Question

When to use PE = mgh and PE = GMm/r?

Answer 1

well..P.E=-GMm/r
let put r=R+h where R is the radius of the earth
P.E=-GMm/(R+h)
=-(GMm/R)*(1+h/R)^-1
=-(GMm/R)*(1-h/R)
=-(GMm/R)+(GMmh/r^2)
=-(GMm/R)+mgh
here h<<R
and if we choose P.E at earth's surface=0
then P.E=mgh
so ehere h<<R and we choose the potential energy at earths's surface is zero...there P.E=mgh

Answer 2

when to use PE = GMm/r
@souvik

Answer 3

where
the distance r is COMPARABLE to R or it may be >R or <R..then we can use it

Answer 4

=-(GMm/R)*(1+h/R)^-1
=-(GMm/R)*(1-h/R)
=-(GMm/R)+(GMmh/r^2)
=-(GMm/R)+mgh

I cannot understand this please tell more clearly @souvik

Answer 5

do you know the en=expansion of (1+h/R)^-1................?
(1+h/R)^-1={1-h/R+higher terms of (h/R)}
we neglect the higher terms as..h<<R