Re: work done by friction

[Leigh Palmer <palmer@SFU.CA>]

> Leigh,
>
> Does your frictional force act on a line of action passing through the center
> of mass of both blocks?  If not, then don't rotational effects need
> consideration?
>
> Bob Carlson

That's a question I hadn't considered. As you point out, it could
matter. But it doesn't. The bricks could be replaced by pieces of
telescope-fitting sewer pipe and the argument would be unchanged,
or they could be freight cars scraping as they pass on parallel
frictionless tracks.

The details of this silly example are unimportant. I just want to
highlight the fact that it is not always useful to designate a
particular interaction as being "work". Friction presents problems
for such analysis; it is not straightforward in the way that other
analyses are. There are tougher ones, too. How much work is done on
a river when a canoeist pulls his paddle through the water? Such a
problem does not tell us anything physical about the system in
question. We can answer questions about well defined physical
properties of the system, like total energy distribution, but the
question of whether mechanical work was done on the river is more
for philosophers than physicists.

Leigh