Originally Posted by **goldsomecriss** Thanks,As i know, gravitational potential energy use the formula of (-GMm/R)...this is the only one i know, but still have no idea for further calculation...
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What kind of course is this? Is it summer school or something? Didn't your text or professor give you an examples of using that formula? I ask these questions, not because I'm stingy or lazy with my help, but because I know from many years of experience that the best way to understand a concept is to force yourself to solve problems, whether they are easy or whether they're hard. I'll give you a little hint to get going.

Hint: Potential energies add up. That means that if you want to find the total potential energy of a system then you add the contributions from each source. Does that help any?