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[Phys-L] Re: Ambiguous Question



Edmiston, Mike wrote:

The dog walks 3 meters toward the dock. After this walk, how far is the
dog from the dock?

The professor made a mistake. It's embarrassing. His "defense" of
his position is vastly more embarrassing. He needs to climb down
ASAP. This is a "learning opportunity" for him.

First of all, I would say to him: Judge not, that ye be not judged. If
he is going to take points away from the students for not seeing the
intended intepretation of the question, then I am going to take points
away from him for not seeing the other interpretation. Since he is
the senior guy and is being paid to set a good example, I'm going to
take a *lot* of points away from him.

Secondly, it is one thing to inadvertently overlook a second interpretation;
people tend to stop looking for additional interpretations as soon as
they see the first acceptable interpretation. We've all been there and
done that. It's nothing to be proud of, but it's not a scandal. In
contrast, for the professor to close his eyes to a second interpretation
*after* it has been pointed out ... that is far, far outside the reasonable
envelope.

The professor says that since they were studying center of mass, it
should have been obvious

Obvious? Obvious??? Whether or not it "should" have been obvious, it
wasn't obvious. If it really had been obvious, we wouldn't be having
thism conversation. The professor thought it would be obvious ... but
he was wrong about this.

In the looking-glass world, characters are allowed to take the attitude
that "a word means what I mean it to mean, nothing more and nothing less."
In the real world, authors are supposed to use words that mean something
to the reader. It is the author's responsibility to disambiguate his
writing. Something is obvious only if all the participants agree it is
obvious.

Furthermore, I see only the weakest of evidence that the intended interpretation
"should" have been obvious. When told something moves 3 meters toward the
dock, most humans would naturally assume it moves 3 meters closer to the dock.

The only pseudo-cue I see is the fact (as M.E. pointed out) that the natural
interpretation makes the problem trivial. But on what stone tablet is it
written that all problems must be nontrivial? As the proverb says, learning
proceeds from the known to the unknown. I often ask trivial questions as
lead-in to less-trivial questions.

Far outweighing this pseudo-cue is the fact that the key sentence mentioned
the dock, thereby certifying the dock as a usable reference. Furthermore,
in elementary physics classes it is common to discourage people from using
non-Newtonian reference frames. Since the boat accelerates and decelerates
during the scenario, this is a very reasonable reason for preferring the
dock frame to the boat frame.

=====

Finally and most importantly, the professor needs to learn something about
physics. It has been understood since the work of Galileo that the laws
of physics are the same in any reference frame.

As a corollary, any statement where the meaning depends on what reference
frame is assumed is not merely ambiguous; it is defective. It is unphysical.

I would be inclined to give this student full credit (as well as all the
others who answered 17 m even if they didn't give the CM information).

At the very least.

In a situation like that, my response would be to have a discussion along
the following lines:

I messed up. Let's make this a learning experience for the bunch of us.
Here's what happened: __________. Here's what I'm gonna do so that I can
recognize this situation (and similar situations), so I don't ever make
the same mistake again: __________. You should pay attention so that you
can avoid making similar mistakes.
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