I am doing a school project to investigate how gravity varies over the earth's surface. I understand that the force is weaker at the equator due to the earth's rotation but there are a few things that I don't quite understand:
1: Firstly, why is an object on the earth's surface only affected by the earth's mass which is within that radius or lower (sorry for the poor explanation but I hope you understand what I mean.
2: If this is the case, then how can you prove that this effect is lesser than the effect of the distance from the centre (because it must be otherwise the gravitational force would be greater at the equator than at the poles)
3: We have also been asked to investigate the effects of mountains, oceans, varying rock densities etc. I can see why they might affect the gravity in a particular region but again, I can't fully understand it

Any help will be greatly appreciated!!