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Re: Friction



Seems to me some fairly simple models DO predict the empirical laws of
friction while at the same time providing an understanding of why these laws
aren't absolute. I like to use a model where you can have the students take
their hands and form a couple 'Cs' (railroad couplers), one upside down the
other right side up. If they place their hands so these 'Cs' are intertwined
but not touching, then you have the 'no horizontal force/no frictional force
situation'. Now pull the hands in opposite directions and one can feel the
'frictional' force increase. If you pull hard enough, your arm strength
will overcome your finger strength and the hands will come apart. The more
intertwined you hold your hands (the more Normal force pushing the surfaces
together) the harder it becomes to separate the two hands. For a given
Normal force, the 'depth' of these hooks (the degree to which they are
intertwined) is a function of the area (or number of such hooks). A large
area will have more hooks but less completely engaged while a smaller area
will push the hooks (hands) together more but there will be fewer such
hooks. If on the other hand you start your hands far apart and move them at
each other so that the finger tips will meet, it is fairly easy to move the
hands by one another (or conversely, harder to stop the hands moving) and
thus kinetic friction is not as strong as the maximum static friction.

This of course is a very rough model, but has the advantage that you can
have the students demonstrate to themselves and they can get immediate
kinesthetic feedback.

Rick


-----Original Message-----
From: Dr. William Newbolt <NewboltW@madison.acad.wlu.edu>

III.Sometimes the empirical laws of friction just don't
work. It is interesting that they work as
frequently as they do since we really have no
compelling reason to suspect that they would.