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NEWTON’S THIRD LAW
Demystifying the Mystery of Action-Reaction Forces
HS SCIENCE CONCEPT PRESENTATION
July 2010
Bianca Mercuri
PRESENTATION OUTLINE
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Background Information
Overview of the Law
Ministry Expectations
Practical Applications
Curriculum Sequence
Learning Difficulties
Lesson Sequence
Assessment & Evaluation Suggestions
Safety Considerations
Resources
WOULDN’T IT BE NICE IF THIS CLIP SAID IT ALL?
BACKGROUND INFORMATION
COURSE: SPH 3U
 UNIT: 2 – Forces
 KEY UNDERSTANDINGS: Conceptual and
mathematical problem-solving
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Newton’s 3rd law is one of the
course’s the most difficult concepts
for students to understand!!!
NEWTON’S 3RD LAW
Can be taught in two main ways:
1.
For every action there is an equal but opposite
reaction (FA = -FB)
1.
Whenever one object exerts a force on a second
object, the second object exerts a force that is
equal in magnitude and opposite in direction back
onto the first object.
Which one do you think may cause misunderstanding
among our students? Why?
MINISTRY EXPECTATIONS
Big Ideas
 Forces - Forces can change the motion of an object.
Applications of Newton’s laws of motion have led to
technological developments that affect society and the
environment.
Overall Expectations
 C1. analyze and propose improvements to technologies that
apply concepts related to dynamics and Newton’s laws, and
assess the technologies’ social and environmental impact;
 C2. investigate, in qualitative and quantitative terms, net
force, acceleration, and mass, and solve related problems;
MINISTRY EXPECTATIONS
Specific Expectations
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C2.1 use appropriate terminology related to forces, including, but not limited
to: mass, time, speed, velocity, acceleration, friction, gravity, normal force,
and free-body diagrams
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C2.2 conduct an inquiry that applies Newton’s laws to analyse, in qualitative
and quantitative terms, the forces acting on an object, and use free-body
diagrams to determine the net force and the acceleration of the object
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C3.2 explain how the theories and discoveries of Galileo and Newton
advanced knowledge of the effects of forces on the motion of objects
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C3.3 state Newton’s laws, and apply them, in qualitative terms, to explain
the effect of forces acting on objects
PRACTICAL APPLICATIONS
Virtually all motion is dependent on Newton’s 3rd
law
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Jumping, walking, running
Driving, riding a bike
Swimming, canoeing
A bird or helicopter flying
A jet engine or rocket
http://dsc.discovery.com/videos/time-warp-water-rocket.html
CURRICULUM SEQUENCE
Newton’s 3rd law is the quintessential culminating topic for
the first two units, most importantly the second unit on dynamics
LEARNING DIFFICULTIES
Two Main Categories of Difficulty:
1) Conceptual Understanding – Thinking
differently
i. Free-body diagrams
ii. Concept questions (question their “beliefs”)
2)
i.
ii.
Mathematical Problem-Solving
Scaffolding
Practice, practice, practice!
LEARNING DIFFICULTY #1
MISUNDERSTANDING: “Action-reaction forces act on
the same body, so why does anything ever move?
TRUTH: They actually act on different bodies.
SOLUTIONS: Free-body, free-body, free-body!
Draw FBD of various action reaction pairs
2) Concept questions on action-reaction pairs
3) Correct definition
1)
SOLUTIONS TO LEARNING DIFFICULTY #1
1)
Drawing FBD of action-reaction pairs
• requires not one FBD, but TWO!
SOLUTIONS TO LEARNING DIFFICULTY #1
2)
Concept questions on action-reaction pairs
Which of the following force pairs is/are not an action-reaction pair?
a)the force you exert on the earth as you stand in your house and the
force that the floor of the house exerts on you
b) the force a bat exerts on a baseball and the force the baseball exerts
on the bat
c) the force a finger exerts on a button and the force the button exerts on
the finger
d)None of the above; a, b, and c are action-reaction pairs.
e) b and c contain force pairs that are NOT equal to each other even
tough they are opposite in direction.
SOLUTIONS TO LEARNING DIFFICULTY #1
3)
Correct Definition
For every action there is an equal but opposite
reaction (FA = -FB)
or
Whenever one object exerts a force on a
second object, the second object exerts a
force that is equal in magnitude and
opposite in direction back onto the first
object.
LEARNING DIFFICULTY #2
MISUNDERSTANDING: “The action force comes first
and is separate from the reaction force. Each
force can be clearly defined.”
TRUTH: Both forces are the action and the reaction
as they occur at the same time.
SOLUTION: Question their “beliefs”?
 Use in-class demos, video clips and real life
situations and ask “Which came first?”
LEARNING DIFFICULTY #3
MISUNDERSTANDING: “Only animate objects exert
forces; inanimate objects (i.e. tables, floors) don’t”
TRUTH: All objects exert force, no matter the nature.
SOLUTION: Question their “beliefs”?
 “How do you stay on your chair?”, “Do you feel the
wind?”, “Do you feel the book on top of your
head?”
LEARNING DIFFICULTY #4
MISUNDERSTANDING: “Larger objects exert more
force than a greater object”. (i.e. the Earth exerts
a greater force on you than you do on it)
TRUTH: Both objects exert the same amount of
force. The effect of the force is different on each
object due to Newton’s 2nd law.
SOLUTION: Vernier Probe-ware Demonstration
 Probe-ware demo shows evidence of truth
SOLUTION TO LEARNING DIFFICULTY #4
LEARNING DIFFICULTY #5
MISUNDERSTANDING: Mathematical solving of
action-reaction problems.
TRUTH: It’s tricky and it takes a lot of practice.
SOLUTION: Scaffolding
 Start with high level of scaffolding for various
problem types and slowly reduce with time and
practice.
SOLUTION TO LEARNING DIFFICULTY #5
Scaffolding via chalk & talk
 Scaffolding reference/worksheets
 Variety of questions
 Practice, practice, practice!
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Ok, now you try!
LESSON 1 – CONCEPT & BASE PROBLEM
1)
Pay attention & Warning!
1)
Definitions and practical uses (student brainstorm)
1)
Demos (in-class & video clips)
1)
Scaffold base problem as class (Socratic)
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Small group/individual problem solving using scaffold
1)
Assessment of one problem prior to leaving
1)
HOMEWORK: Concept questions, base problems
LESSON 2 – ISSUES, GRAVITY & FRICTION
1)
Math and concept issues (student driven)
1)
More demos to help concept understanding?
1)
Scaffold more complex problems with gravity and friction (Socratic)
1)
Small group/individual problem-solving of more complex problems
using scaffold
1)
Assessment of one problem prior to leaving
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HOMEWORK: More complex problems
LESSON 3 – ISSUES & LAB
1)
Math and concept issues (student driven)
1)
Action-Reaction lab
Action-reaction carts or Vernier probe-ware
Possible A&E through an informal lab report
with PR, AI, C as well as problem
solving/calculations
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ASSESSMENT & EVALUATION STRATEGIES
“AS” & “FOR” Learning
1) Question period, walk around, Socratic method
2) G.R.A.S.S. assessment of one problem prior to
leaving
HS Science/Concept Presentation/G.R.A.S.S. Assessment.doc
“OF” Learning
1) Concept and/or problem-solving quiz
2) Action-Reaction lab
SAFETY CONSIDERATIONS
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Physical demos
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Computer equipment
RESOURCES & MATERIALS
Physical/In-Class Demos:
 Action-reaction carts
 Scooter-boards and medicine ball
 Fan-powered vehicle
Technology:
 Video clips
 Probe-ware
 Simulations (gizmos, esamultimedia.esa.int)
http://esamultimedia.esa
.int/docs/issedukit/en/ht
ml/t0205e1.html
RESOURCES & MATERIALS
Labs/Activities:
 Using action-reaction carts
 Using probe-ware
Handouts:
 Scaffolding worksheet
 Concept questions
 Math problem-set
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G.R.A.S.S. assessment template