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A simple pendulum, $20cm$ in length, has the period of $2.7s$ on a certain planet. Find the mass of the planet if its diameter is $18000km$. $G=6.67\times10^{-11}Nm^2/kg^2 $

I have no idea how to get to the solution except for the formula $$g=(GM)/(R^2) $$

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closed as too localized by Brandon Enright, akhmeteli, Waffle's Crazy Peanut, Qmechanic Jun 2 '13 at 22:34

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up vote 3 down vote accepted

The next thing you'll need is time period of a simple pendulum $$T=2\pi\sqrt{\dfrac{L}{g}}$$

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