Friction (and ambuiguous problem statements)

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

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.

John
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  A. John Mallinckrodt      email:  mallinckrodt@csupomona.edu
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