"Big"

is Newton's gravitational constant and gives the constant of proportionality in Newton's Universal law of gravitation which is the basis of our understanding of non-relativistic gravity. The gravitational force**G**between two bodies of mass**F**and**m**_{1}at a distance**m**_{2}is:**R**In SI units,

has the value 6.67 × 10**G**^{-11}Newtons kg^{-2}m^{2}.The direction of the force is in a straight line between the two bodies and is attractive.

Thus, an apple falls from a tree because it feels the gravitational force of the Earth and is therefore subject to "gravity". The acceleration

due to gravity on the Earth can be calculated by substituting the mass and radii of the Earth into the above equation and hence**g=F/m**_{1}= 9.81 m s**g**^{-2}.