Instructors_Guide_ch05 - 5 Dynamics I, Motion Along a Line...

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5 Dynamics I, Motion Along a Line Recommended class days: 3 Background Information Chapter 5 is a continuation of Chapter 4, with the focus now shifting to quantitative problem solving. Learning to recognize and identify forces carefully and accurately is one of the largest hurdles students face. If they’ve begun to get a good handle on this from Chapter 4, then the present chapter becomes more a matter of consolidating and using that information. Chapter 4 introduced free-body diagrams, but likely you didn’t get too much opportunity to have students practice drawing free-body diagrams in class. That needs to be an important focus of the present chapter. Student difficulties with free-body diagrams include: • Identifying a force correctly, but not knowing which way it points. • Including forces exerted by the object, not just forces exerted on the object. • Placing the tips, rather than the tails, of the vectors at the origin, which makes it hard to determine the x - and y -components of the forces. • Including “ ma r ” as a separate force. After all, Newton’s second law says . F ma = r r This error is likely a continued belief in the need for some kind of “force of motion.” • Not using a coordinate system, or using an inappropriate one. • Making errors in determining the component of the force vectors. Fortunately, these are mostly technical errors rather than conceptual errors. Most can be overcome fairly quickly with focused practice and feedback on drawing free-body diagrams. A conceptual issue does arise with mass, weight, and apparent weight. Daily life makes no distinction between mass and weight, so students are accustomed to using the terms interchangeably. They need an opportunity to practice using the distinctions. The author has found the concept of apparent weight to be pedagogically useful. Students associate weight with what a scale reads. They begin to exhibit confusion and uncertainty if you direct their attention to what a scale would read in a rocket taking off, in a roller coaster going over the top, or in an elevator in free fall after the cable breaks. Some will assert that weight changes in these circumstances, but they’re not sure how to reconcile that with weight as a fixed and unchangeable force of the earth’s gravity on the object. It takes quite a bit of discussion for most to accept that they cannot sense the long-range weight force itself, that the sensation they call weight is actually the sensation of the normal force of a surface pressing against them. The magnitude n of the normal force is then defined to be the apparent weight . Its value depends on the acceleration, and students can then recognize that their apparent weight will be more or less than their true weight in an elevator accelerating up or down.
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Instructors_Guide_ch05 - 5 Dynamics I, Motion Along a Line...

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