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(a) calculate the gravatational potential pf a point A on the surface of the Earth

(b) what is the gravatational potential of a point B, 1000km above A?

(c) How much energy is expended if an object of mass 10kg were to just reach point B from A?

I know the eq. for gravatational potential energy is U=-GMm/r ....but what do i use as my 2nd mass ??? please help im really lost