From Steve Kearney
There is a standard demonstration of the Bernoulli principle in any textbook
on flight theory
(5 May, p 40).
You take a piece of paper and hold it in front
of your mouth. You then blow across the upper surface and the paper rises. If
accelerating air across the surface doesn’t reduce pressure and cause the sheet
to rise, then what does?
David Anderson replies: The “Bernoulli strip”, as this is called, is
one of the many common misapplications of the Bernoulli principle, though it is
a good demonstration of the physics of lift. (Other examples are the ping-pong
ball in a jet of air and the curve of a spinning ball. See
www.aa.washington.edu/faculty/eberhardt/Lift_AAPT.pdf.
In brief, a fast-moving, unconfined fluid has the same pressure as the
surroundings. Viscosity causes the breath to follow the surface of the paper
strip. Newton’s first law says that for the air to bend there must be a force on
the air (down) and the third law says that there must be an equal and opposite
force on the strip (up). The breath is diverted from the horizontal to a
downward direction.
Tyers, Victoria
