Re: work done by friction

[Chuck Britton <britton@ODIE.NCSSM.EDU>]

At 5:47 PM +1000 10/25/99, Brian McInnes wrote:
> >More generally, friction between A and B does work on A if B is moving.
>
> John, you have missed the point I was making.  I'm claiming (as does
> Arons in his "Guide to Introductory Physics Teaching.") that that no
> work is done by the frictional force when A and B are in contact and
> are moving relative to one another and, as usually happens, a
> frictional force is present.



Hold on now. Does Arons REALLY say this?

Consider a 'conveyor belt' A, onto which box B is dropped (from an
infinitesimal height and with zero momentum in the direction of the
conveyor belt.

How does this 'no work done by friction' viewpoint treat the gain in
momentum and KE of the box?

FBD of the box only includes 'weight' and friction (initially kinetic).

  -.     .-.     .-.     .-.     .-.     .-.     .-.     .-.     .-.     .-
    \   /   \   /   \ N /   \ C /   \ S /   \ S /   \ M /   \   /   \   /
     `-'     `-'     `-'     `-'     `-'     `-'     `-'     `-'     `-'
Chuck Britton                                   Education is what is left when
britton@odie.ncssm.edu                          you have forgotten everything
North Carolina School of Science & Math         you learned in school.
(919) 286-3366  x224                            Albert Einstein,        1936