....¹

Actually, the acceleration due to gravity varies slightly over the Earth's surface because of the combined effects of the Earth's rotation and the Earth's slightly flattened shape. The acceleration at the poles is about $9.834 {\rm m s^{-2}}$, whereas the acceleration at the equator is only $9.780 {\rm m s^{-2}}$. The average acceleration is $9.81 {\rm m s^{-2}}$.

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... model.²

R.A. Hatch, University of Florida, http://web.clas.ufl.edu/users/rhatch/

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