Re: free-body diagrams

[Peter Schoch <pschoch@NAC.NET>]

We don't get to gravity in the form of Gmm/r^2 until after the
diagrams.  I always introduce every topic with the caveat that we may be
making simplifying assumptions now, but that (hopefully) as we progress
we will be adding layers of complexity that allow us to remove some of
those simplifying assumptions.

Most students take this statement at face value until I get to such
things as putting the Pulley (via moment of inertia and angular motion)
into the problem and make the tensions unequal, etc.  It then all comes
together as to why I make that statement.  From there on out, they tend
to look for the times when we "add back in" some of the complexity.  At
times it turns into a game, but at the very least, I know they are
paying attention!

Peter Schoch
Sussex County Community College
pschoch@nac.net



Joel Rauber wrote:
> 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).
>
> Any thoughts gang?
>
> Joel Rauber
> Joel_Rauber@sdstate.edu