Gravitational Force
Geophysical interpretations from gravity surveys are based on the mutual attraction
experienced between two masses* as first expressed by Isaac
Newton in his classic work Philosophiae naturalis principa mathematica
(The mathematical principles of natural philosophy). 
Newton's law of gravitation states that the mutual attractive force between
two point masses**, m1 and m2, is proportional to one over the
square of the distance between them. 
The constant of proportionality is usually specified as G, the gravitational
constant. Thus, we usually see the law of gravitation written as shown to the
right where F is the force of attraction, G is the gravitational
constant, and r is the distance between the two masses, m1 and
m2.
*As described on the next page, mass is formally defined as the proportionality
constant relating the force applied to a body and the acceleration the body undergoes
as given by Newton's second law, usually written as F=ma. Therefore, mass
is given as m=F/a and has the units of force over acceleration.
**A point mass specifies a body that has very small physical dimensions. That is, the mass can be considered to be concentrated at a single point.
