Instructors_Guide_Ch08

# To identify actionreaction pairs of forces to

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To identify action/reaction pairs of forces. To understand and use Newton’s third law. To understand how to use propulsion forces and tension forces. Pedagogical Approach Chapter 4 began the process of dividing the objects of a problem into the system and the environment . That process now becomes even more important with the need to identify interacting systems , perhaps several. Research has shown that student difficulties with interacting systems often stem from very basic failures to identify the relevant systems or the interaction forces between them. Consequently, the starting point for this chapter is a very basic, but systematic, approach to identifying action/reaction force pairs. The initial strategy asks students to Draw a picture with every object shown as a separate system. Identify, as best they can, every force acting on every object. Express each force in the form F A on B . Identify action/reaction pairs by connecting F A on B to F B on A with a dotted line. If they find an unmatched force F C on D . then they missed a force during the identification stage. But they can find it by reversing the words: F D on C . Verify that every force is paired with one other force, that the forces of a pair act on two different objects, and that the two vectors point in opposite directions. Most students find this surprisingly hard to do until you and they together work through several examples. The initial strategy does not yet incorporate the full Newton’s third law because it says nothing about magnitudes. Other than for long-range forces, students find it reasonable to believe that “If A pushes on B, then B pushes back on A.” Once students can do this systematically, adding additional information about the magnitudes of the forces is straightforward. The verbalization “force of object A on object B” is very important. It compels students to recognize that forces are interactions, not independently existing entities. This verbalization also helps remove remaining misconceptions about what is or is not a force. The initial strategy, illustrated in textbook Examples 8.1 and 8.2, is complete but obviously is cumbersome. After a few practice examples, students readily recognize that only some of the objects in the problem are of interest. These can be identified as “the systems.” Others—the earth, the table, and so on—can be banished to “the environment” from where they exert “external forces” on the systems. Students can now recognize that there are no true external forces, since all forces

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8-4 Instructor’s Guide are interactions, but that it is safe to treat some forces as if they are external forces. A revised strategy is then Separate the objects into “systems of interest” and “the environment.” Draw each system separated from all other systems and from the environment. This is a critical step.
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