初手physics 關於地心吸力

The acceleration due to gravity, g, arises because of the graviational field of the earth. It is, in fact, a measure of the graviational field intensity of the earth. It measures the force acting on a unit mass, and hence, g can also be expressed in uit of N/kg. As it is a field intensity, it doesn't depend on the object (matter) that is placed in the graviational field.

The gravitational force varies with distance from the centre of the earth. The value of g = 9.81 m/s2 is only an average value. Because the earth is not a perfect sphere, the value decreases when one goes from the equator towards the poles. At the equation, g = 9.780 m/s2, but g = 9.832 m/s2 at the poles.

When one goes to higher altitude, g also decrease because of increase distance from the earth centre.

對什麼物質，對多大或多小的東西，也一樣是9.81ms^-2，理論上是正確的。