Re: work done by friction

[Brian McInnes <bmcinnes@PNC.COM.AU>]

John Denker replied to my listing

> > John and Chuck have both presented illustrations to show that
> > frictional forces do work.  John's illustration is the hand wiped
> > across the table top and Chuck's illustration is a box being dropped
> > on to a moving conveyor belt.
> >
> > ... relative motion ...
> >
> > John's and Chuck's examples are not of this [relative motion] kind.
>
> I disagree.

Let's take Chuck's example first.  The box has fallen on to the belt
and after it makes contact with the belt there is surely no relative
motion between it and the belt.

Now to your example

> In my previous note I explicitly stated that the hand was *wiped* across
> the tabletop and explicitly stated that there *was* relative motion between
> the two.

You said you wiped your hand across the table in the direction shown
in the figure and "Guess what.  The table starts to move".  Okay, I've
been walking around the house wiping my hands across tables big and
small (fortunately my wife is away with grandchildren, so she cannot
be concerned with my strange behaviour) and either my hand slips over
the top or sticks to them intermittently.  In the former case the
tables do not move, in the latter case they do start to move but when
that happens there is no relative motion between hand and table.
What am I missing here?

Brian McInnes