Increasing the mass of two objects will increase the gravitational force of attraction between them, assuming no change in the distance separating them. According to Newton's Law of Universal Gravitation the gravitational force of attraction between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. The mathematical form of this law is:
`F = G (m_1m_2)/r^2`
where g is the universal gravitational constant, 6.674 x 10^-11 N m/kg^2, `m_1` and `m_2` are the two masses and r is the distance between them.
Applying this equation, we can see that if one mass doubles the force of attraction will double. If both masses double the force of attraction will increase by a factor of 4.