American Association

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American Association Of Physics Teachers 2006
Summer Meeting Syracuse University
Go Orange!
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TPT and Me
Two balls,
a mirror,
and a puff of air
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Racing Balls
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Racing balls assumptions 1. Friction
small. 2. Balls never leave track. 3. No
loop-the-loop, etc.
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Physics Colloquium The Physics IQ
Test Salisbury State University (Maryland State
University System) January 26, 1995
Asif Shakur and Andrew Pica, On An Ambiguous
Demonstration, TPT 35, 316-317 (1997).
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On an Ambiguous Demonstration
We first witnessed this demonstration a couple
of years ago in a Physics Show on the Road.
It was a curiously innovative apparatus. Two
balls were fired from similar cannons at the same
speed. We reckon that the span of this humongous
piece of equipment was approximately 10 m (33
ft). It was apparently constructed at great
expense at a major university.
literary hyperbole?
Click on picture to see video of demonstration
on web site
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(On an Ambiguous Demonstration)
Fig. 2 (Good)
Fig. 1 (Bad)
It has been shown that the outcome of
demonstrations of the genre depicted in Fig. 1
are ambiguous at best and likely misleading
unless accompanied by several caveats. The
outcome depends on track geometry, initial speed,
and friction.
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Real-World Constraints
. Consequentially, since the second body moves
horizontally as fast or faster than the body on
the bridge, it will cross the valley first.
G. E. Hite Texas AM University TPT 35, 324
(1997) (letter to the editor)
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Image of a Plane Mirror
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Kenneth W. Ford Why is your image in a plane
mirror inverted left-to-right but not
top-to-bottom? TPT 13, 228-229 (1975).
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It isnt!!
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It is inverted front-to-back!
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(No Transcript)
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Questions Some Students Ask A Question of Mirror
reflections
The answer to this peculiar apparent
left-to-right reversal without a corresponding
up-down reversal of image to object is easily
explained in mathematical terms by saying it is
not a question of left-right reversal but a
question of front-rear reversal.
Walter Thumm TPT 10, 346 (1972)
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IMAGE REVERSAL IN A PLANE MIRROR
Shows that an image in a plane mirror is
reversed left to right compared to the object
Taken from an unidentified university Lecture-Demo
nstration Facility.
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Perverted Image
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Bernoulli Effect
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Airfoil Lift Newton vs. Bernoulli?
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Charles N. Eastlake An Aerodynamicists View
of Lift, Bernoulli, and Newton TPT 40, 166-173
(2002).
The production of lift by an airfoil
is described correctly and accurately by A.
Bernoullis law B. Newtons law(s) C. This
article D. All of the above.
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(No Transcript)
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Note air stream deflection, definition of CHORD
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I would like to conclude with a plea to teachers
to emphasize whichever model works more
conveniently in their scenario, without stating
or even implying that the other is wrong. I
always explain lift in terms of Bernoullis law
and have felt comfortable that it made sense to
audiences at many different levels. my emphasis
(Charles N. Eastlake)
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I carefully reviewed several oft quoted
references in the physics-teaching literature and
do not feel that any of them describe a
shortcoming of Bernoullis law that is
technically correct. Besides that, Bernoullis
law is one of the foundations of fluid physics
and is the source of some of my favorite
aerodynamic-toy demonstrations.
(Charles N. Eastlake)
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Reference 6 for the Eastlake TPT article
In physics textbooks two explanations of the
mechanism which enable airplanes to fly are to be
found. The first is based on Bernoullis law
regarding the flow of liquids and gases. This
explanation is most frequently used in textbooks
of school physics and undergraduate physics.
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The second explanation is based on the repulsion
of air pushed downward by the wing. This
explanation is found in monographs on
aerodynamics, e.g. Prandtl, et. al. and it is
mentioned in a few textbooks, e.g. Resnick and
Halliday. To use this explanation in school
physics has first been proposed by Fletcher,
unfortunately without great response.
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An analysis of both explanations shows that the
explanation based on Bernoullis law is
incomplete and that it has a fundamental
drawback The reasoning given is wrong. my
emphasis --Klaus Weltner, AJP 55, 50-54
(1987)
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Dynamic lift must be examined as an external
encounter between air and another object, an
airfoil, for example. In such an examination, it
becomes at once apparent that the law that must
be used to describe this encounter is Newtons
third law covering action and reaction.
Norman F. Smith TPT 10, November 1972 (451-455)
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Bernoullis theorem should be applied only to
cases dealing with an interchange of velocity and
pressure within a fluid under isentropic
conditions. The carburetor, jet pump, and
venturi are all valid applications of Bernoullis
theorem.
(Norman F. Smith)
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For explaining dynamic lift, the result of an
encounter between a fluid and a lifting device,
Newtons laws must my emphasis be used.
Consolidation of all dynamic forces produced in a
fluid propulsion, lift, control, etc. under
Newtons third law is not only correct physics
but also makes the whole business far easier to
teach and to learn.
(Norman F. Smith)
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Incorrect airplane wing explanation
(Norman F. Smith)
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Correct airplane wing explanation
(Norman F. Smith)
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Bernoulli effect assumptions 1. Smooth,
laminar flow 2. Incompressible fluid Neither
of these assumptions applies to an airplane wing.
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If the airfoil generates low pressure at its
upper side and high pressure at its lower side
this causes lateral movements rotating to the
ends of the wing. Below the wing air moves
outwards and above the wing air moves inwards.
Beyond the ends of the airfoil air moves even
upwards. --Physics of Flight reviewed,
Klaus Weltner and Martin Ingelman- Sundberg,
Department of Physics, University of Frankfurt
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Airplane wing vortices
Photo credited to Paul Bowen (Cessna Aircraft
Company) and supplied by Jan-Olov Newborg, KTH,
Stockholm, Sweden.
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The flow near limiting surfaces follows the
geometrical shape of these surfaces. This
behaviour is called Coanda-effect. my
emphasis This is important because this
behaviour holds for all flows limited by smoothly
curved surfaces like aerofoils, streamlined
obstacles, sails and - with a certain reservation
- roofs. --Misinterpretations of Bernoullis
Law, Klaus Weltner and Martin Ingelman-Sundberg,
Department of Physics, University of Frankfurt
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Take a Cessna 172, The wings must lift 2300
lb (1045 kg) at its maximum flying weight. The
path length for the air over the top of the wing
is only about 1.5 percent greater than the length
under the wing. Using the popular description of
lift (Bernoulli effect), the wing would develop
only about 2 percent of the needed lift at 65
mi/h (104 km/h), my emphasis which is slow
flight for this airplane.
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In fact, the calculations say that the minimum
speed for this wing to develop sufficient lift is
over 400 mi/h (640 km/h). If one works the
problem the other way and asks what the
difference in path length would have to be for
the popular description to account for the lift
in slow flight, the answer would be 50 percent.
The thickness of the wing would be almost the
same as the chord length. my emphasis
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Though enthusiastically taught, there is
clearly something seriously wrong with the
popular description of lift.
David F. Anderson and Scott Eberhard Understanding
Flight McGraw-Hill (2001) page 16
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Airfoil Lifting Force MisconceptionWidespread in
K-6 Textbooks Bill Beatty, 1996
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Incorrect explanation compliments of
the Scientific American
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Propeller and jet engines generate thrust by
pushing air backward. In both cases, because the
wing is curved, air streaming over it must travel
farther and faster than air passing underneath
the flat bottom. According to Bernoullis
principle, my emphasis the slower air exerts
more force on the wing than the faster air above,
thereby lifting the plane. Scientific
American, April 2006 (p. 92)
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Retraction compliments of the Scientific American
Numerous readers wrote to correct a common but
faulty explanation of how an airplane wing
creates lift, the complex turning of the
airflow, both below and above the wing, is the
real driver. Mark Fischetti,
editor August 2006, p13-14
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Pro-Bernoulli 1.Charles N. Eastlake, An
Aerodynamicists View of Lift, Bernoulli, and
Newton, TPT 40 (166-173) 2002 2.George Gerhab
and Charles Eastlake, Boundary Layer Control on
Airfoils, TPT 29 (150-151) 1999 (?) 3. John
Denker, See How It Flies http//www.av8n.com/how/
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• Anti-Bernoulli
• NASA Website
• NASA Glenn Research CenterThe Beginner's Guide
  to Aeronautics
• http//www.grc.nasa.gov/WWW/K-12/airplane/
• 2. McGraw-Hill Encyclopedia of Science and
  Technology
• 3.Encyclopedia Britannica
• 4.Encyclopedia of Physics

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Anti-Bernoulli (continued) 5.Norman F. Smith,
Bernoulli and Newton in Fluid Mechanics, TPT 10,
451-455 (1972) 6. Klaus Weltner, A comparison of
explanations of aerodynamic lifting force, AJP
55, 50-54 (1987) 7.Klaus Weltner, Aerodynamic
Lifting Force, TPT 28, 78-82 (1990)
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Anti-Bernoulli (continued) 8.Klaus Weltner,
Bernoullis Law and Aerodynamic Lifting Force,
TPT 28, 84-86 (1990) 9.Chris Waltham, Flight
without Bernoulli, TPT 36, 457-462 (1998) 10.John
D. Anderson, Jr., Ludwig Prandtls Boundary
Layer, Physics Today 58 (12), 42-48 (2005)
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Anti-Bernoulli (continued) 11.Cliff Schwartz,
Numbers Count, Editorial, TPT 34, p536 (1996)
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Other non-Bernouolli applications
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The Coanda Effect
Bad
From Wikipedia
Good
Click here to see video of demonstration on web
Correct explanation for levitating ball and
airplane wing
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The levitating ball
Incorrect Bernoulli explanation often found in
science museums
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Click here for video on web
Correct Explanation The Coanda Effect from John
Denker http//www.sciencetoymaker.org/balloon/li
nks.html
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Water moves down
Water moves up
Water does not move
Click on picture to see video of that case!
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Pro-Bernoulli Harold Cohen and David Horvath,
Two Large-Scale Devices for Demonstrating a
Bernoulli Effect, TPT 41, 9-11 (2003).
Anti-Bernoulli (pro-Coanda) Clifford Schwartz,
Bernoulli and Newton, TPT 41, 196-197 (2003)
letter.
Click for web demo
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The Magnus effect Flettners ship
Incorrect diagram and explanation
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Magnus effect Curve ball
This explanation is INCORRECT.
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Wind tunnel photograph of a "curve ball."
Click here for demos on web
View from above, ball spinning clockwise moving
right to left, shedding vortices down and curving
up in the photograph. (Right-handed pitcher
throws side-arm letting the ball slip off the end
of fingers.)
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University of Maryland demonstration list
F5. PRESSURE IN MOVING FLUIDS (YES MAYBE -
NO) F5-01 BERNOULLI'S PRINCIPLE - TOY CAR AND
BALL F5-02 BERNOULLI'S PRINCIPLE - BALL ABOVE
MOVING CART F5-03 BERNOULLI'S PRINCIPLE - THIN
METAL SHEETS F5-04 BERNOULLI'S PRINCIPLE - LARGE
BALL AND FUNNEL F5-05 BERNOULLI'S PRINCIPLE -
SMALL BALL AND FUNNEL F5-06 BERNOULLI'S
PRINCIPLE - BEACH BALL F5-07 BERNOULLI'S
PRINCIPLE - SPOOL AND CARDBOARD F5-08
BERNOULLI'S PRINCIPLE - MARBLE IN WATER JET F5-09
BERNOULLI'S PRINCIPLE - HAIRDRYER AND PING PONG
BALL F5-10 CHIMNEY DRAW WITH WATER F5-11
AIRPLANE WING F5-12 BERNOULLI'S PRINCIPLE? F5-21
VENTURI TUBE WITH MANOMETERS F5-22 VENTURI TUBE
WITH PING PONG BALLS F5-23 VENTURI TUBE WITH
WATER - GAUGES F5-24 VENTURI TUBE WITH WATER -
MANOMETERS F5-31 MAGNUS EFFECT - FLETTNER'S
SHIP F5-32 CURVE BALL
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The truth shall set you free but first it will
piss you off.
Anonymous, courtesy of Bill Beatty
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Two balls, a mirror, and a puff of air
http//www.physics.umd.edu/lecdem/