Re: work done by friction

[Leigh Palmer <palmer@SFU.CA>]

> ----- Original Message -----
> From: Leigh Palmer <palmer@SFU.CA>
> To: <PHYS-L@lists.nau.edu>
> Sent: Wednesday, October 27, 1999 8:32 PM
> Subject: Re: work done by friction
> > . . .
>
> > I note now that |WA| > |WB| (both WA and WB are negative, and I leave
> > the proof to the reader). It is difficult to imagine how brick A could
> > have scraped a longer path over Brick B than vice-versa in any really
> > meaningful way.  . . .
> > Leigh
>
> Leigh,
> This is not surprising.  Since you have specified MA<MB, doesn't  A have
> to move farther than B to maintain a fixed CM?

Aren't you talking about the friction force "doing" something?
What possible relevance can the CM of anything have? Whatever
work is *done* must be done locally by this friction mechanism
we're talking about. My hypothesis is that the interface between
these bricks is symmetrical (I didn't mention that). How does
the "work" know how to split up? (Reminds me of the look on
Edith Bunker's face when someone asked her how the bathroom
water knows to go to the bathroom and not to the kitchen sink.)

Just because one can find two things to multiply together which
balnce the equation and have the correct units doesn't mean that
the product is work. Torques have the dimensions of energy, too.
If the rules you make up for the application of the concept of
work to different types of forces must differ for different
cases, is it really a single concept you're talking about? Will
it make analysis easier to have a complicated set of rules to
apply? Will the student find it helpful? The final question
(which should have come first): is there an easier way?

Leigh