Gravitational constant

In Newton's law, the gravitational constant, denoted by the capital letter G, is the proportionality constant connecting the gravitational force between two bodies (b1, b2) with the product of their masses (m1, m2) and the inverse square of their distance (r):

Fb1 = Fb2 = G m1m2/r2

Like any other constant, this constant simply exists so that natural phenomena like gravity can be expressed in SI units. It is equal to 0.0000000000667 N m2/kg2, which is a very tiny number that we can rewrite as 6.67 x 10-11 N m2/kg2. The units of G cancel out the units of the masses and radius so that the force is expressed in newtons.

The value of G was not known to Newton when he formulated his universal law of gravitation, but it was determined experimentally about a hundred years later by Henry Cavendish.

Although Newton didn't know the exact value of G, he knew it had to be incredibly small to reduce the gravitational force, or else you’d see a force pulling together most everyday objects. For example, in order for a lemon and an apple to stay on the same table without pulling towards each other, the gravitational force between them must be very small. For this reason, Newton added a constant to his equation, a very small number that would make the gravitational force just a tiny fraction of what you’d calculate otherwise.