Re: Internal or external?

[Leigh Palmer <palmer@SFU.CA>]

> 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?

I didn't say I didn't understand what you were saying; I said
it is a bad idea to tell the students that tension is an
"internal force" in that system. The net force acts in a
direction (down in this case) so the the center of mass
descends, but the net force downward it the total weght minus
twice the tension.

What have you got to say about the force exerted by the pulley
on the mediateor of your internal force?

You will not help the students by likening this to a one
particle system.

Leigh