Instructors_Guide_Ch08 - 8 Newtons Third Law Recommended...

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8-1 8 Newton’s Third Law Recommended class days: 3 Background Information Newton’s third law is profound, much more so than the first two. For students, the third law is a subtle and difficult topic, and they will find this to be a challenging chapter. However, if you spend the recommended three full days and provide students with ample opportunities for practice and feedback, most will end up finding this a rewarding chapter where the concepts of force and motion suddenly begin to “make sense.” Suppose a large truck and a compact car have a head-on collision. During the collision, is the force of the truck on the car larger, smaller, or equal to the force of the car on the truck? This question, and several similar questions on the Force Concept Inventory, is initially missed by 70 to 80% of students in a typical calculus-based physics courses. Conventional instruction makes little improvement, with 60% still missing these questions on the posttest. This is despite the fact that a large majority can “recite” Newton’s third law when they enter the class. Halloun and Hestenes (1985b) have characterized student beliefs about interactions in terms of a dominance principle : the larger (or faster or more active) object exerts a larger force than the smaller (or slower or less active) object. Students tend to view an interaction as a “conflict” in which the stronger wins. It’s not hard to understand how this common-sense view comes about. After all, the effect of the collision on the compact car is much larger than its effect on the truck. Different effects would seem to require different causes , hence different amounts of force. The difference in the masses does not appear to students as a significant factor in drawing conclusions about forces. This basic misconception about interaction forces is likely the most persistent and hard to change of all the student misconceptions in mechanics. Some of the more specific difficulties students have with Newton’s third law and with interacting systems are: • Students don’t believe Newton’s third law. It’s too contrary to common sense. • Students have difficulty identifying action/reaction force pairs: They match two forces on the same object. They place forces on the wrong objects. They don’t believe that long-range forces (e.g., gravity) have reaction forces. • Students confuse equal force with equal acceleration. • Students don’t understand tension: They think that tension is the sum of the forces exerted at the two ends of a string. They think that tension exerts a force only in the direction of motion. They think that tension can pass through an object to another string on the other side. • Students often don’t recognize that objects connected by an inextensible string must have accelerations of equal magnitude.
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8-2 Instructor’s Guide Although the practice is thankfully declining, there are widely-used textbooks with figures such as the one shown here. Students cannot tell from the figure that forces ± n and ± n
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Instructors_Guide_Ch08 - 8 Newtons Third Law Recommended...

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