Re: MOMENT OF INERTIA

[Mervin Koehlinger <tchkoehlimw@CRF.CUIS.EDU>]

On Fri, 18 Dec 1998, Robert Carlson wrote:

> 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?

But, what agent is providing the force that causes the center of mass to
move forward resulting in that torque?

>  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?

Suppose you were standing on a frictionless surface.  Could you lean
forward?  Could you change the position of your center of mass?

Mervin Koehlinger
Physics Instructor
Concordia Lutheran High School
Fort Wayne, Indiana