Re: Friction

[Lowell Herr <herr@catseq.catlin.edu>]

The forces are on the order of 3 to 6 Newtons and the surfaces are 
between a smooth stone lab table and wood blocks where the blocks are the 
size of 1/2 of a brick.  That should give you a visual picture.  We pull 
different numbers of blocks across the table surface and definitely find 
that the frictional force is proportional to the number of wood blocks.

When we first begin the pull, there is a major spike (static friction) 
that shows up on the computer monitor.  When the blocks break loose 
from the table surface, the frictional force drops as one would 
expect.  We then stop briefly and repeat the routine.  This stopping 
and pulling at a constant speed goes on for 25 - 40 seconds.  We 
eliminate the very first spike as it is generally higher than all the 
subsequent spikes.  We average these peaks to determine the static 
frictional force for a given number of blocks.  The kinetic frictional 
force jumpes around slightly but if one is careful to pull with a 
steady speed, it is quite consistent.  We average a number of the kinetic 
force values (a bit difficult to explain without seeing what is 
happening) and come up with a kinetic frictional force.

To restate my original question, what is surprising is that the kinetic 
frictional force is not affected by either the velocity or surface area 
but the static frictional force is much larger for a larger surface 
area.  Why is the static frictional force larger for a larger surface 
area.  I don't recall seeing this in any text.

Lowell Herr

Mon, 2 Feb 1998, Joseph Bellina wrote:

> Could you say more about the surfaces you have in contact, and roughly 
> the magnitude of the normal force?
>
> On Mon, 2 Feb 1998, Lowell Herr wrote:
>
> > 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
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> > Portland, OR  97225