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



This is actually one of the questions on the FCI (at least on the original)
and one I omit when I use the test (primarily because we have not talked
about air pressure by the time I give the test.)

The problem with the question, IMO, is that it depends....depends on how
'air-tight' the seal between the book and the table is. If you really
had a good seal, then yes the air pressure on the top of the book needs to
be included. But in most real circumstances there is enough air that seeps
in under the book to cancel _almost_ all of the difference. Of course to
really nit-pick this, there is a difference between the top and bottom
forces on the book due to the miniscule difference in air pressure due to
the 'depth' of the top and bottom of the book in the atmosphere (and some
people argue that this effect should be considered to choose the answer).
But ignoring that, I would go by experience that a 8"x10" book sitting on a
table does not seem to require a half-ton of force to lift off the surface.
;-)

Rick

****************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556
rtarara@saintmarys.edu

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FREE: Windows and Mac Instructional Software
www.saintmarys.edu/~rtarara/software.html
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[Original Message]
From: SSHS KPHOX <kphox@CHERRYCREEKSCHOOLS.ORG>
To: <PHYS-L@LISTS.NAU.EDU>
Date: 1/12/2005 6:55:58 PM
Subject: Forces acting on a book

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
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