Re: MOMENT OF INERTIA

[Bob Sciamanda <trebor@VELOCITY.NET>]

(I just can't resist)
Robert, it all depends on the meaning of the word "work" :)
We thrashed out this issue some time ago, and I submitted several verbose
posts on my view.
Since there are so many differing definitions of the word "work", I
proposed never to use it!

I then detailed what I called the universally applicable (within
Newtonian mechanics) "Mechanical Energy Theorem" (MET):  The line
integral of the net external force on a many particle system calculated
over the CM trajectory of that system is numerically equal to the change
in the translational CM kinetic energy of that system (all done from an
inertial frame).  This does not imply that the agent(s) of these external
forces are necessarily the source of an energy transfer to/from the
system.

Bob Sciamanda
Physics, Edinboro Univ of PA (ret)
trebor@velocity.net
http://www.velocity.net/~trebor
-----Original Message-----
From: Robert Carlson <Raacc@AOL.COM>
To: PHYS-L@LISTS.NAU.EDU <PHYS-L@LISTS.NAU.EDU>
Date: Friday, December 18, 1998 11:53 PM
Subject: Re: MOMENT OF INERTIA


> I'm not sure whether Bob is saying static friction does or does not do
work in
> his comment below.  I suggest it does not, at least in the floor's frame
of
> reference.  When I start to walk it always involves a leaning forward.
In
> this sense, my weight is providing a torque about my center of mass.  I
> believe that the frictional force, not doing work, is providing a pivot
point
> for this torque.  Am I wrong?  However, I would not use this explanation
in an
> introductory course until torque is discussed.  Is this another case
where we
> lead introductory students astray in that static friction does work and
> accelerates objects, or am I completely wrong here?  For example, most
texts I
> have seen explain that when a person walks, they place a force back on
the
> floor.  By Newton's 3rd, they then say that the floor places a force
forward
> on the person.  In this explanation, the conclusion is that the static
> frictional force is causing the acceleration.  But, if it is static
friction,
> then how can it do work and cause the acceleration?
>
> Bob Carlson
>
> In a message dated 12/18/98 9:10:40 PM Central Standard Time,
> trebor@VELOCITY.NET writes:
>
> > The force of static friction in the rolling wheel performs much the
same
> >  function as does the force of static friction between your foot and
the
> >  floor when you walk.  Think of a wheel as the limit of an infinite
number
> >  of spokes, each with a shoe at its distal end!
>
> And Martha Takats writes:
>
> > But I still worry about why
> > there are no non-conservative forces doing work--for example the force
> > of static friction, which prevents slipping. Can anyone give a SIMPLE
> > explanation of why we don't have to include it in the work-energy
> > equation?