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Re: MOMENT OF INERTIA



Martha brings up an interesting question that I have recently pondered. One
might ask, when does static friction do work? I have reached my own
conclusions, but will pose these cases to the list for some thought.

1. A rope accelerates without slipping over a pulley with a nonzero
rotational inertia. Hence, there is a static frictional force between the
surface of the rope and the surface of the pulley. Does this static
frictional force do work on the pulley?

2. A box is in the bed of a truck and the truck accelerates. The box does
not slip in the bed of the truck. Does the static frictional force do work on
the box?

3. The drive wheels of an automobile do not slip on the road when the
automobile accelerates. Does the static frictional force do work on the car?

4. You use a crutch with a rubber tip as a vertical lever to move a table.
Hence, the lever is perpendicular to the floor for the first instant. The
rubber tip, acting at the pivot, does not slip on the floor, yet the table
accelerates. Does the static frictional force between the rubber tip and the
floor do work on the table?

5. Can one treat a rotating object on an incline as an infinite number of
perpendicular levers that are all instantaneously perpendicular to the
incline's surface? If so, does the static frictional force, at the pivot, do
work? If not, and the object's kinetic energy does change, then what force
does do work?

Bob Carlson


In a message dated 12/18/98 1:23:29 PM Central Standard Time,
mtakats@ACAD.URSINUS.EDU 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?