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Re: Newton's 2nd law Lab

Concerning Rick's comments:
Again you complicate this beyond the Introductory level--especially
non-physics majors.

I didn't complicate it. You, er, whoever designed the experiment did.
It was not my idea to do a lab that proports to show N2 for a composite
system by having different parts of the system move in different
directions. I just pointed out the complications that were already
present in the Lab setup that were (IMO) illegitmately ignored. If a
simple experiment showing N2 is desired, I would suggest one where there
is one accelerating mass of interest with the net force on that mass
independently measurable from the acceleration of the object, and then a
sufficient repetition of the experiment for many mass, force, and
acceleration values. If this setup must be used, I would suggest, at
least, that the focus be placed on the N2 behavior of the individual
masses, and not the composite system.

With this group the pulley SIMPLY changes the direction
of the force from vertical to horizontal.

The *tape* changes direction when it is run over the pulley. The only way
to change the direction of a force applied by a fixed agency is by applying
some other (noncolinear) force to add to it. Do you think that this is
beyond intro students' understanding? (I'm not sure I want to hear the
answer to that one.)

We NEVER complicate things by
determining the 3-dimensional force vectors that accomplish this.

I never suggested 3-d vectors; that would be an excessive complication.
But 2-d vectors are essential for the 2-d motions characterizing the
experiment as described. If you want to use only 1-d vectors, then you
ought to confine the motions to 1-d.

Your
comments and concerns are appropriate to a 2nd course in mechanics, NOT the
intro physics courses for which these labs are designed.

So the students must wait, yet again, until some later course find out how
they were swindled in their introductory course? For a concept as
fundamental as N2 is to introductory physics, I hope that this is not what
others would advocate.

David Bowman
dbowman@georgetowncollege.edu