J & K CET Engineering J and K - CET Engineering Solved Paper-2003

  • question_answer
    If a planet of given density were made larger (keeping its density unchanged) its force of attraction for an object on its surface would increase because of increased mass of the planet but would decrease because of larger separation between the centre of the planet and its surface. Which effect would dominate?

    A)  Increase in mass

    B)  Increase in radius

    C)  Both affect the attraction equally

    D)  None of the above                     

    Correct Answer: B

    Solution :

    The relation between mass and density of earth is given from Newton's law of gravitation, according to which \[{{M}_{e}}=\frac{gR_{e}^{2}}{G}\] where \[{{M}_{e}}\] is mass of earth, G the gravitational    constant, \[{{R}_{e}}\] the radius of earth and g the I     acceleration due to gravity. Also,  \[mass=volume\times density\] \[\therefore \] \[g=\frac{G\times volume\times density}{{{R}^{2}}}\] Assuming spherical shape of earth volume \[=\frac{4}{3}\pi {{R}^{3}}\] \[g=G\times \frac{4}{3}\frac{\pi {{R}^{3}}}{{{R}^{2}}}\rho \] \[\Rightarrow \] \[g=G.\frac{4}{3}\pi R\rho \] Hence, increase in radius would dominate.


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