Re: work done by friction

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

>

> At 11:21 AM +1000 10/26/99, Brian McInnes wrote:
>
> > "frictional forces do not do work"
>
> If I drop a penny onto a slowly rotating turntable, I see that the
> penny begins rotating at the same rate. What force diagram describes
> how the coin gained this rotational KE?
>
>      `-'     `-'     `-'     `-'     `-'     `-'     `-'     `-'     `-'
> Chuck Britton



Chuck, I'm not claiming that forces (in particular a net non-zero
force) do not bring about changes in kinetic energy (or that net
unbalanced torques do not bring about a change in rotational kinetic
energy.  They DO.
As John Malinckdrobt put it so well earlier today: "the simple
integrated form of Newton's Second Law establishes a clean and useful
relationship between the net force, the motion of the center of mass,
and the change in center of mass
kinetic energy (and which has often been called the pseudowork-kinetic
energy theorem)".

What I am claiming is that frictional forces may not do work and, in
many situations, they do not do any work, even though the same forces
are involved in changes in kinetic energy.

Brian McInnes