Answer: g depends on the mass of the planet and on the distance from its centre (so it changes with height, depth and latitude).
The force of gravity on a body of mass m is \(F = \dfrac{GMm}{R^2}\), and this force produces an acceleration \(g = \dfrac{F}{m} = \dfrac{GM}{R^2}\), where M is the mass of the planet and R the distance of the body from its centre.
- Mass of the planet: g is directly proportional to M, so g on the moon is about one-sixth of that on the earth.
- Distance from the centre: g is inversely proportional to R², so g decreases as we go higher above the surface and also decreases as we go deeper into the earth (it is zero at the centre).
- Shape and rotation of the earth: the earth is flattened at the poles, so R is least at the poles and g is greatest there; it is least at the equator.