Re: free-body diagrams

[John Denker <jsd@MONMOUTH.COM>]

At 11:13 AM 10/10/99 -0500, Joel Rauber wrote:
> I want to find out how others handle the following minor point.
...
> 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).

Here's how I would handle it.

1) Physics is a natural science, not an exact science.  If students insist
on exactitude, they should drop physics and stick to arithmetic.

2) The skill of knowing what's negligible -- and what's not -- is one of
the most important skills a student should learn in physics class.

3) So, just rephrase the instructions:  "We must identify all the
significant forces in order for the free-body procedure to work...."  No
fist-pounding is required.

OK?

______________________________________________________________
copyright (C) 1999 John S. Denker             jsd@monmouth.com