Answers

2015-09-25T16:11:01+05:30
g_e = 9.81\ m/s^2\\g_m = 1.67\ m/s^2

Mass of earth is 81 times mass of moon. So
M_e=81\ M_m

We need to find  \frac{R_e}{R_m}

 \frac{g_m}{g_e}= \frac{GM_m}{R_m^2} /\frac{GM_e}{R_e^2} \\ \\ \Rightarrow \frac{1.67}{9.81}= \frac{GM_m \times R_e^2}{GM_e \times R_m^2} \\ \\ \Rightarrow  \frac{1.67}{9.81}= \frac{M_m}{M_e}  \times (\frac{R_e}{R_m})^2 \\ \\ \Rightarrow \frac{R_e}{R_m}= \sqrt{\frac{1.67}{9.81} \times \frac{M_e}{M_m} } =\sqrt{\frac{1.67}{9.81} \times 81} \\ \\ \Rightarrow \frac{R_e}{R_m}= 3.71

Ratio of radius of earth to moon is 3.71
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