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A spherical rock weighs $1000\ N$ on a certain planet. The planet is a billion times as massive as the rock and has a radius that is a billion times as great as that of the rock's radius.
How much force does the rock exert on the planet?
A thousand-billion Newtons.
A thousand-billion-billion Newtons.
A thousand billionths of a Newton.
A thousand billion-billionths of a Newton.
A thousand Newtons.