Newton's law of gravitation

Newton's law of gravitation assumes that the gravitational force between any two points objects is 

• always an attractive force,
• proportional to the mass of each object
• proportional to 1/ r², where r is their distance apart.

These last two requirements can be summarised as:

 

Gravitational force F= Gm₁m₂/r²

where m₁ and m₂ are masses of the two objects

The constant of proportionality,G, in the above equation, is called the universal constant of gravitation. 

G can be given units of Nm²kg¯². The value of G is 6.67 x 10¯¹¹  Nm²kg¯².