Gravity or Gravitation is a phenomenon which gives an idea about the existence of a force between any two objects having some mass. The force due to gravity is always attractive. 

Law of Gravity states that if two masses, m₁ and m₂, are kept at a distance of r from each other, then force due to gravity is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

If ‘F’ is the force due to gravity, then mathematically,

F�

α�

m1m2�1�2   and 

F�

α�

1r21�2

Therefore,

F�

α�

m1m2r2�1�2�2

Where G = Gravitational constant = 6.673 x 10-11 N-m²/Kg² (or) m³/Kg-s². On Earth, the force due to gravity between Earth and an object on Earth can also be related to the same formula. 

Let mass of the Earth is M and mass of the object is m and radius of earth is r. So, force due to gravity on an object on Earth’s surface is the force between both the masses is given by:

F=GMmr2�=����2

mg=GMmr2��=����2

g=GMmr2=9.8m/s2�=����2=9.8�/�2