Re: friction

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

My colleague here (Joe Bellina) who does surface physics, is always quick to
point out that we really don't have a good handle on the physics of
friction--especially at the microscopic level.  I also know that simple
models for friction are out of vogue since they usually have some serious
flaws.  However, that doesn't deter me--at least in my 'liberal arts'
class...

I use a hook model and the way I do it is to have the students take their
hands  and cup them in a 'C' shape, one above the other and inverted.  The
upper hand represents the object, the lower hand the fixed surface.   Then:
1)  Have the students place their hands so that only the finger tips overlap
(to the first knuckle).  Hold the hands near each other but not touching.
There is NO FORCE then between the hands.
2)  Now move the top hand against the bottom and slowly increase the pull on
the top hand.  They will feel the force between the hands increasing.
3)  Pull hard enough and eventually the strength of the fingers will be
exceeded and the 'bond' will be broken and the top hand will move.  They've
exceeded the maximum static friction.
4)  Now place the hands several centimeters apart, but still positioned so
that only the finger tips are lined up.  Now move the top hand quickly
towards the fixed lower hand.  On contact they will feel a force between the
hands, but the bottom hand won't be able to stop the top hand and the
magnitude of the force should be less than what it was to break the static
'bond' situation.  Kinetic friction is less than the maximum static
friction.
5)  Now set the hands close again, but this time push the top hand down
until the fingers overlap to the second knuckle.  Try to pull the hands
apart again (probably can't).  This is the effect of increasing the 'Normal
Force'.
6)  You can use (5) to explain why the area of contact is not critical.  If
the 'Normal Force' is spread out over a large area, you get more 'bonds' but
they are all of the 1-knuckle type.  If you have a smaller area, there are
fewer bonds but they are of the 2-knuckle type.
7)  To deal with the coefficients of friction, go back and have the student
repeat these demos but with the fingers straight rather than cupped--this
models smaller coefficients.  Then have them make extreme 'C's to model high
coefficients.

I let the class know that this is a very crude model and won't hold up to
close scrutiny, but it does permit us to relate the observed behavior of
friction to something they can 'feel' and understand.  The next step, for
physicists, it to refine such a model to better reflect the microscopic
'reality'.

Rick

**********************************************
Richard W. Tarara
Associate Professor of Physics
Saint Mary's College
Notre Dame, IN 46556
rtarara@saintmarys.edu

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----- Original Message -----
From: "Promod R. Pratap" <pratapp@UNCG.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, October 18, 2000 8:44 AM
Subject: Re: PHYS-L Digest - 17 Oct 2000 to 18 Oct 2000 (#2000-374)


> I was always under the impression that the equation: F_st =mu_st * N only
> applied when the object is about to slide.  Otherwise, F_st is independent
of
> N.  Take the case of a light box and a heavy box sitting on the floor.
Now
> apply the same small force to each box; this force is smaller than mu_st*N
> for either box (with mu_st) being the constant evaluated just before the
> boxes slide).  In that case, the F_st must be the same for both boxes, and
is
> therefore independent of N.  Of course, one can say that mu_st increases
> differently for the two boxes, but that would mean the mu_st is a function
of
> N.  In the latter case, mu_st must be proportional to 1/N when the box is
> stationary.
>
> Any comments?
>
> Promod Pratap