Demystifying the Mystery of Action-Reaction Forces

HS SCIENCE CONCEPT PRESENTATIONJuly 2010

Bianca Mercuri

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

COURSE: SPH 3U UNIT: 2 – Forces KEY UNDERSTANDINGS:

Conceptual and mathematical problem-solving

Newton’s 3rd law is one of the course’s the most difficult concepts for students to understand!!!

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?

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;

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 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

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

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

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

Two Main Categories of Difficulty:1)Conceptual Understanding – Thinking

differently i. Free-body diagramsii. Concept questions (question their

“beliefs”)

2)Mathematical Problem-Solvingi. Scaffoldingii. Practice, practice, practice!

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!

1) Draw FBD of various action reaction pairs2) Concept questions on action-reaction pairs3) Correct definition

1) Drawing FBD of action-reaction pairs• requires not one FBD, but TWO!

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.

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.

3) Correct Definition

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?”

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?”

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

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.

Scaffolding via chalk & talk Scaffolding reference/worksheets Variety of questions Practice, practice, practice!

Ok, now you try!

1) Pay attention & Warning!

1) Definitions and practical uses (student brainstorm)

1) Demos (in-class & video clips)

1) Scaffold base problem as class (Socratic)

1) Small group/individual problem solving using scaffold

1) Assessment of one problem prior to leaving

1) HOMEWORK: Concept questions, base problems

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

1) HOMEWORK: More complex problems

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

“AS” & “FOR” Learning1)Question period, walk around, Socratic

method2)G.R.A.S.S. assessment of one problem prior

to leaving HS Science/Concept Presentation/G.R.A.S.S. Assessment.doc

“OF” Learning1)Concept and/or problem-solving quiz2)Action-Reaction lab

Physical demos

Computer equipment

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/html/t0205e1.html

Labs/Activities: Using action-reaction carts Using probe-ware

Handouts: Scaffolding worksheet Concept questions Math problem-set G.R.A.S.S. assessment template