Re: Internal or external?

[Richard Tarara <rtarara@SAINTMARYS.EDU>]

----- Original Message -----
From: Leigh Palmer <palmer@SFU.CA>

> 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?

I can..using a trick picked up from some text I used early on--probably
Miller's College Physics.  You turn the problem into a one dimensional one
because the connecting cord requires coupled motion and therefore you know
that if the system moves it will move either up on the left and down on the
right or down on the left and up on the right.  You then draw a big curved
arrow around the outside of the original diagram defining a positive
direction (up from the left side, over the top, down on the right--for
example.)  Now a free body diagram can provide the answers.  The same trick
works on the 'modified Atwood's machine.

I like to work these programs BOTH ways as it does show that there is not a
single, algorithmic way to solve all problems.

> Just because something looks the same as something else, or has
> the same units, it ain't necessarily the same.
>
> Leigh

Rick

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Richard W. Tarara
Department of Chemistry & Physics
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rtarara@saintmarys.edu

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