Law of gravitation

The idealized observation of Galileo that all bodies in free-fall accelerate equally implies that the gravitational force causing acceleration bears a constant relation to the inertial mass.

 According to Newton’s postulated law of gravitation, two bodies of mass m₁ and m₂, separated by a distance r, exert equal attractive forces on each other (the equal action and reaction of the third law of motion) of magnitude proportional to m₁m₂/r². The constant of proportionality, G, in the gravitational law:

 F = Gm₁m₂/r²,

 is thus to be regarded as a universal constant, applying to all bodies, whatever their constitution. The constancy of gravitational acceleration, g, at a given point on the Earth is a particular case of this general law.