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Re: Friction

Just a quick comment about Tom's experiment. We have lots of trouble
getting students to let go of their presuppositions about motion...to
move from Aristotle to Galileo (a simplification I realise, but bear with
me)...here is case in which their presupposition about bonding is right,
what is confused is their notion of smoothness. Few people think about
the notion that the earth is smoother than a billiard ball. In the case
of friction, I would applaud their understanding of the role of bonding
and then raise the question of why the macroscopic result does not agree
with their valid microscopic model. That leads to an interesting
discussion of what the interface must look like.
If you have access to the web, you then go to IBM Almaden's visualization
site and see a real copper surface, which atomically looks like the Rocky
Mountains.

cheers

On Mon, 2 Feb 1998, Tom McCarthy wrote:

I just finished a lab on friction and I had my students using the
dual-range force sensor with about a 4"x6"x1" piece of wood. I had them put
a piece of cardboard on the bottom to change area, starting with a piece
that completely covered the bottom. They stacked about 3 kg of weights on
top of the block to set the weight. After each pull they would remove the
cardboard, cut off a fifth, and re-center the remaining piece. The
resulting data should both kinetic and maximum static friction to be
independent of area, with fairly low random error.
I like this experiment because students have preconceived ideas that
there is a dependence. I believe some of the results that appear are
attempts to make the experiment fit the data! Another thing I really like
is the display of force versus time using the sensor. It is so incredible
to see the whole process on the screen. It really helps students see it
clearly, too.

Tom McCarthy
Saint Edward's School
1895 St. Edward's Drive
Vero Beach, FL 32963
561-231-4136
Physics and Astronomy