Why radius of equatorial region is longer tan pole
Wherever there is circular motion, there is a interplay between centripetal (inward) and centrifugal forces (outward).
Consider a stone being whirled round on a string, in a horizontal plane. In a frame of reference rotating with the stone around the same axis as the stone, the stone is stationary. However, the tension in the string is still acting on the stone. If Newton’s laws were applied in their usual form, the stone would accelerate in the direction of the net applied force; towards the axis of rotation, which it does not do.
Now Earth is not an ideal rigid body. The rotation of the Earth leads to more centrifugal force in equatorial regions than in the polar regions. This causes a bulge near equator and relative flatness near poles. Hence the difference in radius in the two regions.
