Is acceleration due to gravity always 9.8 m/s^2 on Earth?

Answer 1

No. In fact there are people who have the amazing job of going around the world and measuring the gravity with a very high degree of accuracy. But, for all realistic purposes, every physicist will use 9.8 or maybe 9.81 (if three sig figs are need), for any calculation.

Answer 2

It isn't. Gravity varies very subtly around the Earth's surface. You can see an image illustrating the (exaggerated) effect in my citations. The reason for this is that the Earth's density is not uniform.

The Earth has multiple layers- the Crust (relatively dense, made of cool rock), the Mantle (also made of rock, but less dense because of its high temperature) and the Core (made of metal, mostly iron, and thus very dense).

The crust can vary in thickness from 10 to 50 kilometers. Where it's thicker, there is more mass under you, and thus more pull of gravity. Additionally, the core isn't precisely even either; where higher "mountains" rise on its surface, there is also more mass under you, and therefore more gravity again. Transitory 'bubbles' of higher and lower density in the liquid mantle, and the rise or collapse of objects on the core's surface, can also influence things over time.

Of course, scientists can measure these differences, but for all practical purposes they are too small to make any difference. Even very sensitive experiments meant to measure gravity itself rarely have to take them into account.

(Note: This all reflects the best research I know about, but it's all very theoretical, since it involves the largely unknown inner crust. The "real" answer may change in the future!)

Answer 3

Yes, for any significant degree of accuracy.

The gravitational force is dependent on the mass of the two objects being compared (mass of earth and mass of object), their distance (center of Earth to center of object), and the gravitational constant (G=6.6742 × 10-11 N·m^/kg^2)

Answer 4

On its surface, yes, approximately.