\]                 \[=510t\]                 \[t=\frac{1}{2}S\] Time taken by the stone to reach the edge of the wall is \[\frac{1}{2}s+\frac{1}{2}s=1s\] Time taken to reach the water level\[=3s1=2s\] Depth of the water level is \[h=ut+\frac{1}{2}g{{t}^{2}}\]                                 \[=5\times 2+\frac{1}{2}\times 10\times {{(2)}^{2}}\]                                 \[=30\text{ }m.\]            


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