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

[John Denker <jsd@AV8N.COM>]

I wrote:
> "... to a _paddle_ made by gluing paper to a similar hoop,
> rather than just resting paper on some object, the point
> being to ensure that the paper is not left behind when
> you wave the object around."


Bernard Cleyet wrote:

> and newspaper laid on a flat stick won't be left behind even
> if it's not held flat by atmospheric pressure.

Have you done the experiment recently?

1) In my experience, a piece of newspaper draped over a
  broomstick has a considerable tendency to centrifuge
  itself off the end.  I can prevent this using careful
  technique, but
   a) my life is much easier if I tape the paper to
    prevent this, and
   b) if I'm letting students wave the thing around,
    I'd be crazy not to tape it.

2) Perhaps "left behind" isn't exactly the right word,
  perhaps I should have said "trail behind", but the
  fact is that newspaper draped over a broomstick
  will assume a 'flag' configuration, trailing behind
  the stick.  This might be OK for demonstrating
  skin-friction drag ... but that's not the same as
  pressure, and I'm pretty sure this thread was supposed
  to be about pressure.  And even if I'm wrong about
  that, the pressure-drag you can develop using a
  paddle configuration is a whole lot bigger than the
  skin-friction drag you can develop in the flag
  configuration.  An easy way to ensure a nice big
  frontal area is to use a hoop-like stick.  A Y-shaped
  stick is almost as good, and can be considered a
  split-open hoop.  Badminton rackets work great and
  usually aren't too hard to borrow.

I'm not saying a hoop is the only way to demonstrate
pressure drag, but it's well within the envelope of
reasonable approaches.  In any case I cannot imagine
why anyone would recommend draping paper over a simple
stick.
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