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RE: Friction (and ambuiguous problem statements)

Hi John-
While your point about careful wording is well taken, the
quote that you are addressing was meant as a sketch of the problem
for teachers; it is not verbatim from a test. Most of the comments
seemed to be addressed to the problem as intended.
Regards,
Jack
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Hey Jack, you wrote:

Consider the following problem, which I always
include on friction quizzes and tests:
A 75 Kg skier stands on horizontal, snowy surface. Coef of
friction between skis and snow is .02. The skiers friend pushes her
forward with a steady force of 1 N. Calculate the acceleration of
the skier.
About 80% of all students have the skier accelerating
backwards.

Your implication is that that answer is not acceptable and a subsequent
flurry of responses attempted to figure out just what was going on in the
heads of those nutty students!

If I were one of your students and were marked wrong for giving this
answer I'd probably scream. If the skier has a mass of 75 kg and is
being pushed "forward" with a force of 1 N on a horizontal surface with a
"coef of friction" of .02, then I'd say the skier's acceleration *is* 0.2
m/s^2 "backwards," i.e., opposite the "forward" direction referred to in
the problem. That is, the skier is slowing down because the push is not
hard enough.

If the problem had read "*Static* coef of friction" and "pushes *on* her
with a steady force" (without the word "forward"), then I might believe
that the skier was intended to *be* "at rest." I do see that you describe
the skier as "stand"ing on the snow, but the implications of that word
are, in my mind, overruled by your subsequent language. Exam questions
should be concise, but it is even *more* important that they be unambiguous.
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