From: "JACK L. URETSKY (C)1998; HEP DIVISION, ARGONNE NATIONAL LAB ARGONNE, IL 60439" <JLU@HEP.ANL.GOV>
Date: Mon, 11 Oct 1999 21:39:01 -0500
Hi all-
Joel Rauber writes:
********************************************************
In teaching the use of free-body diagrams and applying Newton's 2nd to them
I stress very heavily that one must identify *all* the forces acting on each
object for which you have a free-body diagram. So I draw the the diagram
for m_1, the forces are:
F_g --> force of gravity of earth on m_1
N_e --> normal force of earth bottom horizontal surface) upwards on m_1
N_2 --> normal force of bottom surface of m_2 pushing down on m_1
This is where most texts stop. I've now taken to at least mentioning that
there is another force, the force of gravity of m_2 pulling up on m_1.
*********************
Say what?
As I understand it, m_2 is on top of m_1 and you are doing a
free-body diagram of m_1. That diagram shows only forces acting on m_1.
You are addressing a force acting on m_2, which does not belong in this
free-body diagram, if it exists.
Shame on the people who have already given thoughtless answers.
Regards,
Jack
"I scored the next great triumph for science myself,
to wit, how the milk gets into the cow. Both of us
had marveled over that mystery a long time. We had
followed the cows around for years - that is, in the
daytime - but had never caught them drinking fluid of
that color."
Mark Twain, Extract from Eve's
Autobiography