Re: Internal or external?

[Bob Sciamanda <trebor@VELOCITY.NET>]

This seems to be an argument in favor of the two free body analysis.  Your
conclusions involve applying F=ma to each mass, individually - you have
not employed the mentioned "system approach".

I think that what should be said is that the solution to this problem
happens to be also the solution to a quite different (carefully contrived)
one dimensional "two-connected-mass" problem . . . blah, blah, etc. If
done carefully, something can be learned about developing an intuition for
seeing "many problems in one".  (The danger of encouraging reliance on
undeveloped intuition lurks here. )

Also the tempting confusion of this "system" approach with the application
of F=ma to the CM is needs to be "unexplained".

Bob

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

----- Original Message -----
From: Ed Schweber <edschweb@IX.NETCOM.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, October 08, 1999 4:37 PM
Subject: Re: Internal or external?


> Ed Schweber (edschweb@ix.netcom.com)
> Physics Teacher at The Solomon Schechter Day School, West Orange, NJ
> To obtain free resources for creative physics teachers visit:
> http://www.physicsweb.com
> ----- Original Message -----
> . . .
>    The system approach also allows students to build up a quantitative
> intuition in other ways. Think of a hanging block connected by a string
over
> a pulley to another block on a frictionless horizontal surface. What
happens
> to the tension and the acceleration if we increase the mass of the block
on
> the table.?
>
>    The acceleration decreases - easy enough. But students will then
> frequently focus on the block on the table and become stymied. Its
> acceleration is less so they typically think the tension is less. But
then
> they come to realize that the mass is also greater and they don't know
which
> effect is more important.
>
>    If they focus on the hanging block, it has less acceleration but the
same
> mass. So the upward tension in the string must have increased.
> . . .
>  Ed Schweber