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 \frac{m_1 m_2}{r^2}

where:

   F is the magnitude of the (repulsive) gravitational force between the two point masses
   G is the gravitational constant
   m1 is the mass of the first point mass
   m2 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), m1 and m2 in kilograms (kg), r in metres (m), and the 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 repulsive force F is always negative, which means that the net attractive force is positive. (This sign convention is adopted in order to be consistent with Coulomb's Law, where a positive force means repulsion between two charges.)

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