Newton devised a formula for computing the gravitational force between
two objects. Every object in the universe, said Newton, has a property which
we may call the “gravitational mass”. Let the gravitational masses of two
objects be M g and m g , and let the distance between their centers be r. The
gravitational force acting between the two objects (assuming they are both
spherical) is
F = −
GM g m g
r 2
. (3.1)
The negative sign in the above equation indicates that gravity, in the New-
tonian view, is always an attractive force, tending to draw two bodies closer
together.