1. Gravitational constant and Gravitational acceleration

According to the law of universal gravitation, for a mass (m) on the earth,the attractive force (F) between the mass and the earth is proportional to the product of their masses (m and M), and inversely proportional to the square of the distance (r) between them:

F=GMm/r/r                                                          （1）

Where G is Gravitational constant.

G was firstly measured by Cavendish in the Cavendish experiment by using a horizontal torsion beam with lead balls whose inertia (in relation to the torsion constant) he could tell by timing the beam's oscillation. The value of G he obtained is 6.74 × 10−11 m3 kg−1 s−2, which differs by only 1% from the currently accepted value: 6.674 × 10−11 m3 kg−1 s−2. He also obtained the Earth's density which was 5.448 ± 0.033 times that of water

By using Newton's laws of motion，the attractive force (F)can also be written as

F=mg                                                                    (2)

Where g is gravitational acceleration, which is the acceleration on an object caused by gravity.

(1) and （2） produce:

g=GM/r/r                                                               (3)

r is approximately the radius of Equatorial radius of the Earth，which is 6378137 m. Earth mass (M) is 5.9722 × 1024 kg，  g can thus be calculated as approximately as 9.81 m s-2. It should be exactly called standard gravity or standard acceleration due to free fall. The acceleration of a body near the surface of the Earth is due to combined effects of gravity and centrifugal acceleration. The total (the apparent gravity) is about 0.5 percent greater at the poles than at the equator.

2. Geopotential