American Association Of Physics Teachers 2006 Summer Meeting: Syracuse University Go Orange!

Two balls, a mirror, and a puff of air TPT and Me

Racing Balls

“Racing balls” assumptions: 1. Friction small. 2. Balls never leave track. 3. No “loop-the-loop,” etc.

Asif Shakur and Andrew Pica, On An Ambiguous Demonstration, TPT 35, (1997). Physics Colloquium: “The Physics IQ Test” Salisbury State University (Maryland State University System) January 26, 1995

“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

“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.” (On an Ambiguous Demonstration) Fig. 1 (“Bad”) Fig. 2 (“Good”)

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, (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-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)

“Shows that an image in a plane mirror is reversed left to right compared to the object” IMAGE REVERSAL IN A PLANE MIRROR 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, (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)

“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.” Reference #6 for the Eastlake TPT article

“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, (1987)

Norman F. Smith TPT 10, November 1972 ( ) “ ‘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.”

“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 Ingelman- Sundberg, Department of Physics, University of Frankfurt

Photo credited to Paul Bowen (Cessna Aircraft Company) and supplied by Jan-Olov Newborg, KTH, Stockholm, Sweden. Airplane wing vortices

“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 ( ) George Gerhab and Charles Eastlake, Boundary Layer Control on Airfoils, TPT 29 ( ) 1999 (?) 3. John Denker, See How It Flies

Anti-Bernoulli: 1.NASA Website NASA Glenn Research Center: The Beginner's Guide to Aeronautics 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, (1972) 6. Klaus Weltner, A comparison of explanations of aerodynamic lifting force, AJP 55, (1987) 7.Klaus Weltner, Aerodynamic Lifting Force, TPT 28, (1990)

Anti-Bernoulli (continued): 8.Klaus Weltner, Bernoulli’s Law and Aerodynamic Lifting Force, TPT 28, (1990) 9.Chris Waltham, Flight without Bernoulli, TPT 36, (1998) 10.John D. Anderson, Jr., Ludwig Prandtl’s Boundary Layer, Physics Today 58 (12), (2005)

Anti-Bernoulli (continued): 11.Cliff Schwartz, Numbers Count, Editorial, TPT 34, p536 (1996)

Other non-Bernouolli applications

The Coanda Effect Correct explanation for levitating ball and airplane wing Bad Good From: Wikipedia Click here to see video of demonstration on web

The levitating ball Incorrect Bernoulli explanation often found in science museums

Correct Explanation: The Coanda Effect from John Denker: Click here for video on web

Click on picture to see video of that case! Water moves downWater moves upWater does not move

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, (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." 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.) Click here for demos on web

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 University of Maryland demonstration list

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