J & K CET Engineering J and K - CET Engineering Solved Paper-2004

  • question_answer
    Two planets of radii in the ratio \[2:3\]are made from the material of density in the ratio\[3:2\]. Then, the ratio of acceleration due to gravity \[\frac{{{g}_{1}}}{{{g}_{2}}}\] at the surface of the two planets will be

    A)  \[1\]            

    B)  \[2.25\]

    C)  \[4/9\]           

    D)  \[0.12\]

    Correct Answer: A

    Solution :

    The acceleration due to gravity (g) is given by \[g=\frac{GM}{{{R}^{2}}}\] where M is mass, G the gravitational constant and R the radius. Since, planets have a spherical shape \[V=\frac{4}{3}\pi {{r}^{3}}\] Also, mass (M) = volume (V) \[\times \] density \[(\rho )\] \[g=\frac{G\frac{4}{3}\pi {{R}^{3}}\rho }{{{R}^{2}}}\] \[\Rightarrow \] \[g=\frac{4G\pi \rho R}{3}\] Given,     \[{{R}_{1}}:{{R}_{2}}=2:3\] \[{{\rho }_{1}}:{{\rho }_{2}}=\frac{3}{2}\] \[\therefore \] \[\frac{{{g}_{1}}}{{{g}_{2}}}=\frac{{{\rho }_{1}}{{R}_{1}}}{{{\rho }_{2}}{{R}_{2}}}=\frac{3}{2}\times \frac{2}{3}=1\]


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