Newton’s law gives F=r2GMm between point masses, or outside spherical masses. Use centre-to-centre separation, not altitude. The pair forces have equal magnitude and opposite directions, although the two accelerations can differ. Doubling one mass doubles the force; doubling separation reduces it to a quarter.
Gravitational field is force per unit test mass: g=mF=r2GM, in Nkg−1 or equivalently ms−2. It does not depend on test mass. An altitude h above radius R means r=R+h. Fields add as vectors; between unequal masses, the zero-field point lies nearer the smaller mass.
Example 1
A planet has mass 6.00×1024kg and radius 6.40×106m. Use G=6.67×10−11.