If the earth is supposed to be a sphere of radius R, ${\mathrm{g}}_{{30}^{\mathrm{o}}}$ is the value of acceleration due to gravity at latitude of 30° and g at the pole, the value of $\mathrm{g}-{\mathrm{g}}_{{30}^{\mathrm{o}}}$ is (where $\mathrm{\omega }$ is the angular velocity of earth)