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



SSHS KPHOX wrote:
[1]
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).

Good!

[2]
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.

That's true ... but it's in no way evidence against point [1].

How do we resolve this debate?

Perhaps by distinguishing static pressure from dynamic pressure.
-- Point [1] is entirely concerned with static pressure.
-- Point [2] is entirely concerned with dynamic pressure.

Static pressure: locally: P(h) = constant + rho g h
where rho is the density of air, and h is the height.

Dynamic pressure: Q(v) = .5 rho v^2
where v is the velocity

Total pressure := P + Q

For speeds well below the speed of sound, Q << P.

For details, see
http://www.av8n.com/how/htm/airfoils.html#sec-pressure
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