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Re: MOMENT OF INERTIA



On Sat, 19 Dec 1998, Jerome Epstein wrote:

All of this is usually covered in some depth in a basic physics
sequence, but I will try to say a few words that may be useful.

IN terrrestrial situations the contact force is typically from the
weight pushing the two surfaces together. If I have a book on a table
the contact force is the VERTICAL force the book exerts on the table and
its reaction force the VERTICAL force the table exerts on the book.
Without this vertical contact force, there can be no friction. WHile
there are no horizontal forces pushing the book the frictional force is
zero. When an outside agent, such as your hand, begins to push the book
horizontally (let's say toward the East), the force of static friction
appears. It is a HORIZONTAL force, directed opposite to the applied
force (i.e. to the WEST) and exactly equal to the applied force, thus
preventing acceleration. It has, however a limit, namely mu-s
(coefficient of static friction) times the normal (contact) force, which
in this case is the weight of the book. If the applied force
(horizontal) exceeds the maximum available force of static friction
(also horizontal and opposite to the applied force) then the book begins
to move.
J. EPstein

If the contact force is a vertical force (and I agree that it is), then it
cannot cause an acceleration in the horizontal direction. The only force
in the horizontal direction is the friction force. Therefore, it is
responsible for the horizontal acceleration.

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