Which of the following is
not one of Kepler’s laws?

The reason the moon does not fall into the Earth is that

The "escape velocity" of a planet is the speed needed for a rocket to
go from the surface of the planet into orbit.

Ralph noticed the negative sign
in the general equation for gravitation potential energy, *PE *= -*GMm*/*r*,
and he read the book's statement (8th edition, chapter summary) that "this expression
reduces to *PE *= *mgh *close to the surface of Earth". He is very
confused, because one equation has a negative sign and the other one doesn't!
How can they possibly be equivalent? What can you tell Ralph to help him out?