BHU PMT BHU PMT (Mains) Solved Paper-2009

  • question_answer
    The speed with which the earth have to rotate on its axis so that a person on the equator would weigh\[{{(3/5)}^{th}}\] as much as present. [Radius of earth = 6400 km]

    A)  \[4.83\times {{10}^{-3}}rad\,{{s}^{-1}}\]              

    B)  \[5.41\times {{10}^{-3}}rad\,{{s}^{-1}}\]

    C)  \[7.82\times {{10}^{-4}}rad\,{{s}^{-1}}\]              

    D)  \[8.88\times {{10}^{-14}}rad\,{{s}^{-1}}\]

    Correct Answer: C

    Solution :

     The apparent weight of person on the equator (latitude\[\lambda =0\]) is given by                 \[w'=w-mR{{\omega }^{2}}\]                 Here,\[w'=(3/5)w=(3/5)mg\]\[[\because w=mg]\]                 \[\therefore \]  \[(3/5)mg=mg-mR{{\omega }^{2}}\]                 Or \[mR{{\omega }^{2}}=mg-(3/5)mg\left( \frac{2}{5} \right)mg\]                 Or\[{{\omega }^{2}}=\frac{2g}{5R}\]                 \[\therefore \]\[\omega =\sqrt{\frac{2g}{5R}}\]                 Here,\[g=9.8\,m{{s}^{-2}}\]and                 \[R=6400\text{ }km=6400\times {{10}^{3}}m\]                 \[\therefore \]\[\omega =\sqrt{\left( \frac{2}{5}\times \frac{9.8}{6400\times {{10}^{3}}} \right)}rad{{s}^{-1}}\]                 \[=7.82\times {{10}^{-4}}rad{{s}^{-1}}\]


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