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Re: [Phys-l] FCI answer?



John,
That's what I thought, too, initially. Then I began thinking, "If this is an implusive force directed perpendicular to the instantaneous velocity, then it does zero work. That means the speed doesn't change, only the direction." Am I missing something?

Thanks,
Bill

"John Clement" <clement@hal-pc.org> 2/13/2009 2:14 pm >>>
Assuming you have the same version, the answer to #9 from Mazur's book is #5
in the answer key. This is the question about the "speed" of the puck
after the kick.

The main problem is that Vk has not been defined in the version in Mazur's
book, but it should be evident that this would be the velocity if the ball
were not already moving. And both concepts give the same answer because v0
is perp. To Vk. So the speed is greater than either V0 or Vk, but less than
the arithmetic sum of them. I would say it is testing vector addition, but
that depends on the student understanding that the change in velocity Vk is
in the direction of the kick which is perpendicular to the initial velocity.
So there are actually 2 concepts in this question.

I would agree with Mazur's answer key, and his answers are numbered 1 to 5
rather than A to E.

John M. Clement
Houston, TX


I'm trying to avoid "spoiling" the FCI answers, but I'd like some opinions
on one of the questions and whether it should be included. Can we do that
here?

I'm NOT INTERESTED in opinions on the FCI in general, just on question #9.
I don't have the "official" answer sheet, but I'm not sure whether the
intent is to test vector addition or relative direction of acceleration
and velocity. One would expect different answers depending on the concept
being tested. The first time I read the test, I said "Oh, this is vector
addition." But a closer look makes me think of "instantaneous"
perpendicular impulse.

What do you think is intended?

Bill

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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l