Law of gravitation:

Everybody in the universe attracts every other body with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.

Explanation:

Consider two bodies of masses m₁ and m₂. The distance between the centers of masses is d.

According to the law of gravitation, the gravitational force of attraction F with which the two masses m₁ and m₂ separated by a distance d attract each other is given by:

$F \: \alpha \: m_{1}m_{2}$   ……………   (i)

$F \alpha\frac{1}{d^{2}}$……………   (ii)

By combining (i) and (ii) we get

$F \: \alpha \:\frac{m_{1}m_{2}}{d^{2}}$

or

$F=G \: \frac{m_{1}m_{2}}{d^{2}}$

The universal constant of gravitation (G):

Here G is the proportionality constant. It is called the universal constant of gravitation. Its value is the same everywhere.

In SI units the value of G is  $6.673\times10^{-11}\: Nm^2{kg^{-2}}$