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Thanks for all the interesting discussion. I agree that kinetic energy,
work, momentum and impulse are probably not in the view of what the
developers wanted to test. However, the published research shows that on
#9, the normalized gain is negative for the test groups. I would be
interested to see what the "incorrect" answers were on the post-test; I
speculate that many students interpreted the question in a work-energy
viewpoint. (It is still possible to get the correct answer to #8 with that
perspective since the arrows shown aren't drawn to scale. There need not
be a change in speed to have a change in direction.)
In addition, whether the writers intended to test only for vector
addition, the reality is that "good physics" must consider the work-energy
implications of the drawing and description. I take this position in light
of many problems in college texts (Young, for example) who describe
impulsive forces that don't change the KE, only the direction of the
velocity.
The fact of the kick on a stationary puck causing a speed of V_k
notwithstanding, the kick is NOT on a stationary puck, but a moving one.
And "swift kick" is physics problem talk for "instantaneous," isn't it?
Kind of like saying "a light string" in describing Atwood machines.
I think I understand the intent of the question, but I think it is poorly
described. Maybe it should be reworded as a "horizontal kick of short, but
finite, duration and constant direction." Then I would agree that it's
unambiguous.
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