American Association
Of Physics Teachers
2006 Summer Meeting:
Syracuse University
Go Orange!
TPT and Me
Two balls,
a mirror,
and a puff of air
Racing Balls
“Racing balls” assumptions:
1. Friction small.
2. Balls never leave track.
3. No “loop-the-loop,” etc.
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).
“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
(On an Ambiguous Demonstration)
Fig. 1 (“Bad”)
Fig. 2 (“Good”)
“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.”
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 A&M University
TPT 35, 324 (1997)
(letter to the editor)
Image
of a
Plane Mirror
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).
It isn’t!!
It is inverted front-to-back!
Questions Some Students Ask…
A Question of Mirror reflections
“The answer to this peculiar apparent left-toright reversal without a corresponding updown 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)
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-Demonstration Facility.
“Perverted Image”
“Bernoulli Effect”
Airfoil Lift:
Newton vs. Bernoulli?
Charles N. Eastlake:
An Aerodynamicist’s 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. Bernoulli’s law
B. Newton’s law(s)
C. This article
D. All of the above.”
Note air stream deflection, definition of “CHORD”
“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 Bernoulli’s law and
have felt comfortable that it made sense
to audiences at many different levels.”
[my emphasis]
(Charles N. Eastlake)
“I carefully reviewed several oft quoted
references in the physics-teaching
literature and do not feel that any of
them describe a shortcoming of
Bernoulli’s law that is technically correct.
Besides that, Bernoulli’s law is one of the
foundations of fluid physics and is the
source of some of my favorite
aerodynamic-toy demonstrations.”
(Charles N. Eastlake)
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
Bernoulli’s law regarding the flow of
liquids and gases. This explanation is
most frequently used in textbooks of
school physics and undergraduate
physics.”
“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.”
“An analysis of both explanations shows
that the explanation based on Bernoulli’s
law is incomplete and that it has a
fundamental drawback: The reasoning
given is wrong.” [my emphasis]
--Klaus Weltner, AJP 55, 50-54 (1987)
“ ‘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 Newton’s third
law covering action and reaction.”
Norman F. Smith
TPT 10, November 1972 (451-455)
“Bernoulli’s 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
Bernoulli’s theorem.”
(Norman F. Smith)
“For explaining dynamic lift, the result of
an encounter between a fluid and a lifting
device, Newton’s laws must [my
emphasis] be used. Consolidation of all
dynamic forces produced in a fluid –
propulsion, lift, control, etc. – under
Newton’s third law is not only correct
physics but also makes the whole
business far easier to teach and to learn.”
(Norman F. Smith)
Incorrect airplane wing explanation
(Norman F. Smith)
Correct airplane wing explanation
(Norman F. Smith)
Bernoulli effect assumptions:
1. Smooth, laminar flow
2. Incompressible fluid
Neither of these assumptions
applies to an airplane wing.
“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 IngelmanSundberg, Department of Physics,
University of Frankfurt
Airplane wing vortices
Photo credited to Paul Bowen (Cessna Aircraft Company) and
supplied by Jan-Olov Newborg, KTH, Stockholm, Sweden.
“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 Bernoulli’s Law,
Klaus Weltner and Martin Ingelman-Sundberg,
Department of Physics, University of Frankfurt
“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.”
“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]
“…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
Airfoil Lifting Force Misconception
Widespread in K-6 Textbooks
Bill Beatty, 1996
Incorrect explanation
compliments of the
Scientific American
“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 Bernoulli’s 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)
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
Pro-Bernoulli:
1.Charles N. Eastlake, An
Aerodynamicist’s 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/
Anti-Bernoulli:
1. NASA Website
NASA Glenn Research Center:
The 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
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)
Anti-Bernoulli (continued):
8.Klaus Weltner, Bernoulli’s 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
Prandtl’s Boundary Layer, Physics
Today 58 (12), 42-48 (2005)
Anti-Bernoulli (continued):
11.Cliff Schwartz, Numbers
Count, Editorial, TPT 34, p536
(1996)
Other
non-Bernouolli
applications
The Coanda Effect
Bad
From: Wikipedia
Click here to see video
of demonstration on web
Good
Correct explanation for
levitating ball and airplane wing
The levitating ball
Incorrect Bernoulli explanation
often found in science museums
Click here
for video on web
Correct Explanation: The Coanda Effect
from John Denker:
http://www.sciencetoymaker.org/balloon/links.html
Water moves down
Water moves up
Water does not move
Click on picture to see video of that case!
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
The Magnus effect: Flettner’s ship
Incorrect diagram and explanation
Magnus effect: Curve ball
This explanation is INCORRECT.
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 sidearm letting the ball slip off the end of fingers.)
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
The truth shall set you free;
but first it will piss you off.
Anonymous,
courtesy of Bill Beatty
Two balls,
a mirror, and
a puff of air
http://www.physics.umd.edu/lecdem/