Good Morning everyone,
We all know that the earth is not flat and most of us also know that it more has the form of an egg than an actual ball. That causes that so some areas are further away from the center of the earth which causes less gravitational force. That means if you are on top of the Mount Everest your Fg will be a little bit smaller than in a submarine on the bottom of the seas. The reason for that is the equation for force due gravity: Fg=(Gm1m2)/r2 and as r gets greater the force will get smaller. That means as further you get away from the center of the earth as smaller the gravitational force will be.