Using class time three days are recommended for

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Using Class Time Three days are recommended for Chapter 8. The first day focuses on identifying action/reaction force pairs and on propulsion; the second day on tension, acceleration, and comparing forces; and the third day on extended examples of problem solving. DAY 1: Most students readily accept that if A pushes/pulls B, then B pushes/pulls back on A. If you ask a student to stretch a spring, she can “feel” that the spring pulls on her hand at the same time she pulls on the spring. Long-range forces are more troublesome because students don’t yet understand the role of mass in the “outcome” of an interaction. The earth clearly pulls down on a ball that is dropped, but there’s little evidence of the ball exerting a force on the earth. You can make this idea plausible by demonstrations with magnets. If a student holds two fairly strong magnets, he can feel that each is pulling (or pushing) on the other. If you attach magnets in the repulsive orientation to two gliders on an air track, both gliders move apart when released. More importantly, if you now
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8-6 Instructor’s Guide weight the gliders differently, students can see that there is a mass effect and that the lighter glider does most of the motion. Now it’s a much smaller step to accepting that the ball really does exert a reaction force on the earth. With this as a starting point, it’s good to spend an entire class asking students to follow the steps in Tactics Box 8.1 for identifying and labeling action/reaction pairs and for drawing free-body diagrams. As trivial as it seems, it’s worth starting with a block sitting at rest on the floor (ask them to consider the floor to be part of the earth) and have students draw a picture showing separately the block, the surface, and the earth as a whole. Although somewhat artificial, it’s important to distinguish “the surface” where contact forces occur from “the earth” that exerts the gravitational force. Have them draw all force vectors, label them as F A on B , and connect all action/reaction pairs with dotted lines. Even this simple situation will rapidly lead to conflict for many students who want w and n to be an action/reaction pair. Then place a second block on top of the first. The lower block now experiences two normal forces, one from above and one from below. Again, this apparently trivial situation is initially difficult for many students. Fortunately, most will “get it” after just a few such examples. The text uses Example 8.1 (a person pushing a crate across a floor with friction) to introduce the important idea of propulsion. The different roles of friction for the person and for the crate are sufficiently confusing for many students that it’s worth working through this example in class. As noted above, many students find propulsion to be a difficult idea. You can convey the idea by asking them to imagine what would happen on a frictionless surface. Also ask them to imagine what direction loose gravel would be “kicked” as they sprint forward or a car accelerates forward. A related example is to consider a box in the back of a truck as the truck accelerates forward. Here
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