NEWTON’S THIRD LAW

1. Demystifying the Mystery of Action-Reaction Forces NEWTON’S THIRD LAW HS SCIENCE CONCEPT PRESENTATION July 2010 Bianca Mercuri

2. Background Information • Overview of the Law • Ministry Expectations • Practical Applications • Curriculum Sequence • Learning Difficulties • Lesson Sequence • Assessment & Evaluation Suggestions • Safety Considerations • Resources Presentation outline

4. Wouldn’t it be nice if this clip said it all?

5. COURSE: SPH 3U UNIT: 2 – Forces KEY UNDERSTANDINGS: Conceptual and mathematical problem-solving Background information Newton’s 3rd law is one of the course’s the most difficult concepts for students to understand!!!

6. Can be taught in two main ways: For every action there is an equal but opposite reaction (FA = -FB) 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. Newton’s 3rd law Which one do you think may cause misunderstanding among our students? Why?

7. 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

8. Specific Expectations 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 C2.2 conduct an inquiry that applies Newton’s lawsto 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 C3.2 explain how the theories and discoveries of Galileo and Newton advanced knowledge of the effects of forces on the motion of objects C3.3 state Newton’s laws, and apply them, in qualitative terms, to explain the effect of forces acting on objects Ministry expectations

9. Virtually all motion is dependent on Newton’s 3rd law 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 Practical applications

10. Curriculum sequence Newton’s 3rd law is the quintessential culminating topic for the first two units, most importantly the second unit on dynamics

11. Two Main Categories of Difficulty: Conceptual Understanding – Thinking differently Free-body diagrams Concept questions (question their “beliefs”) Mathematical Problem-Solving Scaffolding Practice, practice, practice! Learning difficulties

12. 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 Concept questions on action-reaction pairs Correct definition Learning difficulty #1

13. Drawing FBD of action-reaction pairs • requires not one FBD, but TWO! Solutions to Learning difficulty #1

14. Concept questions on action-reaction pairs SOLUTIONS TO Learning difficulty #1 • Which of the following force pairs is/are not an action-reaction pair? • 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 • 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.

15. For every action there is an equal but opposite reaction (FA = -FB) or Solutions to learning difficulty #1 Correct Definition 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.

16. 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 #2

17. 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 #3

18. 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 Learning difficulty #4

19. Solution to learning difficulty #4

20. 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. Learning difficulty #5

21. Scaffolding via chalk & talk • Scaffolding reference/worksheets • Variety of questions • Practice, practice, practice! Solution to learning difficulty #5 Ok, now you try!

22. Pay attention & Warning! Definitions and practical uses (student brainstorm) Demos (in-class & video clips) Scaffold base problem as class (Socratic) Small group/individual problem solving using scaffold Assessment of one problem prior to leaving HOMEWORK: Concept questions, base problems Lesson 1 – CONCEPT & BASE PROBLEM

23. Math and concept issues (student driven) More demos to help concept understanding? Scaffold more complex problems with gravity and friction (Socratic) Small group/individual problem-solving of more complex problems using scaffold Assessment of one problem prior to leaving HOMEWORK: More complex problems Lesson 2 – issues, gravity & friction

24. Math and concept issues (student driven) • 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 Lesson 3 – Issues & lab

25. “AS” & “FOR” Learning Question period, walk around, Socratic method 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 Concept and/or problem-solving quiz Action-Reaction lab Assessment & evaluation strategies

26. Physical demos Computer equipment Safety considerations

27. 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) Resources & MATERIALS

28. http://esamultimedia.esa.int/docs/issedukit/en/html/t0205e1.htmlhttp://esamultimedia.esa.int/docs/issedukit/en/html/t0205e1.html

29. Labs/Activities: • Using action-reaction carts • Using probe-ware Handouts: • Scaffolding worksheet • Concept questions • Math problem-set • G.R.A.S.S. assessment template Resources & MATERIALS