Gravitational forces are central forces, which implies that the gravitational force between any two particles is along the line connecting their mass centres. Also, since the sphere’s surface increases with r2.
Where r is the radius.
The Gravitational field must be uniformly distributed over the sphere’s surface. We can assume that the gravitational field is proportional to 1/R2.
Consider the radius of the earth at the equator to be 6400 km. If an aeroplane flies at a distance of 40 km above the earth’s surface then the gravity, g2 is calculated by equating the ratio of acceleration due to gravity, g1 and g2 to square of ratio of the radius. This gives g2 as 9.67 m/s2.
Therefore, it can be said that the gravitational force decreases by 1.2% for an aeroplane which is flying at 40 km distance.