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I don't have an issue with the ANSWER to this question. The question
never stated that the force continuously changed direction to remain
perpendicular to the instantaneous velocity. So I think this problem is
about independence of velocity components and also about the result of
vector addition, and not about what a centripetal force does. But I do
have a question about the item:
Is there a common misconception about forces that this item is being used
to shed light upon? I can see that other FCI questions are designed to
work that way. (For example, students believe that a ball thrown upward
retains some force until it reaches its peak.) But while this item
requires some sophistication to answer correctly and would be a good item
on a 2-D chapter test, I wonder how it fits in with the FCI. Is there a
predominantly chosen wrong answer among those who take this test as a pre-
test? I would guess that it would be a pretty even split among all the
answers with most students just guessing.
________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of LaMontagne, Bob
[RLAMONT@providence.edu]
Sent: Saturday, February 14, 2009 7:51 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] FCI answer?
??? It can't be perpendicular to both - how does reference to the previous
problem clarify that?
Bob at PC
________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu on behalf of John Clement
Sent: Sat 2/14/2009 4:12 PM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] FCI answer?
Since it makes reference to the previous problem, it is quite clear that
the
change in velocity is only perpendicular to the original velocity so a
component is added in the Y direction on the diagram. Also the notation
makes it clear that you are adding a velocity.
Incidentally the test is actually available in toto on the web if you look
hard enough.
John M. Clement
Houston, TX
applied
The preamble to the problem tries to get around issues of impulse
atof
an instant, I think. I don't want to be too specific in an open forum,
course, but do others think that the description before the figure dealsvelocity,
with the issue that Bill brought up?
On preview, I see that JD has brought up the similar point.
sincerely,
Krishna
Krishna Chowdary
Faculty, Math & Physics
The Evergreen State College
Olympia, WA 98505
On Sat, Feb 14, 2009 at 11:38 AM, Bill Nettles <bnettles@uu.edu> wrote:
John,is
That's what I thought, too, initially. Then I began thinking, "If this
an implusive force directed perpendicular to the instantaneous
puckthen it does zero work. That means the speed doesn't change, only theis
direction." Am I missing something?
Thanks,
Bill
Assuming you have the same version, the answer to #9 from Mazur's book"John Clement" <clement@hal-pc.org> 2/13/2009 2:14 pm >>>
#5
in the answer key. This is the question about the "speed" of the
becauseafter the kick.Mazur's
The main problem is that Vk has not been defined in the version in
book, but it should be evident that this would be the velocity if theball
were not already moving. And both concepts give the same answer
v0less
is perp. To Vk. So the speed is greater than either V0 or Vk, but
addition,than
the arithmetic sum of them. I would say it is testing vector
butVk
that depends on the student understanding that the change in velocity
isto
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
5question
rather than A to E.that
John M. Clement
Houston, TX
opinions
I'm trying to avoid "spoiling" the FCI answers, but I'd like some
on one of the questions and whether it should be included. Can we do
here?
I'm NOT INTERESTED in opinions on the FCI in general, just on
the#9.
I don't have the "official" answer sheet, but I'm not sure whether
accelerationintent is to test vector addition or relative direction of
conceptand velocity. One would expect different answers depending on the
vectorbeing tested. The first time I read the test, I said "Oh, this is
_______________________________________________addition." But a closer look makes me think of "instantaneous"
perpendicular impulse.
What do you think is intended?
Bill
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