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Re: N2



Maybe that's why when I was at MIT (26 years after Jack!), they had NO
freshman physics lab! I agree with Jack, and research in physics education
does show, that developing "kinesthetic intuition" is essential before
formalizing the ideas as mathematical relationships.

Alex





Jack Uretsky <jlu@HEP.ANL.GOV>@lists.nau.edu: Forum for Physics Educators"
<PHYS-L on 07/04/2000 12:44:46 PM

Please respond to "phys-l@lists.nau.edu: Forum for Physics Educators"
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Subject: Re: N2


Hi all-
I'm going to try to explain what there is about this whole thread
that troubles me. In doing so I am trying to project back 59 years to
the year I first walked into a physics lab (I don't think there was a lab
component to my high school physics course). What I recall is that
freshman physics lab (at MIT) was a distinctly negative experience.
That, perhaps, is why, when I discovered physics in grad school, I became
a theorist.
I think the first "experiment" was the one with a mass falling
along an electrified wire. Equal time-interval sparks made marks on
sensitized paper. Measurement of distances between marks then let us make
the position-velocity-acceleration graphs with the usual write-ups
(probably involving significant digits, error analysis, etc.).
So where was the focus of my attention? It was on the mechanical
chores of measuring those distances, calculating the velocities and
accelerations, making the graphs, and thinking of things to say in the
write-ups. To say that all of this was making me sensitive to F=ma, is
to totally misconstrue the reaction of a 17 year-old mind.
It is not an answer to say that today most of the drudge work is
done by computers, so that in today's lab I could just focus on the
result - the effect of constant acceleration on a mass. The point I think
I am making is that that quantitative relationship F=ma (emphasis on the
=) isn't even interesting until I have grasped the qualitative
relationship; F increases with both m and a. When I have grasped that
fact, then I am ready to be led to the question: What is the quantitative
relationship? But the relationship betweeen the fact and the question
is by no means trivial.
To put it differently, there is (for me, at least) a kinesthetic
intuition that underlies that mathematical relationships of mechanics.
The first step of an effective physics lab develops that kinesthetic
intuition. The second step, which can be delayed, is to develop
kinesthetic intuition into mathematical relationships - that is what I
think physics is really all about.
Dick Hake, when he was at Indiana, developed an N2 lab that
involved students pushing masses around and making sketches to
describe their observations and reactions. This type of activity is,
I think, much more effective in developing kinesthetic intuition than
are the "experiments" I've seen described on this thread. Perhaps Dick
can be persuaded to put his N2 lab on the net so that you can see what
I am talking about.
Regards,
Jack

Adam was by constitution and proclivity a scientist; I was the same, and
we loved to call ourselves by that great name...Our first memorable
scientific discovery was the law that water and like fluids run downhill,
not up.
Mark Twain, <Extract from Eve's Autobiography>

On Tue, 4 Jul 2000, Mark Sylvester wrote:



But if you *do* use a force probe then you can make the lab more
elementary
because you no longer have to analyze an Atwood Machine. The pulley and
the
mass at the end of the string are merely a way to apply the force, which
is
measured at the point of application. No need for mg either. The next
step
is to get rid of the string and just push and pull the cart randomly by
the
force sensor, while logging F and a. Pasco should modify the software so
that the force probe can be calibrated in arbitrary units. For this
experiment you don't want to measure force in newtons.

Related to this, I found something really nice when we were playing with
the Pasco track a few months back. The idea was to show that the
acceleration of a body projected upwards doesn't change as it goes up and
comes down again. The v-t graph should be a straight line, starting with
positive velocity, passing through zero at the top of the motion, and
going
negative as the body comes down. (Of course a terribly difficult graph to
predict, sometimes even for people with a lot of basic physics behind
them). So we set the track at an incline and pushed the cart up, letting
it
come down again, while logging the motion with the little ultrasound
sensor. We got a distinct kink in the graph as it passed through zero
velocity. The acceleration while the cart is going up has bigger
magnitude
than while it's coming down, and the graph makes two pretty good straight
lines. Assuming that this is because the friction changes sign at the top
of the motion, one can go on to estimate the size of the friction force
from this graph (you also need the mass of the cart). I was struck by how
easy it was to see and measure the effect with the datalogging stuff.

Mark