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Earth is bulged out due to the outward radial push of the centrifugal force.
For an inertial observer, there must be some force causing this outward push other than the centrifugal force (since centrifugal force doesn't exist for this inertial observer).
So, I wanted to know how would an inertial observer explain the bulged out shape of the earth (oblate spheroid)? And by how, I mean, by attributing it to which force? (maybe be interior forces or something else)
Explanation:
Newton's first law.
An object, released to free motion, will move in a straight line, unless prevented from doing so by a force.
youtube video uploaded by Yuri Kovalenok, demonstrating bulging of an elastic spherical object when subjected to rotation.
Initially the parts of the sphere close to the equator do not experience sufficient centripetal force, so their initial motion is not purely circumnavigating motion. Instead their motion is a vector sum of a component of circumnavigating motion and a component of motion in a straight line. As a consequence the "equator" recedes from the axis of rotation.
The object is elastic. The bulging deforms the object and the elasticity starts to oppose that deformation. After some bulging has set in the elasticity of the object provides the required centripetal force to prevent further bulging.
Consider a small sample of surface sea water at a middle north latitude. Gravity pulls it toward the center of the earth, and the normal force from the water below pushes it up. Because the earth is rotating, the resultant of these two must provide a small centripetal accelertion toward the axis of the earth. To give this resultant, the normal force (which is perpendicular to the surface) must lean slightly to the north. Conclusion: The surface of the sea (and the once moltant earth underneath) must rise as you approach the equator.
