The Law of Universal Gravitation
The force of attraction F between two objects with respective masses m_{1} and m_{2} separated by a centertocenter distance R, anywhere in the universe, is described by Newton's Law of Universal Gravitation:
The two postulates of this law are:

The force of attraction is directly proportional to the product of the objects' masses

The force of attraction is inversely proportional to the distance (taken from their respective centers) squared
If we combine the above statements mathematically, we obtain the following relation:
This relationship is known as an inversesquare law. This means that one factor (F) is inversely proportional to another factor (R) squared
We translate the above relationship into a definite equation by removing the "proportionalty symbol" () and replacing it with the "equal sign" (=) and a constant. The symbol for this constant is "G"
Therefore:
Where:
 F is the force of attraction between two masses m_{1} and m_{2} anywhere in the Universe, and
 and, G is the Universal Gravitational Constant: G = 6.67 X 10^{11} Nm^{2} /Kg^{2}
The animation below is a very simple illustration of the inverselaw law
Notes: When you use the above equation
 Observe proper units (mass in kg, distance in m, Force in [N]
 The distance R has to be centerto center; i.e. from the center of one mass to the center of the other
 Don't forget to square the distance R
 Remember the significance of the Inverse Square Law