Question

why the value of (g) is not same everywhere on the surface of the earth​

Answer 1

Explanation:

The value of g is not same everywhere on surface of earth because of the altitude or the height of the surface above the sea level is not same everywhere.

And at poles g is more and near the equator g is less

Answer 2

The variation in apparent gravitational acceleration (g) at different locations on Earth is caused by two things.

1. The Earth is not a perfect sphere it's slightly flattened at the poles and bulges out near the equator, so points near the equator are farther from the center of mass. The distance between the centers of mass of two objects affects the gravitational force between them, so the force of gravity on an object is smaller at the equator compared to the poles. This cause variation.
2. The rotation of the Earth causes an apparent centrifugal force which points away from the axis of rotation, and this force can reduce the apparent gravitational force. The centrifugal force points directly opposite the gravitational force at the equator, and is zero at the poles. Together, the centrifugal effect and the center of mass distance reduce g at the equator compared to the poles.

Hope this was helpful....