**Gravitational
force** -an attractive force that exists between all objects with
mass; an object with mass attracts another object with mass; the magnitude of
the force is directly proportional to the masses of the two objects and
inversely proportional to the square of the distance between the two objects.

Stated mathematically:

Where *G* is the universal gravitational
constant (meaning it has the same value throughout the universe), *m _{1 }*and

G = 6.67 x 10^{-11} N m^{2} / kg^{2
}

Cavendish found the universal gravitation constant, allowing the earth to be
"weighed." As we examine this equation we can note that the larger the
masses, the larger the gravitational force.
The farther apart the masses are the smaller the force.
Because the force is proportional to 1/d^{2}, If we double the
distance between two masses, the gravitation force is not halve but 1/4 of the
original value. The other thingto
note is the distances are based on the center of the mass (center of gravity)
and so even though I am standing on the earth, I an quite a distance from the
earth’s center. (R=6400 km or
3980 mi.)