Question

why is the value of g not constant on earth​

Answer 1

The value of acceleration due to gravity(g) is 9.8 or 10.

Answer 2

As the acceleration due to gravity (g) is not an independent physical quantity it is derived from the other physical quantities it is not constant on earth. Factors affecting acceleration due to gravity (g):                                                                1.  As the height (altitude) increases acceleration due to gravity (g) decreases .
2. As the depth increases acceleration due to gravity (g) decreases .
3. Local conditions affect the acceleration due to gravity (g) slightly by the geological deposits, massive concrete buildings and topography .
4. At the poles acceleration due to gravity (g) is maximum.
5. At the equator acceleration due to gravity (g) is minimum.

Hope it will be helpful...