Re: MOMENT OF INERTIA

[Robert Carlson <Raacc@AOL.COM>]

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?