--- Craig & Margaret Lucanus <lucanus@iinet.net.au>
wrote: SNIP
or when a book on a
shelf falls and loses PE,
This seems to imply that the book "has" PE. Not so. In
the book/Earth system (one always needs to define the
system when analyzing energy changes), the work done
by raising the book to the shelf results in an
increase of energy stored in the Earth's gravitational
field.
It may also be helpful to remember that since
computation of system energy is always reference
frame-dependent, the only thing that really matters is
CHANGE in the energy of a system.