In classical mechanics, we assert that the attractive force of gravitation between two point masses is given by:
where the direction of the force is along the line joining the two masses and is Newton’s constant of universal gravitation. Note the similarity to Coulomb’s law.
The masses should be very small relative to their separation. For bodies that are spherical and of uniform density, we can assume the mass of the body is concentrated at its centre, as if the body is a point mass.