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Re: Newton's 2nd law lab



-----Original Message-----
From: phys-l@lists.nau.edu: Forum for Physics Educators
[mailto:PHYS-L@LISTS.NAU.EDU]On Behalf Of Rick Tarara
{When this kind of lab is done with my science majors, they have worked with
the modified Atwood's machine problem prior to the lab and thus are familiar
with this idea of transferring the vertical force to the horizontal
direction. For my non-science majors, they never question the assumption
that the falling mass provides the force that is pulling horizontally on the
cart. ;-) }


This has been an interesting discussion today. NII is a key experiment in an
introductory physics course, yet it is a hard one to do cleanly for the
typical student. As a student I was asked to do the half-Atwood machine,
_used_ Newton II to infer the tension force, and then measured the
acceleration of the cart using, I think, spark timers. The logic there is
obviously circular, although I don't know if I realized it at the time. Still,
the exercise had value to me because I had to apply the concept of NII
repeatedly.

I've come to feel that the half-Atwood is a great discussion and experimental
device, but it isn't my choice for introducing NII because of the issue raised
by other posters. It is just too darn complicated if you really think about
it.

That's why I like the force sensor/accelerometer lab. There are no strings, no
complex bodies, just one cart that is moving according to the force applied by
my fingers. The bigger F is, the bigger a is. The cart's mass is the
proportionality constant, and I can verify that.

This lab trades the complexity of the Atwood for the black-box behavior of the
accelerometer. Is this a good thing? I think so. The accelerometer is another
tool used during the year, and students will develop an understanding of that
tool. Initially, acceleration is just what that thing the teacher calls an
accelerometer measures. That's just fine to start out with. As they do more
experiments, their understanding deepens and the tool's behavior becomes
clearer.

It is very, very hard to design experiments that are complete in that they
don't depend on other, as yet-undefined concepts. That's the problem with the
half-Atwood lab as often taught, and it can be a problem with using black box
tools. That's ok, however, as a given experiment is rarely alone. There are
other activities that come back and use and amplify the ideas.

JEG
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John E. Gastineau john@gastineau.org KC8IEW
900 B Ridgeway Ave. http://gastineau.home.mindspring.com
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