Force of Gravity
The force of gravity exists between massive objects without them being in contact.
This force is proportional to the masses of
the two interacting objects and inversely
proportional to the square of their separation.
Newton's Universal Law of Gravitation: 
F_{g} 
= 
 G m M
 
r^{2}


The factors G, M, and r are the same for all masses on the Earth's
surface. Combine these factors into g, the acceleration due to gravity.
g 
= 
G M
 
r^{2}


= 
6.67 x 10^{11} N m^{2}/kg^{2} * 5.98 x 10^{24} kg
 
(6.37 x 10^{6} m)^{2}


= 
9.8 m/s^{2}=22mph/sec 
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.