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 dryice 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 algebrabased 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 algebrabased course remained at 0%. The investigators did
not use students from a regular, nonhonors section of calculusbased 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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 Spring '10
 kant
 Conservation Of Energy, Momentum, Collision, conservation laws

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