When the earth rotates about its own axis with an angular velocity $\omega$, every particle of the earth experiences a centrifugal force equal to $mr\omega^2$ where $m$ is the mass of the particle and $r$ is its distance from the axis of rotation. The value of $r$ increases from zero for particles at poles to maximum for those at the equator. Thus, the centrifugal force is zero at the poles and maximum at the equator. This force causes the earth to bulge out at equator and flatten at the poles.

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Gravitation Reasonings
Explain why the earth is flattened at the poles.