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Can a system's mass vary?



There has been a thread discussing systems of variable mass. I don't
advocate that approach, following reasoning very similar to that of
John Mallinckrodt. I've posted how I handle these situations on my book
discussion site <http://homepage.mac.com/beichner/Book_Discussion/>.

Progress on the book has been very slow, but it is still progress. (I'm
not teaching next semester, so hopefully I'll be able to get more
writing done.) I'll soon be working on energy topics. I know that's a
ripe area for discussion. I've already had some useful conversations
with Bruce Sherwood. His favorite example: you apply a force F and
slide a block a distance d across a rough surface. The work you've done
is just Fd. Meanwhile, a frictional force, f, is opposing the block's
motion. If there is no acceleration, F and f have the same magnitude.
Since the change in KE is zero, the net work done is zero. Most people
indicate this as Fd - fd = 0. All the energy is accounted for...but the
block gets warmer! Where does that energy come from?

Anyone want to share their favorite energy examples or approaches?

Bob