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Re: Friction



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
-----Original Message-----
From: Lowell Herr <herr@catseq.catlin.edu>
To: phys-l@atlantis.UWF.EDU <phys-l@atlantis.UWF.EDU>
Cc: phys-l@atlantis.UWF.EDU <phys-l@atlantis.UWF.EDU>
Date: Monday, February 02, 1998 6:26 PM
Subject: Friction


Over the past several years I have had my students do a friction
experiment using a Vernier Force Sensor where different masses are pulled
across a smooth horizontal surface. We check to see if mass is a factor
(it is), if velocity makes a difference (it does not), and if the surface
area is a factor. Here is my question.

With 'identical' surfaces, the kinetic friction remains nearly the same
when we double the surface area that is in contact with the table top.
This is not a surprise but what does surprise me is that the static
friction is much greater (nearly doubles) when the surface area is
increased. I have not seen this factor show up in a friction equation but
it
is certainly visible when we do the experiment. What is the logic behind
this behavior?

Lowell

PHYSLab Home Page - http://physlab.catlin.edu

Lowell Herr
The Catlin Gabel School
Project PHYSLab II
8825 SW Barnes Road
Portland, OR 97225