Question

calculate the height above the surface of earth at which the value of acceleration due to gravity is half of what it is on the earth

Answer 1

M/R^2. This is also the force per unit mass on an object in the gravity field. If you multiply by m for an object of mass m you get the gravitational force on a mass m due to a large mass M. At the surface of a spherical planet this will be g= GM/R1^2 where R1 is the Earth's radius