gravity

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

where:

   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.

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