and 
separated by a distance 
as
was first formulated: by Isaac Newton (1642-1727). [P2.3, P3.1]
states: that every particle of matter in the Universe attracts every other particle with a force that is directly proportional to the product of the masses of the particles and inversely proportional to the square of the distance between them. [P2.3, P3.1]
can be expressed: for two masses and separated by a distance as
as the force on due to
where 
is Newton's universal gravitational constant and 
is a unit vector pointing from to 
[P2.3, P3.1, P3.2]
also known as: universal law of gravitation.
Copyright 1997, The Open University