I want to find out how others handle the following minor point.
Take a typical introductory situation of block m_2 on top of block m_1 on
top of a horizontal surface. I don't care if there are frictional forces or
not at the interfaces (but lets assume none for simplicity in the below
discussion).
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.
Of course, it is utterly negligible; and I comment on that. It just bothers
me that I pound my fists on the table saying that you *must* identify all
forces in order for the free-body procedure to work and then one manifestly
leaves out some forces that we know are present (even if negigible).