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Instructors_Guide_ch04

# Instructors_Guide_ch04 - 4 Force and Motion Recommended...

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4 Force and Motion Recommended class days: 2 When asked to draw a force diagram for some simple situation, most students emerging from any level of introductory physics course are likely to draw objects which look like a porcupine shot by an Indian hunting party—the number and direction of pointed entities being essentially stochastic. Arnold Arons (1979) Background Information The chapter on Physics Education Research has already provided much of the background information on student difficulties with force and motion. The major reference is Halloun and Hestenes (1985b). Other reference are cited in Reddish (1994). This section will summarize the earlier information. There are two basic issues, both of which cause serious difficulties for students: What is a force? What is the connection between force and motion? What is a force? Students don’t have a clear idea of just what a force is. They tend not to distinguish between force, inertia, energy, power, or even velocity, often using these terms interchangeably. In addition: Some students believe that only animate objects can exert forces. They don’t believe that a table exerts a upward force on an object; the table simple “gets in the way of the object wanting to fall.” Forces recognized by physicists are often seen by students as simply influences on an object’s motion, not as forces. Thus friction is not a force but merely “what makes it stop.” Gravity is not a force but simply “what makes it fall.” A majority of students believe in an impetus theory of motion . In throwing a ball, the hand imparts a “force of the throw” to the ball. This is a property of the ball (an inherent force ) and travels with the ball “to keep it moving.” Typically 75% or more of students beginning calculus-based physics think that a ball tossed upward has an upward “force of the throw” or “force of your hand” on it after it leaves your hand. Students tend to view forces from the perspective of the applier of force rather than from the perspective of the object experiencing force. That is, they recognize the pushes and pulls they must apply to move an object, but they don’t recognize that the object may experience additional inanimate forces of friction, gravity, and so on. This is one of the major reasons they think that motion requires a force. How are force and motion connected? The prevailing student belief is that motion requires a force . This belief is based on much common-sense evidence, and it is a belief that is highly resistant to change. More specifically, the “student version” of the laws of motion is: 4-1

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4-2 Instructor’s Guide If there’s no force on an object, the object is at rest or will immediately come to rest.
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Instructors_Guide_ch04 - 4 Force and Motion Recommended...

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