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Re: MOMENT OF INERTIA



On Sat, 19 Dec 1998, Jerome Epstein wrote:

I am utterly unable to see how friction can do positive work on a body.
Friction (kinetic friction) always acts opposite to velocity. Thus
slowing it down. I missed the early part of this thread, but there has
to be some confusion here.

It would be more accurate to say that friction forces oppose both *motion*
and *impending motion*. That is, kinetic friction acts so as to terminate
the existing motion of a moving object, and static friction acts so as to
prevent the motion of a stationary object.

As we all know, motion is relative. Thus, another way to think of this
strange situation: kinetic friction forces tend to eliminate the relative
motion between objects while static friction forces tend to prevent the
relative motion between objects. The latter situation can actually result
in motion, as when I depress the "accelerator" on my car and the tire
tread attempts to remain stationary in contact with the road, propelling
the car forward. More commonly, when I take a step, the sole of my shoe
attempts to remain stationary in contact with the floor, and the friction
force of the floor on the sole propels my body forward. My experience has
been that when friction forces are eliminated, as on ice, neither my car
nor my body gets anywhere.

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