Whenever two objects are touching, they usually exert forces on each other. The force of gravity, on the other hand, is an example of a force that exists between objects without them having to be in contact.

Objects with mass exert forces on each other via the force of gravity. This force is proportional to the mass of the two interacting objects, and is inversely proportional to the square of the distance between them.

Newton's Universal Law of Gravitation:

Fg

=

- G m M r2

The factors G, M, and r are the same for all masses at the surface of the Earth. We roll those factors together into the constant g, which we call the acceleration due to gravity.

g

=

G M r2

=

6.67 x 10-11 N m2/kg2 * 5.98 x 1024 kg (6.37 x 106 m)2

=

9.8 m/s2

At the Earth's surface the gravitational force exerted on an object of mass m by the Earth has a magnitude mg and is directed down.