Newton’s Law of GravitationNewton’s Law of Gravitation

This describes the forces between any objects that have mass. It states that the force between two point masses is:

Directly proportional to the product of the masses,
Inversely proportional to the square of their separation, $Fpropto frac{1}{r_{2}}$

These can be combined into a single equation:

$F= -frac{GMm}{r_{2}}$

Since gravitational field strength is defined as $g = frac{F}{m}$ , it can also be expressed as:

$g=-frac{GM}{r_{2}}$

Gravitational field strength is uniform close to the centre of the Earth. This is because the field lines are almost parallel. On the surface of the Earth, gravitational field strength is numerically equal to the acceleration of free fall.