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Re: [Phys-l] A challenge.



After rereading, I can't
imagine the kind of question that he is concerned with.

check out the original question -- answer and explain why.

-------------------------------------
A student in a lab experiment jumps upward
off a common bathroom scale as the lab
partner records the scale reading.

What does the lab partner observe during the
instant the student pushes off?
A The scale reading will remain unchanged
during the entire time the student is in contact
with the scale.
B The scale reading will increase momentarily
then will decrease as the student is moving
upward from the scale.
C The scale reading will increase during the
entire time the student is in contact with
the scale.
D The scale reading will decrease momentarily
then will increase as the student is moving
upward from the scale.

Is this really a fair question to ask??? What assumptions are we making? The picture shows two pictures: 1) Guy crouched on scale 2) Guy in midair.
I'm making the following assumptions: 1) We ignore the mechanism of the scale and assume it displays the real-time force (and anyone who has used a traditional analog bathroom scale knows that's not true). I'm making this iffy assumption based on the iffy assumption that whoever created this question has good enough judgment not to assess a student on the mechanism of a bathroom scale on top of their assessment of N2L/N3L. 2) Based on the picture, we're beginning our measurement as the guy accelerates upward, NOT including his action of lowering himself to a crouch 3) He accelerates upward at a constant rate because non-constant acceleration isn't in the standards

Using these assumptions, I'm modeling the person as an object accelerating upward at a constant rate. Therefore, there is a constant unbalanced force being exerted upward on him by his feet/ legs (N2L). (Wow, this is getting really complicated in my head... I hope I'm not needlessly overcomplicating this.) Due to N3L, his body is exerting a constant downward force on his feet/legs. His feet/legs are still, meaning forces on them are balanced, meaning there's a constant force upward on his feet from the scale. N3L now says that constant force must also be exerted downwards by his feet on the scale. Then I'm sure I'm making some reasonable simplification by saying this is the force that the scale measures.

Following this logic, I would say the scale measures a constant force greater than the weight of the guy. Except that is not a choice. Fine. The next closest answer is C, that the force is constantly increasing, which would mean the guy's acceleration is increasing. Still plausible.

The answer is B.

WTF.

Frank
-----------------------------------------------------------------

The attachment w/ two drawings makes a diff., but


bc still disagrees

On 2009, Nov 28, , at 16:14, Jack Uretsky wrote:

Dan is correct. I missed his stated concern. After rereading, I can't
imagine the kind of question that he is concerned with. Are there, in
fact, such questions that are systematically missed by the "higher scoring
students" and answered correctly by the lower scoring students?
I suspect that we may have a disagreement about what is the
"correct" answer .
Regards,
Jack

"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley




On Fri, 27 Nov 2009, Dan Crowe wrote:

I think that Jack has missed the point of my concern. I'm not concerned about the competitive nature of the test; I'm concerned about the fairness of the test. There could very well be _valid_ questions that would tend to increase the scores of students who otherwise would receive lower scores on the _biased_ test that results when such questions are systematically rejected. Such biased tests are less valid because they don't measure what they purport to measure. The criteria for question rejection should not reduce the validity of the test.

Daniel Crowe
Loudoun County Public Schools
Academy of Science
dan.crowe@loudoun.k12.va.us
Jack Uretsky <jlu@hep.anl.gov> 11/27/09 10:21 PM >>>
Put your question in the context of applicants for positions on
the varsity squad of an athletic team. Perhaps, then, the question will
answer itself.
But, you might say, academic grades are not competitive. But what
fraction of applicants to. say, MIT, Harvard, Stanford, or Rice are
accepted? And to what extent does the college of one's degree affect the
job potential?
My expeience is that real life is highly competive, so the world
of athletics is a relevant model for the world of survival outside of
athletics. Give your students a break. Let them know that grade
inflation does not prepare them for real life.
Regards,
Jack

"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley




On Fri, 27 Nov 2009, Dan Crowe wrote:

Is anyone else bothered by the policy that reinforces the "smart" vs. "dumb" kids scores? I can imagine that there are valid questions that otherwise higher-scoring students miss more frequently that otherwise lower-scoring students. If such questions exist, then systematically rejecting such questions reduces the validity of the entire test.

Daniel Crowe
Loudoun County Public Schools
Academy of Science
dan.crowe@loudoun.k12.va.us
Bernard Cleyet <bernardcleyet@redshift.com> 11/26/09 11:35 PM >>>
Posted on PTSOS by Dean Baird:

The process by which an item makes it on to the CST is ... involved.
Add a few more steps for those that get released.

Items are developed by ETS and screened by the California Department
of Education and by its Assessment Review Panel. If that all goes
well, the item is field-tested. The item must perform well on a
series of psychometric measures. Most importantly, the item must be
neither too easy nor too hard, based on student performance on the
item. And the item must discriminate well. That is, students who
perform well on the test overall should perform well on the item. And
students who don't perform well on the test overall should perform
poorly on the item. There are always some items that "smart" kids get
wrong and "dumb" kids get right. Those items are rejected.

<snip>
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