Physics Education as an academic discipline
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February 11, 2020
Colloquium at the Department of
Physics and Astronomy
Physics Education as an academic discipline –
The case of weight concept
Igal Galili
Faculty of Mathematics and Natural Sciences,
The Amos de Shalit Science Teaching Center
1925
The Hebrew University of Jerusalem
Physics is a web of concepts and conceptions
The concept of weight is among the oldest in science.
It possesses anthropomorphic nature and currently, it is
not in use in advanced physics. Yet, in the introductory
physics it is among the most fundamental.
Across different cultures, it universally possessed the
meaning of gravity heaviness evaluated by perception
and measurement (weighing)
Along the history, different theories explained weight:
Aristotle, Buridan, Descartes, Newton, Einstein
There is a problem with this concept in physics teaching
To consider a concept in physics teaching one is required
addressing various perspectives
Physics
knowledge
History of Philosophy
Science of Science
None of them is univocal
Physics teaching
Cognitive Science Pedagogy
Learning theories Didactics
Sociology
Physics educators are pushed to consider and decide by their own…
Brief history and philosophy of weight concept
Hellenic
Aristotelian cosmos
Sub-lunar world:
heaviness and lightness
radial arrangement
Supra lunar world: CW
weightlessness
Weight-gravity manifests itself as intention to fall towards CW and
restore the order according with gravity (the heavier – the faster).
Weight is not a force.
Hellenistic
Weight an operationally defined
quality
Euclid -3C
“Weight is a measure of the heaviness and
lightness of one thing, compared to another,
by means of a balance”.
Weight may compete with force
Archimedes -3C
(with buoyant force) (lever)
Medieval
Weight pondus, gravity, mass
Oresme 14c
impetus increase in falling weight change
Weight
Natural weight Accidental weight
Students: sense evaluation of weight (disc-ball confusion)
(Galili & Bar, 1997)
17th century
. . as has been often remarked, the medium diminishes
the weight of any substance immersed in it . . . Galilei 1638
Weight is heaviness, perceived/measured
Falling is determined not by weight but by
density difference with medium
Weight the residual push to the center of
rotation in the whirl of compound material plenum
Descartes
1640
Model: Pieces of cork
immersed among
metallic spheres (beads)
in a spinning barrel
Centripetal residual push on
the cork pieces causes weight
Mathematical Principles of Natural Philosophy
Newton 1687
Fs-e Fe-s
Fs-e= Fe-s
The universal law of gravitational attractionGravitational force causes weight
Gravitational force is weight
Gravitational force = Gravity/weight
Weight “towards”…
weight appears in pairs of forces
WSE WES WEM WMS
cause is equated with effectNewton answered to Galileo:
The true weight of the object in
water does not change
“. . . bodies placed in fluids have a two-fold gravity: the one true
and absolute, the other apparent, common, and comparative. . .
The common weight is nothing but the excess of the true weight
above the weight of the air”
Newton 1687: Book II, Proposition 20, Theorem 15, Cor. VI
Principle: knowledge guides
interpretation of measurement
Many modern textbooks today
adopted the approach of TRUE and
APPARENT weights of NewtonWeighing is gravitational g varies with latitude
(neglecting all the rest…)
force never correct!
gpole = 9.832 m/sec2
geq = 9.789 m/sec2
g = 0.043 m/sec2
g/g = 0.0044
Is that much?
e.g. 1 ton cargo from Stockholm
to Somalia “looses” about 4 kgthe Principle of Equivalence (1916)
Gravitational Inertial
force force
Hans Reichenbach
(1927)
Weighing by
an observer
Weighing cannot reliably inform about
the gravitational force applied on a body
Gravity (of a body) ≠ Gravitational Force (applied on the body)
The need to distinguish between the gravitational force
and weight as different conceptsThe radical change in
physics epistemology
Discovery of the Construction of the
natural order of reality picture of reality as
(observer independent) observer-determined
Understanding of gravity (weight)
Discovery versus knowledge constructionExample of the old physics approach:
National curriculum of Ireland, p. 13:
A feature of the work in both the middle and senior
classes is that pupils will investigate falling objects.
They will discover that objects fall because of the
force of gravity. They will measure force by constructing their own
spring balances. By the end of sixth class some children may recognize
that the amount of pull or force is measured in newtons.
There is no way to discover gravitational
force since it presents a conventional concept
– a theoretical tool invented by Newton.
16 George BerkeleyA summary of the conceptual history of weight
17th c Scientific 20th c. Scientific
Revolution Revolution
Heaviness Gravitation force Gravitational force
Gravity, weight, =Weight force
mass, pondus Weight force
Mass Mass
Newton 1687
Einstein 1916
Reichenbach 1927Newton’s rationalism
Absolute space, absolute time and unique observer
Gravity is explained by gravitation the law of
universal gravitation: weight is gravitation
Weighing results can be weight or not… (in falling)Einstein’s
operationalism
●
All observers are equally legitimate
Each observer builds physics knowledge based
on his measurement
Standard weighing provides weight for any observerIntroduction of different observers
T
H Fcf
Christian Huygens
O
Huygens inferred and proved:
P
Centrifugal force is identical in
action to gravitational one N
The implicit result:
Weight is determined by both forcesWeightlessness dialogue No weightlessness Yes, the weight is zero
“Weightlessness” Weightlessness
The Newtonian approach The Einsteinian
presumes the external approach draws on the
(informed) observer who local measurement by
explains weighing the observer
Newtonian definition Einsteinian definition
There is weight, The weight is zero,
weightlessness is regardless the
an illusion… gravitation“Newtonian” weightlessness
zero net gravitational force
WE WM
At all other locations the astronauts possessed Newtonian
weight towards the Earth and the Moon varying from point
to point
Students’ frequent misconceptionSpace medicine: Israel, Italy to launch joint
mission conducting zero gravity experiments
Israel and Italy are to jointly carry out a space mission to be launched at the end of March for
the purpose of carrying out medical experiments that have the potential to produce a
breakthrough. According to the report, the launching of the unmanned satellite is one of the
manifestations of closer ties that have been developing between the two countries. The
president of the Italian Space Agency, Giorgio Saccoccia, visited Israel on the occasion of
Space Week at the end of January.
February 3, 2020
Spacecraft (Shutterstock/NASA)The impact of the state of Weightlessness: 1. De-calcination and bones mass loss 2. Vestibular and vision problems 3. Blood vessels problems 4. Muscular atrophy 5. Metabolism (chemical reactions, drugs) 6. Functioning of microbes 7. Psychological impact The subject of intensive ongoing research
History informs us
1. There were multiple accounts of weight within different
theories.
2. The gravitational weight (Newton) suffers from
epistemological pitfall: weighing does not provide weight.
3. The operationally defined weight requires to distinguish
between weight/gravity and gravitation (interaction).
4. The modern physics (epistemology) requires coherent
operational and nominal definitionsWeight in Textbooks
Definitions of weight
Theoretical definition Operational definition -
- (17th C) (20th C)
Weight is the Weight is
gravitational force the result of
exerted on the body weighing
Measurement illustrates Measurement defines
(to be explained)
Weight is the force exerted
on the supportTextbooks Situation in the US
Gravitational definition Operational definition
Weight is
True and apparent the result of
weights weighing
True weight is Weight is the
Gravitational force exerted
force on the support
Gravitational force acts between masses
NASA term: microgravityA split in introductory physics teaching
Gravitational definition Operational definition
Taylor, L.W. (1941) Baruch, A. & Vizansky, A.(1937)
Sears, F. W. & Zemansky, M. W. Chaikin, S. E. (1947/1963)
(1982) Orear, J. (1967)
Young, H. D. & Freedman, R. A. Marion, J. B. & Hornyack, W. F. (1982)
(2008) French, A. P. (1971)
western world… Lerner, S. L. (1996)
Keller, F. J. et al. (1993)
Halliday, D., Resnick, R. (1988) Halliday, D., Resnick, R., & Walker, J.
(2001)
Walker, J. (2008)
Hewitt, P. G. (2002). Hewitt, P. G. (2012).
Knight, R. (2004) Knight, R. (2013)The choice to make by a curricular designer:
Fgr = W g*
The
Common in Common in international
many countries a few countries committee of
standards (SI)Gravitational approach
Weight is the gravitational
force exerted on the body
Fgr = W
W=mg
“The weight of a body is the total gravitational force
exerted on the body by all other bodies in the universe”
Sears & Zemansky, 1987; Young & Freedman, 2003; …….
This definition is useless (no way to measure,
calculate or apply to problem solving)!IIId stage: high school (AP level) (minority of students)
Weight
True weight Wt Apparent weight Wa
0
Fgr = Wt 1
2
3
Wa
The true weight is not defined operationally
The apparent weight is defined operationallyWeight operationally defined
ω
Fc.p.
R
W
Weight mg mω2 R
gω R
2Physics Teaching Research about weight
In light of the dichotomy
of weight definition
Subject to investigate:
How to teach weight in presence
of two different definitions?
(1) Are the two definitions – gravitational and
operational – equally appropriate theoretically?
(2) Are they equally effective pedagogically?Physics Education Research inforns:
1. Students and teachers very often confuse the meaning
of: gravity, weight, gravitation, weighing results
2. Being taught the gravitational definition, students often
develop alternative conceptions and accounts
3. Being taught the operational definition,
students show better conceptual knowledge
in various settingsScheme-facet structure of knowledge
Question 1
Facet of Knowledge
Students' Answers
Scheme
Question 2
Facet of Knowledge
Students' Answers Scheme
Facet of Knowledge
Question 3
Scheme
Students' Answers Facet of Knowledge
DATA
Addressing different Context dependent Context independent
situations conception conceptionWeight schemes with the affiliated facets of knowledge
Distance determines weight and weighing
(facets: weightlessness in a satellite, on the Moon, in
space, increase inside the Earth, infinite in the
center of the Earth)
Weightlessness (diminished weight) is caused by Vacuum
(facets: in a satellite, on the Moon, in space, under
glass dome in the laboratory)
Gravitation is nullified by buoyant force
(facets: weightlessness in a pool, weight loss in swimming)
Weighing registers the gravitational pull
(facets: weight of objects due to the Moon, Sun)Weight-weighing result
Linear historical presentation
Principle of
equivalence
Gravitational force
Centrifugal force is equal
to gravity
Centripetal push of the medium
Natural and accidental gravity
Measured heaviness
Natural inclinationDiscipline-Cultural perspective The Dialogue on Weight
Natural and Heaviness
accidental weight measured
Centripetal push
of medium Natural
inclination
One cannot
distinguish…
Centrifugal
force is like
gravity…
Measurement We need two
Force of determines concepts
gravitation1. Teaching the conceptual split between weight and
gravitation supported by historical narrative - effective
in pedagogical and cognitive perspectives
2. The new teaching of weight emphasizes the role of
measurement and the historical discourse of weight-
gravity understanding
3. The new teaching addresses a variety of physical
situations (a representative set, esp. falling) to apply
conceptual variation and comparison to establish
cultural content knowledgeResearch reports on students’ knowledge Children's schemes, Galili & Bar, 1997, IJSE Students’ scheme-facets, Galili & Kaplan, 1996, Sc. Ed. Operational teaching in 7th grade, Stein, Galili & Schur, 2015, JRST, Operational teaching in 9th grade, Galili, Bar, & Brosh, 2017, Sc & Ed
Conclusion • Weight concept presents a paradigmatic case of the key importance of the epistemology in physics • The cultural knowledge of weight and its synergic teaching with tides possess various advantages. • The case of weight illustrates the complex role of physics education as an academic discipline.
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