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Re: Controversial Exam Questions - Not Ohm's Law



On Thu, 11 May 2000, Michael Edmiston wrote:

I think the question and answer are correct. If the woman's weight is the
gravitational force the earth exerts on her, then the reaction force is the
gravitational force she exerts on the earth. Answer B is correct.

The problem is that many students will view her weight as the contact force
her feet exert on the earth.

... as do I and many other nonstudents.

The third-law reaction to the contact force of
her feet on earth would be answer A. In order to make answer A correct, the
problem would have to be reworded to replace "opposite force to her weight"
with the words "opposite force to the force she exerts on the ground."

I don't agree. I would say that to make the answer *unambiguously* B, the
problem would have to be reworded to replace "opposite force to her
weight" with the words "opposite force to the force exerted on her by the
earth due to gravity." Admittedly, this gets a bit unwieldy.

I believe weight is defined as the gravitational force the earth exerts on a
body, and if that is true, the question is okay and B is the correct answer.

Now, the question is this... is that what the exam writers wanted to find
out? Were they trying to see if the students could make this distinction?
It appears to me thy might indeed be after exactly that.

I think you might be right. If they really wanted to know if the students
understand the *third law*, they could have written a far less ambiguous
question.

John Mallinckrodt mailto:ajm@csupomona.edu
Cal Poly Pomona http://www.csupomona.edu/~ajm