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Re: Internal or external?



Returning to the same (more or less) topics each year reflects
the nature of our work. This is not deterioration. We should
be able to look at physics through the eyes of our students and
discuss it as if we were students. In that spirit, let me share a
way (new to me) of introducing internal and external forces; it
popped in a class yesterday, unexpectedly .

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,
even if it may be a repetition of a thread I do not remember.
Ludwik Kowalski

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.

Leigh