Instructors_Guide_Ch09

# Instructors_Guide_Ch09 - 9 Impulse and Momentum Recommended...

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9 Impulse and Momentum Recommended class days: 2 minimum Background Information To the experienced physicist, conservation laws seem the “obvious” way to tackle many problems in physics. Yet students find conservation laws to be rather mysterious, and they are reluctant to use a conservation law unless explicitly asked to do so. Most instructors find that a majority of students will elect to use Newton’s laws to solve (or try to solve) an exam problem that could have been solved more easily with a conservation law. My students, in informal questioning, report that they feel “comfortable” using the tangible ideas of forces and acceleration, but they find conserved quantities—especially energy—to be too abstract to hold any meaning for them. Conservation laws are at the center of our modern under- standing of physics, so an important pedagogical task is to provide students with a learning environment in which they come to “own” the ideas of conservation of energy and momentum. Give students a choice of a rubber ball or a sticky clay ball (of equal mass) that they are to throw against an upright wood block in order to knock it over. A large majority will chose the clay ball, feeling that sticking to the block is somehow more effective at applying a force than is bouncing off the block. Students have little intuition for the idea of an impulse , and most are very surprised at the outcome of a demonstration showing that the elastic collision has more “effect” on the block than the inelastic collision. There has been little systematic research on students’ understanding of the ideas of impulse and momentum. The only paper I know of is that of Lawson and McDermott (1987). They used a stream of compressed air to accelerate two dry-ice pucks of significantly different mass through a fixed distance. Then they asked students (who had completed instruction in energy and momentum conservation) to make comparisons of the final momenta and final kinetic energies of the pucks. To answer the question, students had to reason on the basis of work (equal for both pucks due to equal forces over equal distances) and impulse (larger for the heavier puck, which accelerated more slowly and thus took more time). Thus the two pucks ended with equal kinetic energies, but the heavier puck—although slower—had a larger momentum. Only 25% of honors students responded correctly on the momentum comparison, and no stu- dents in an algebra-based course gave the correct answer. Students who answered incorrectly were given hints to focus their attention on the fact that both pucks experienced equal forces over equal distances. Following the hints, the correct response rate of honors students went up to 58%, but the correct response rate of students in the algebra-based course remained at 0%. The investigators did not use students from a regular, non-honors section of calculus-based physics, but their response could be expected to fall between the two groups tested. The most common incorrect response was that the final momenta would be equal. Students

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Instructors_Guide_Ch09 - 9 Impulse and Momentum Recommended...

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