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?