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The amount of acceleration an object near the surface of the earth exhibits due to Earth's gravitational field, earth gravity is equal to 9.8 meters/second2. Also known as a 'G', as in "the pilot pulled 3 Gs". In this case, it is used as a unit.

It is often best to consider gravitational accelerations as a force per mass, since we perceive gravity only indirectly, as the force which counteracts it (the floor pressing up on your feet, the chair pressing up on your bum, whatever). This acknowledges that we are not actually accelerating at 9.8 m/s2. Phrased in this manner, earth gravity is 9.8 Newtons/Kilogram.

Evilkalla: if you're talking about spacegoing objects, well of COURSE g is going to be different. That's hardly on the surface anymore, is it? But even more deeply than that you are correct - that the earth's surface has all the same potential merely means that the direction of the gradient of the potential at any point will be straight down, and not that its strength will be the same everywhere. Still, the value is, to two significant figures, 9.8 meters per second.

The acceleration due to Earth Gravity, lowercase, g is commonly stated to be equal to 9.8 meters/second2. When dealing with space-going objects with ballistic trajectories (such as ballistic missiles), using this assumption will often result in the prediction of incorrect trajectories.

This is because the Earth is appreciably oblate, the equatorial diameter being 42.77 km greater than the polar diameter. Changes in the gravity field caused by this oblateness creates an asymmetric potential in earth's gravitational field. The cyclical characterization of this perturbation requires a rather high level of degree and order in the spherical harmonic expansion representation of the gravity field in order to predict precise effects on ballistic trajectories or satellite orbits.

As a result, there exist a host of gravitation models for Earth, the complexity of which is dependent on the required navitational precision. These can range from treating the Earth as a perfectly smooth sphere, a perfectly smooth oblate spheroid, or a much more complex topographically generated empirical model of very high order.

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