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Re: plug and chug



Jack Uretsky wrote:

That is nitpicking IMO. You are free to reword the details. The
point is that many students turn off their brains and answer with a
negative acceleration.

And I would say that you are being *way* too harsh in reaching this
conclusion. If the skier really is being "pushed forward" with a
force of 15 N (rather than being pushed *on* in the forward
direction), then the skier's acceleration *will* be "negative"--that
is, the skier will be slowing down since the push isn't even hard
enough. All one has to do to reach this conclusion is fail to
appreciate the fact that the skier was at rest when the push was
first applied and I think the wording is needlessly confusing and/or
self-contradictory on this point. I don't doubt that many students
would still reach the same answer with less ambiguous wording, but
I'd reserve judgment until this alternate explanation had been ruled
out.

On Sun, 28 Jul 2002, John Mallinckrodt wrote:

>1. The same skier comes off the hill and stops on a level spot. Her
>friend pushes the skier forward with a force of 15.0 N. Find the
>skier's acceleration. (The coefficient of friction was given as
>.0500, the skier's mass as 75.0 kg).

<NITPICK> I think we may have discussed this one before. My best
answer would be that the problem is ill-posed. Assuming that the
> skier is initially at rest (which the problem seems to be clear
> about), then her friend may "push on the skier's back" with a force
of 15 N but her friend can't "push the skier forward" with a force
> that small.</NITPICK>


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