INVESTIGATING STUDENT DIFFICULTIES WITH CONCEPTS OF GRAVITATION

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INVESTIGATING STUDENT DIFFICULTIES WITH CONCEPTS
OF GRAVITATION

• Or...

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...DO YOU UNDERSTAND THE GRAVITY OF THIS
SITUATION?

• Jack Dostal and David Meltzer
• Iowa State University

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OUTLINE I. The Physics Education Research Group
at ISU (ISUPERG) II. Why Gravitation? A. Some
attention at grade school level B. Minimal
treatment at HS-college level (aside from
Newtons laws) C. Middle ground between physics
and astronomy D. Applicable to entry-level
astronomy courses III. Pretesting Student
preconceptions/misconceptions A. The
MGDT What is it? Examples (of
questions) Results B. The DGDT What is
it? Examples (of questions) Results OUTLINE
(continued) IV. New Curriculum A. How can it
be used? B. Potential partners HCC --
Introductory Astronomy (gen. ed.) UNI --
Astronomy (gen. ed.) ISU -- Physics 111-112 V.
Post-testing and results (which we dont
have) VI. Student Interviews (which we dont
have) VII. Ultimate goal of this research
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ABSTRACT Teaching the concept of gravity and the
universal nature of gravitation in any physics or
astronomy classroom can be difficult. Unlike
subjects such as magnetism or optics for example,
gravitation is difficult (or impossible, on
astronomical scales) to demonstrate in a student
laboratory. We have developed diagnostic
instruments to identify students' initial
concepts of gravitation, and have administered
them in community-college and university
settings. We are also developing materials to
enable students to recognize incomplete and
incompatible concepts, and to strengthen their
understanding of the nature of gravitation. The
target audience for these materials is the
typical population of introductory astronomy and
physics courses both science and non-science
majors. We will report diagnostic data, as well
as preliminary data from class testing of the
curricular materials. Talk follows
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• Why Gravitation?
• Lack of research on students concepts of gravity
• Similar in nature to Coulombs Law
• Subject of interest in both physics and astronomy

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• Previous Research
• Not much! But here are a couple of examples
• Nussbaum, late 1970s - early 1980s (Israel)
• Emphasis Childrens concepts of the earth
• What does Earth look like? Which way is down?
• Instructional method Audio-tape lessons
• Gunstone and White, 1981 (Australia)
• Emphasis First-year college students
  understanding of gravity
• Students were presented a physical situation and
  asked to predict what would happen if a certain
  action was taken.
• Students made observations and were asked to
  reconcile any discrepancies between their
  prediction and observation.

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• Our Goals
• To investigate students thoughts about
  gravitation
• To develop instructional materials which enable
  students to learn about gravitation more
  effectively
• To quantitatively evaluate the effectiveness of
  the materials produced

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• Pretesting What are the students thinking?
• We have used two main diagnostic instruments
• Multiple choice questions
• 11-question pretest addressing many different
  aspects of gravitation (inverse square law,
  Newtons 3rd Law, more)
• Used at Southeastern Louisiana University, Iowa
  State University, and Hawkeye Community College
• Free-Response questions
• 10 question pretest, all free-response and
  drawing
• Used at Hawkeye Community College will use at
  other schools in the near future

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Pretesting What are the students
thinking? Class profiles SE Louisiana Second-se
mester algebra-based physics students (N gt
100) Multiple choice questions given on the first
day of class Iowa State Second-semester
algebra-based physics students (N 79) Multiple
choice questions given on the first day of
class Hawkeye CC General education astronomy
class (N 21) Several weeks into course
(Newtons Laws and Universal Law of Gravitation
mentioned, but not thoroughly covered)
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Selected results from multiple choice
questions Multiple choice Q1 The mass of the
sun is about 3 x 105 times the mass of the earth.
How does the magnitude of the gravitational
force exerted by the sun on the earth compare
with the magnitude of the gravitational force
exerted by the earth on the sun? The force
exerted by the sun on the earth is A. About 9 x
1010 times larger B. About 3 x 105 times
larger C. Exactly the same D. About 3 x 105
times smaller E. About 9 x 1010 times smaller
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Selected results from multiple choice
questions Multiple choice Q1 The mass of the
sun is about 3 x 105 times of the mass of the
earth. How does the magnitude of the
gravitational force exerted by the sun on the
earth compare with the magnitude of the
gravitational force exerted by the earth on the
sun? The force exerted by the sun on the earth
is A. About 9 x 1010 times larger B. About 3 x
105 times larger C. Exactly the same D. About 3
x 105 times smaller E. About 9 x 1010 times
smaller
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• Selected results from multiple choice questions
• Multiple choice 1
• Student concepts
• The GREATER the mass of an object, the GREATER
  its force on another object.
• Which is great! But
• When one object is LARGER than another, it exerts
  MORE force on the smaller object than the smaller
  object exerts on the larger object.

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Selected results from multiple choice
questions Multiple Choice Q1 SE Louisiana less
than 10 correct Iowa State 14 correct Hawkeye
CC 19 correct Students in various
settings/backgrounds/courses have the same
misconception
Student thinking Objects of unequal masses
exert unequal forces on one another!
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Selected results from multiple choice
questions Multiple choice Q8 Which of the
following diagrams most closely represents the
gravitational forces that the earth and the moon
exert on each other? (Note The mass of the
earth is about 80 times that of the moon)
(A) (C) (E)
(B) (D) (F)
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Selected results from multiple choice
questions Multiple choice Q8 Hawkeye CC results
and Iowa State results
(A) 76, 38 (C) 14, 6 (E) 10, 1
(B) 0, 44 (D) 0, 0 (F) 0, 0
Student thinking Objects of unequal masses
exert unequal forces on one another!
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Selected results from free-response questions
Earth
asteroid
Free Response Q2 Refer to the picture above.
State whether the magnitude of the force exerted
by the Earth on the asteroid is larger than,
smaller than, or the same as the magnitude of the
force exerted by the asteroid on the Earth.
Explain the reasoning for your choice.
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Selected results from free-response questions
Earth
asteroid
Free Response Q2 Refer to the picture above.
State whether the magnitude of the force exerted
by the Earth on the asteroid is larger than,
smaller than, or the same as the magnitude of the
force exerted by the asteroid on the Earth.
Explain the reasoning for your choice.
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Selected results from free-response questions
Same as 10 Smaller than 5 Larger than 86
76 of all students explicitly reasoned that
because the Earth was larger or more massive, it
exerted a greater force on the asteroid.
LARGER objects exert MORE force on SMALLER
objects...
...than SMALLER objects exert on LARGER objects.
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Selected results from free-response questions
Free Response Q8 Imagine that you are standing
on the surface of the moon holding a pen in one
hand. A) If you let go of the pen, what happens
to the pen? Why? B) After you let go of the
pen, what happens to you? Why?
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Free Response Q8 Imagine that you are standing
on the surface of the moon holding a pen in one
hand. A) If you let go of the pen, what happens
to the pen? Why?
45 31 24
DROPS TO THE SURFACE
FLOATS
FLOATS AWAY
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Free Response Q8 Imagine that you are standing
on the surface of the moon holding a pen in one
hand. A) If you let go of the pen, what happens
to the pen? Why?

• DROPS TO THE SURFACE
• Most cite the moons lesser gravity
• Like the astronauts

• FLOATS or FLOATS AWAY
• Low gravity
• Not enough gravity
• No gravity
• Weightlessness

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Free Response Q8 Imagine that you are standing
on the surface of the moon holding a pen in one
hand. B) If you let go of the pen, what happens
to you? Why?
71 5 5 19
YOU STAY ON THE SURFACE
YOU FLOAT
YOU FLOAT AWAY
OTHER
Students do not automatically consider
gravitation to be universal!
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Selected results from free-response questions
Earth
Free Response Q1 In the picture above, a
person is standing on the Earth holding a ball in
one hand. Draw the direction of the force
exerted by the Earth on the ball in the picture
above.
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Earth
Free Response Q1 Correct (downward) Incorrec
t (upward) Incorrect (balanced)
Do students really think that the ball will fall
up? No. This appears to be a confusion of what
is meant by a force exerted by the Earth on the
ball.
67 29 5
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Selected results from free-response questions

• Free Response Q7(A)
• Two large masses M1 and M2 are in space as shown.
• Where (if anywhere) can you place a third mass M3
  to increase the total gravitational pull on M2?
  (This would increase the push or pull experienced
  by M2.)
• Draw M3 in that position on the picture above and
  explain your reasoning for choosing that position.

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Free Response Q7(A) Correct answers 67 Correc
t answers with correct reasoning (superposition
of forces) 14
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• Free Response Q7(A)
• Closeness M3 is closer to M2 than M1, so there
  is more force on M2

• Tension response M2 must now pull both M1
  and M3, so there more force on M2

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Selected results from free-response questions

• Free Response Q7(B)
• Two large masses M1 and M2 are in space as shown.
  
• Where (if anywhere) can you place a third mass M3
  to decrease the total gravitational pull on M2?
  (This would decrease the push or pull experienced
  by M2.)
• Draw M3 in that position on the picture above and
  explain your reasoning for choosing that position.

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Selected results from free-response questions
Free Response Q7(B) Correct
answers 38 Correct answers with correct
reasoning (superposition/cancellation of
forces) 29
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• Free Response Q7(B) -- Reasoning for some
  incorrect answers
• Force Distribution I M1s force is used up
  on M3, so M1 pulls on M2 with less force.

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• Free Response Q7(B) -- Reasoning for some
  incorrect answers
• Force Distribution II M2 has to pull two
  objects, so the pull is less.

M1s force on M2 depends in some way on M3!
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• Whats next?
• Create workbook-style instructional materials to
  teach gravitation
• Field-test instructional materials in both
  physics and astronomy classroom settings
• Evaluate the effectiveness of these materials
• Further investigate student understanding through
  interviews and additional diagnostics