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Re: third law forces cancel?



At 7:14 PM -0500 10/11/04, John Clement wrote:

Students need to identify third law pairs as being opposite, on different
interacting objects, and of the same kind, and as having switched objects
and agents.

I think Knight does an excellent job of getting the students to correctly
identify the 3rd-law pairs (all through chapter 8). He even has them draw
a dotted line connecting the forces in each 3-rd law pair (and those dotted
lines must necessarily go from one FBD for one system/object to a different
FBD for another system/object).

Now Knight's objective may be OK, in the context of momentum, but it is
difficult to tell in a snippet. The statement should be judged in context.
What page is it on, Chapter&section?

The statement is in section 9.3 on Conservation of Momentum, subsection
"Law of Conservation of Momentum," specifically in the middle of page 248.
The discussion makes complete sense on its own. I guess it is just a
question of carefully instructing students when 3rd-law pairs cancel and
when they don't; the pedagogy of this was my original question.

Thanks,
Larry