JEE Main & Advanced JEE Main Paper (Held on 12-4-2019 Afternoon)

  • question_answer
    The ratio of the weights of a body on the Earth's surface to that on the surface of a planet is 9 : 4. The mass of the planet is\[\frac{1}{9}\]th of that of the Earth. If 'R' is the radius of the Earth, what is the radius of the planet ? (Take the planets to have the same mass density) [JEE Main 12-4-2019 Afternoon]

    A) \[\frac{R}{3}\]                                   

    B) \[\frac{R}{2}\]

    C) \[\frac{R}{4}\]                       

    D) \[\frac{R}{9}\]

    Correct Answer: B

    Solution :

    Since mass of the object remains same \[\therefore \]Weight of object will be proportional to 'g' (acceleration due to gravity) Given \[\frac{{{W}_{earth}}}{{{W}_{planet}}}=\frac{9}{4}=\frac{{{g}_{earth}}}{{{g}_{planet}}}\] Also, \[g{{ & }_{surface}}=\frac{GM}{{{R}^{2}}}\](M is mass planet, G is universal gravitational constant, R is radius of planet) \[\therefore \]\[\frac{9}{4}=\frac{G{{M}_{earth}}R_{planet}^{2}}{G{{M}_{planet}}R_{earth}^{2}}=\frac{{{M}_{earth}}}{{{M}_{planet}}}\times \frac{R_{planet}^{2}}{R_{earth}^{2}}=9\frac{R_{planet}^{2}}{R_{earth}^{2}}\] \[\therefore \]\[R{{ & }_{planet}}=\frac{{{R}_{earth}}}{2}=\frac{R}{2}\]


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