I agree that the rocket problem is best dealt with by considering the
mass of the rocket and the fuel as one system, and then using momentum
conservation to derive the thrust on the rocket.
But, what about more mundane problems where it seems to me that it is
much easier to consider the mass of the system to be varying. Examples
include conveyor belts and escalators, and the simple problem of keeping
a hopper car moving at constant velocity while it is moving under a
grain elevator that is dumping grain into it. F=dp/dt works for those
problems when you allow the mass vary.
-----Original Message-----
From: Robert J. Beichner [mailto:beichner@NCSU.EDU]
Sent: Sunday, November 09, 2003 2:40 AM
To: PHYS-L@lists.nau.edu
Subject: 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?