Re: free-body diagrams

[Bob Sciamanda <trebor@VELOCITY.NET>]

There would be no end to this approach!  What about the force of Mars, the
moon, the local air, etc on m_1?

The course must begin with the caveat that we do not solve any real
problems, we solve MODELS of the real situation - models which we must
create.  The question is "what are the SIGNIFICANT forces on the object?"

The essence of doing physics (and engineering) is the art of representing
a real situation (unsolvable in its entire existential reality) by a model
which is 1) solvable and 2) useful - this is of course an iterative
process.

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor

"In the hard sciences we mostly talk about models rather than laws.  And
if you talk to the people who are working on foundations of mathematics,
they also talk about models.  It's certainly true of physics and astronomy
in particular that a law is just a model that we've got used to.   . . . I
have enormous respect for Stephen Hawking, but I sometimes think he
doesn't know the difference between a model and the real thing.  That's an
occupational disease of theoretical physicists."
Freeman Dyson in ("A Glorious Accident", W. Kayzer ed.)

----- Original Message -----
From: Joel Rauber <Joel_Rauber@SDSTATE.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Sunday, October 10, 1999 12:13 PM
Subject: free-body diagrams
> . . .
> 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).
> . . .
> Joel Rauber