Re: [Phys-l] index of refraction

["LaMontagne, Bob" <RLAMONT@providence.edu>]

While I agree with a lot of what you have said, I would like to make the case for "verification" labs (I don't call them experiments because they really aren't). Students learn about force components, centripetal force, conservation of momentum, etc., in class. but for many students, it doesn't become real for them until they set up forces on a force table, actually measure the tension in a string tied to a whirling mass, or follow the collision of steel balls or marbles. Some students need to "see it" to believe it. It's often in the middle of a "verification" lab that a student will say "Now I get it!".

Bob at PC
________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Hugh Haskell [hhaskell@mindspring.com]
Sent: Monday, May 11, 2009 9:48 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] index of refraction

At 12:49 -0500 05/11/2009, Paul Lulai wrote:
> That would make it a fairly productive lab.  Additionally, they
> could calculate % difference btn their values and the 'accepted'
> values.
I would argue that any lab in which the goal is to measure the %
difference between their value and the "accepted" value teaches the
students nothing of value, and in most cases does just the opposite.
First, it teaches the students that experiments are not to find out
anything new, but to verify what we already know, and second it leads
to what we used to call in the Navy, "gundecking" the results--that
is, making the results give the "right" answer. Furthermore, the
"accepted" value is the accumulated best experimental results, and
not in any sense "correct." so calculating the % difference between
the two is pretty meaningless, and particularly confusing when the
"accepted value is zero, since any % difference from zero is
automatically infinite.

It is much better to design experiments that have no pre-known
answer, and show them how to 1) estimate a statistical uncertainty
value, and 2) look critically at the experimental setup and try to
figure out what, if any, systematic error might be present due to the
experimental design.

When I have something that I want them to do that does involve
comparing their results to a "known" value or idea, I try to design
the experiment so that the expected result is zero, and then tell
them to estimate their "error bars" and see if that range of values
includes the "known" value. If so, they should be disappointed, since
they didn't discover anything new. If not, then they need to figure
out what they did that might have led them to get a result that
doesn't agree with the consensus, and if they can't find something
that might have gone wrong, then they need to offer a possible
hypothesis for testing that "explains" the difference between their
result and the "accepted" result.

Furthermore, using the phrase "human error," to explain any anomalous
result, earns them an automatic failing grade for that lab. They can
say "I figured out that I measured A using an incorrect method,"
(provided they describe the incorrect method, and how they could have
done it right) but they cannot say, "I must have measured A
incorrectly."

The sooner students learn that science experiments are not for the
purpose of verifying what we already know, but for trying to discover
something new, or at least to confirm a previously unconfirmed
hypothesis, the better off they will be.

Hugh

--
Hugh Haskell
mailto:hugh@ieer.org
mailto:hhaskell@mindspring,.com

So-called "global warming" is just a secret ploy by wacko
tree-huggers to make America energy independent, clean our air and
water, improve the fuel efficiency of our vehicles, kick-start
21st-century industries, and make our cities safer. Don't let them
get away with it!!

                                        Chip Giller, Founder, Grist.org
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