[Physltest] [Phys-L] Re: Forces acting on a book

["Richard A. Lindgren" <ral5q@VIRGINIA.EDU>]

Dear Ken,

The air pressure does act on all sides just as you
suggested. This gives rise to a buoyant force acting
upward which we normally neglect because it is very small.
On an average size man (150 lbs) you can simply calculate
the buoyant force to be a an ounce or so. This buoyant
force is the same one given by archimedes principle. It is
very small because the density of air is small.

I would guess that the paper holds the stick down because
the paper can't move very fast due to the large air
resistance as a result of the large surface area of the
paper.

Richard Lindgren




On Wed, 12 Jan 2005 16:52:20 -0700
  SSHS KPHOX <kphox@CHERRYCREEKSCHOOLS.ORG> wrote:
> We are having a debate on whether air pressure has a net
> effect on a book
> at rest on a table top. If it does, is it up or down?
>
> My colleague cited the demo done often to show the
> strength of the air
> pressure force by placing a stick so one end is under a
> sheet of newspaper
> with the other extending over the edge of the table. It
> is easy to break
> the stick with a sharp whack on the unsupported end. This
> is ostensibly
> because the air pressure on the paper keeps the "under
> the paper" end at
> rest so the pair of torques snaps the stick.
>
> I am thinking that if this is so that when we calculate
> the friction
> force, the "air pressure force" should be included in the
> normal force.
> But we never do that as we think the only forces acting
> on the book are
> the "normal" and "gravity" forces. I have thought that
> the air pressure
> manages to apply forces to all sides of the book (of
> normal size) equally
> (for all practical purposes).
>
> Is this to be my first pubic faux-pas of 2005? How do we
> resolve this
> debate?
>
> Cheers.
>
> Ken Fox

Dr. Richard A. Lindgren
Research Professor of Physics and
Director of Masters of Arts in Physics Education Program
Institute of Nuclear and Particle Physics
Department of Physics
University of Virginia
P.O. Box 400714
Charlottesville, VA 22904-4714
Office 434-982-2691
Fax    434-924-4576
email  ral5q@virginia.edu
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