Based
on Newton's law of universal gravitation, the equation for the
gravitational force exerted by an object on another object is given by:
F = Gm1m2/(r^2)
where G is the universal gravitational constant, F is the gravitational
force exerted, m1 is the mass of the first object, m2 is the mass of the
second object, and r is the separation distance between the two
objects.
If the mass of both objects were doubled, then we would have: m1' * m2' =
(2m1) * (2m2) = 4m1m2. Assuming r stays constant (G is a constant so
that won't change anyway), then this means that the new force will be 4
times greater, ie 8N * 4 = 32N of gravitational force.
