Re: Conservation of ME and nonconservative forces

["John S. Denker" <jsd@MONMOUTH.COM>]

> > But in order to use cons. of ME, we need to be certain that there is no
> > work done by nonconservative forces.


At 10:06 PM 6/28/01 -0500, Herb Schulz wrote:
> I haven't followed all that has been said about this but here's my take.
>
> When rolling downhill the static friction force does negative work as
> far as linear motion is concerned but it does an equal amount of
> positive work as a torque acting about the CM of the ball.  The net
> amount of work done, when the total kinetic is taken into account
> (Linear KE of CM + rotational KE about the CM) those two works add to
> zero.
>
> Going uphill the static friction force actually reverses and the
> signs of both works switch so the total still remains zero.

I don't think that answers the question.  At the very least, it needs to be
prefaced by a big "IF".

        IF the forces are 100% nondissipative, then going uphill
        undoes whatever happened going downhill.

        That's what happens IF that's what happens.

But that begs the original question:  How do we know whether the forces
are  nondissipative?

Indeed as Marc Kossover (04:35 PM 6/28/01 -0500) and others have pointed
out, in many cases assuming no slip and no dissipation is a _not_ safe
assumption, nor even a decent approximation.

> Going uphill the static friction force actually reverses and the
> signs of both works switch so the total still remains zero.

Please do not assume this.  Please do not try to "prove" this.  It's bad
luck to prove things that aren't true.