9th Class Science Gravitation and Floatation Question Bank Gravitation and Pressure

  • question_answer
    Points \[P,\,\,Q\] and \[R\] are in a vertical line such that\[PQ=QR\]. A ball at the top-most point \['P'\] is allowed to fall freely. What is the ratio of the times of descent through \[PQ\] and\[PR\]?

    A) \[\frac{3}{2}\]              

    B)        \[\frac{3}{\sqrt{2}+1}\]

    C) \[\frac{1}{\sqrt{2}-1}\]    

    D)        \[\frac{5}{2}\]

    Correct Answer: C

    Solution :

     For motion from \[P\] to \[Q\]             \[y=\frac{1}{2}g{{t}_{1}}^{2}\]                                  ? (1) For motion from \[P\] to\[R\]:             \[2y=\frac{1}{2}g{{({{t}_{1}}+{{t}_{2}})}^{2}}\]                 ? (2) From (1) and (2), \[2{{t}_{1}}^{2}={{({{t}_{1}}+{{t}_{2}})}^{2}}\] \[\Rightarrow \]   \[{{t}_{1}}+{{t}_{2}}=\sqrt{2}{{t}_{1}}\Rightarrow {{t}_{1}}\left( \sqrt{2}-1 \right)={{t}_{2}}\] \[\Rightarrow \]   \[\frac{{{t}_{1}}}{{{t}_{2}}}=\frac{1}{\sqrt{2}-1}\]


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