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[Physltest] [Phys-L] Re: Friction?



Ludwik Kowalski wrote:

The surface is not smooth. Looking through a magnifying glass I see a
lot of bumps, like on leather.

The texture is there for a good reason: it creates
channels for the _air_. A pair of smooth surfaces
wouldn't work nearly as well, because you'd have an
unintended "air bearing" between them.

The forces responsible for adhesion are very short-ranged.

Do the math, using the known viscosity of air. You'll
find that to squeeze out the last couple microns of air
from between two flat surfaces would take a lonnng time.

There are other solutions to the same problem. For
instance, cellophane tape is neither textured nor
air-permeable. But note that you make use of its
flexibility, applying it starting from one end and
gradually progressing ... not stamping it on broadside.

As a tangent, the same logic explains why static electricity
is popularly associated with textiles and powders: most
other objects would be prevented by the air layer from
getting close enough for appreciable contacte electrification.
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