A spacecraft of mass m at a distance of 4R(earth) from the center of the planet. a) What is the formula for the gravitational force on the vehicle at that distance? b) What is the weight of the vehicle at a distance of 4R(earth)? c) Alternatively, express the weight in terms of the acceleration due to gravity at 4R(earth). d) Write an expression for the gravitational acceleration at 4R, in terms of R(earth), G, and M(earth).
Have a look at this. Especially the summaries in the two blue boxes. http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html