When the body is hanged on a spring F=−kx=mg . . . (i) acts on it, where x is extension in spring. Let 800km above the Earth's surface, the stretch in thelength of spring is x′ and value of g is g′. g=
GM
R2
and g′=
GM
(R+h)2
So, g′=
g
(1+
h
R
)2
. . . (ii) where, R= radius of Earth From Eq. (i), we get kx=mg⇒x∝g . . . (iii) Therefore,