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Re: [Phys-l] FCI answer?

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


The preamble to the problem tries to get around issues of impulse applied
at
an instant, I think. I don't want to be too specific in an open forum, of
course, but do others think that the description before the figure deals
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,
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
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_______________________________________________
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Phys-l@carnot.physics.buffalo.edu
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Forum for Physics Educators
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