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Re: Friction



At 18:00 2/2/98 -0800, you wrote:
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


Lowell,
I think you've been cheating! It sounds rather like you are dragging a
'train' of two or more blocks around, each independently resting on the lab
table surface.
This is inappropriate if true.
The general idea is that a bearing surface of widely varying area only
bears at 3 points.
Two independent blocks obviously bear at six points.
This would evidently affect the unstick force.
Other factors that might vary the three-point bearing idealization are a
coating of wax or very carefully smoothed bearing surfaces.

Sincerely.
brian whatcott <inet@intellisys.net>
Altus OK