“Background Information” text from the Teacher’s Edition

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It is recommended that you read the section Physics
Talk: Newton’s Third Law of Motion in the student
text for this activity before proceeding in this
section.
The explanation of forces involved in walking given
in the teacher’s Background Information for
Activity 4 will serve to explain the forces involved
with walking and running brought up in this
activity. You may wish to review the Background
Information for Activity 4 before proceeding.
The pairs of equal and opposite forces identified
during earlier activities to explain friction andwalking are examples of Newton’s Third Law ofMotion, often stated as: “For every action there is
an equal and opposite reaction.” Another equal andopposite pair of forces arises during this activitywhen a student standing on a skateboard setshimself into motion by using a leg and foot to pushoff from the wall.
Inevitably, forces exist in equal and opposite pairs, and often the force which we identify as the forceresponsible for motion is not the correct one.
For example, a person who says, “I pushed down on the
trampoline with a mighty force, and my forcelaunched me upward in a high jump,” is mistaken; it was the equal and opposite reaction force
provided by the trampoline that launched the person upward.
Active-ating
the Physics InfoMall
In addition to looking for information on Newton’s
Third Law (look at problem 4.14 in Schaum’s 3000
Solved Problems in Physics, in the Problems Place),
perform a search using “force diagrams” as the
keywords, and the first hit is a great one! It is,
again, from Arons’ A Guide to Introductory Physics
Teaching: Elementary Dynamics, Chapter 3. Section
3.12 is on Newton’s Third Law and Free Body
Diagrams. Arons mentions common problems and
suggests solutions, including suggestions of what
not to do.
Arons also notes that “Students do not really begin
to understand the concept of force until they


 


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become able to apply the third law correctly and
draw proper, isolated force diagrams of interacting
objects,” in his article “Thinking, reasoning, and
understanding in introductory physics courses,” in
The Physics Teacher, vol. 19, issue 3, 1981. Check
out this article.
This same search produces the warning that
“Introductory textbooks are liberally decorated with
diagrams, but they fail to convey to students the
essential role of diagrams in problem solving or,
indeed, to distinguish the roles of different kinds of
diagrams” from “Toward a modeling theory of
physics instruction,” in the American Journal of
Physics, vol. 55, issue 5, 1987. It is clear that the
practice and ability to draw force diagrams are
important.
Stretching Exercise: Add the word “elevator” to the
search above (so now it is “force diagrams” AND
“elevator*”) for some nice discussions related to the
Stretching Exercise.