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



On Jan 12, 2005, at 6:52 PM, SSHS KPHOX 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.

Ken:

Air pressure does act on all sides of the book as it can get between
the book and the table. If we make the surfaces so smooth as to not
admit air then you have made something like a suction cup. The demo
you mention involves moving a lot of air quickly, something a little
different.

There is a net effect creating buoyancy but it's trivial for a book in
air.

My students and I nigh-always treat this situation in mechanics as part
of wondering when the force / resistance due to air are significant
and when they aren't. This is a central question to the "at rest"
condition.

Dan M

Dan MacIsaac, Assistant Professor, Physics, SUNY Buffalo State College
222 SCIE BSC 1300 Elmwood Ave, Buffalo NY 14222 716-878-3802
<macisadl@buffalostate.edu> <http://PhysicsEd.BuffaloState.edu>
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