Correct Option
The correct option is Because the distance from the centre at the equator is greater than that at the poles..
Explanation
The Earth is not a perfect sphere but an oblate spheroid (Geoid). Due to the centrifugal force generated by its rotation, the Earth bulges at the equator and is flattened at the poles. This shape results in a variation in the distance from the Earth's center to the surface at different latitudes.
Detailed Analysis
- Gravitational Force and Distance: According to Newton’s Law of Universal Gravitation, the force of gravity ($g$) is inversely proportional to the square of the distance from the center of mass ($R$). The formula is $g \propto \frac{1}{R^2}$.
- Equatorial vs. Polar Radius: The equatorial radius of the Earth is approximately 21 km longer than the polar radius.
- At the Equator, the surface is farther from the center, resulting in a lower gravitational force.
- At the Poles, the surface is closer to the center, resulting in a higher gravitational force.
- Role of Rotation (Option a): While the Earth's rotation creates a centrifugal force that also reduces the net gravity at the equator, the difference in distance (shape) is the dominant static factor responsible for the variation in gravitational acceleration between the poles and the equator.
Key Takeaway: Gravity is strongest at the poles and weakest at the equator primarily because the equator is farther from the Earth's center than the poles.