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The universal law of Gravitation

Force of attraction (F) = G.(m1m2) / d2

Let us consider two bodies with mass m1 and m2 at the distance of d from their center.
According to Newton's law of gravitation, the force of attraction between two bodies is directly proportional to the product of their masses and inversely to the square of the distance between them (their center).

So,    F ∝ m1m2
         F ∝ 1/d2

         F ∝ (m1m2) / d2

Therefore,    G.(m1m2) / d2
where G is the gravitational constant whose value is 6.67 x 10-11 Nm2/Kg2 and is the same over the whole universe.

Gravitational constant (G)
G is defined as the force of attraction between two bodies of mass 1 kg each at the distance of 1 m from their center.

Things to remember:

Me = 6 x 1024 Kg
Re = 6.4 x 106 m
G = 6.67 x 10-11 Nm2Kg-2
ge = 9.8 ms-2
gm = 1.67 ms-2