There are many answers to the question why is the Earth bulged at the equator, see e.g. here, but almost all of them involve centrifugal force. Since it's a fictitious force, how to we explain this effect in an inertial frame. I guess there would be a similar answer to when we rotate a stone tied to a string, as we rotate it faster, the stone moves away from the axis of rotation. Please answer why does this happen, and don't involve the use of centrifugal force.