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Re: [Phys-l] FCI answer?

In the statement for problems 8-11 it clearly states that V_k is the
velocity the puck would have starting from rest, and it clearly shows the
force in only one direction perpendicular to the initial motion. I missed
the problem statement because in the copy in Mazur's book it is on the
previous page. Mea culpa, that was my mistake. The problem statement is
exact and clear. Then only 1 trajectory is correct in problem 8, and when
you pick that, the answer to problem 9 is also clear. There is no
ambiguity.

The problem can be answered without thinking about momentum, and just
understanding that the velocity will only change in the direction of the
force. Then you have to understand that the horizontal and vertical
velocities do not affect each other. As I recall there are no questions on
the published FCI or FMCE that require any understanding of momentum or
energy. The FMCE from the author has a few very simple energy questions.

I should have looked at my copy because it has the problem statement and the
3 questions on the same page to avoid problems when students read it. And
the current FCI has had some slight reworking to make it more clear and
exact.

John M. Clement
Houston, TX


??? 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
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

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

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


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