Re: Internal or external?

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

The combined mass in the Atwood's machine moves in the
direction of the net force; it can be treated, mathematically,
as if it were moving along an axis. Yes, an axis bending
over a pulley is an abstraction. So what? How do we decide
which abstractions are good and which a bad?
Ludwik Kowalski

Referring to:

> > There are two possible approaches to pulley problems. One is
> > to consider the total mass (m1+m2) accelerated by the net force.
> > In our example (Atwood's machine) it was the difference between
> > two weights. The other approach is to consider two free body
> > diagrams (each mass is acted upon by two forces: its own weight
> > and the string tension) . In the first approach we simply ignore
> > tensions, they are internal forces, as far as the whole system is
> > concerned. In the second approach tensions can not be ignored,
> > they are external forces for m1 and m2. Both approaches lead to
> > the same value of a. Trivial? Yes. Worth sharing? I hope so...

Leigh Palmer wrote:

> A very bad idea in my opinion. Two bodies; two free body disgrams!
> Ask the question: "What is the direction of the net force which
> act on your two-body system?" Then draw the "free body" diagram.
> Can't do it, eh?
>
> Just because something looks the same as something else, or has
> the same units, it ain't necessarily the same.