Gravity is the tendency of objects with mass to accelerate toward each other.
Newton's law of universal gravitation states the following:
Every point mass attracts every other point mass by a force directed along the line connecting the two. This force is proportional to the product of the masses and inversely proportional to the square of the distance between them:
F = - G M m / r2
F is the magnitude of the gravitational force between the two point masses G is the gravitational constant M is the mass of the first point mass m is the mass of the second point mass r is the distance between the two point masses
Assuming SI units, F is measured in Newtons (N), M and m in kilograms (kg), r in metres (m), and Newton's gravitational constant G is approximately equal to 6.67 × 10−11 N m2 kg−2 (Newtons times metres-squared per kilogram-squared).
It can be seen that the force F is always negative. This sign convention is consistent with its electromagnetic equivalent Coulomb's Law, where a positive force means repulsion between two charges.