Gravitational constant

G is the universal gravitational constant:

$G=6.67408(31)\times 10^{-11}\ {\mbox{N}}\ {\mbox{m}}^{2}\ {\mbox{kg}}^{-2}\$

It is used in Newton's formula for the gravitational force between two objects, which is:

$F=G{\frac {m_{1}m_{2}}{r^{2}}}$

where

${F}\$ is the force between the two objects

${m_{1}},{m_{2}}\$ are the masses of each object

${r}\$ is the distance between the two objects.

Acceleration due to Gravity

The lower case $g$ is known as the acceleration due to gravity. It is the acceleration of a body experiences due to Earth's gravitational field at the Earth's surface. It varies slightly across the surface of the earth due to differences in altitude and local geology. The rotation of the earth also affects it, causing the earth to bulge around the equator, increasing the distance between the surface and the centre of the earth. It has a value of approximately $g=9.81\ {\mbox{m}}\ {\mbox{s}}^{-2}$.