By using a universal gravitational force, we can get that

.
For note : r is a distance from center of the earth to the object.
Then, if

is 6.400 km, and if

is 12.800 km,
we can say that

is

In first equation we can say that :

then in second equation we can say that :

,

(because

is

)

,
we can say that :

so, by plugging first equation into second equation, we can get
