Re: work done by friction

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

 Bob Sciamanda wrote

> As in other instances of these "work-energy" threads, I stress that one
> should distinguish between the two very different questions:
>
> 1) Does a given force do work?  and
> 2) Is the mechanism or agent represented by a given force a source or sink
> of energy?
>
> Eg.: When I push myself away from a wall, the force from the wall does
> work (defined as its line integral over the displacement of my CM), but
> the wall is not an energy source

This is the rub.  HOW DO WE DEFINE WORK?

The traditional definition of work done by a force is NOT the line
integral of the force over the displacement of the CM but the line
integral of the force over the displacement of the point of
application of the force.
The reason for this is that when we are extending our definition of
work from the particle situation to the system of particles that make
up an extended body, the work is calculated in terms of the force
applied to a particular particle, here the one that is in contact with
the wall and which is not moving.  The particle located at the CM has
no external force acting on it.

  Brian McInnes