Solid-Solid Friction Hypothesis

Sat, Apr 30, 2005 at 12:25 AM


Why would solid-solid friction, a property that supposedly tied to
unchangeable structures, varies according to the velocity of a moving
object(lessen in most of the case)?
I have two routes for explanation, they are both from the
microscopic perspective of movement. My first perspective is related
to the natural vibration of each atom/molecule that constitute the
object. As we know, each atom/molecule is in constant vibration at all
3 planes, therefore the boundary of that object is constantly
changing. As we drag the object forward, some atom/molecules would
encounter obstacle in the path of movement. Since the location of that
atom/molecule is constantly changing, an atom/molecule from the fix
surface would only be an obstacle IF it is happen to be on the way,
thus causing friction. If the horizontal velocity of an object’s
movement has increase, the number of atom/molecule that it went across
would increase. An from the perspective of natural probabilities, the
net number of atom/molecule happen to be in the way would decrease
compare to lower velocity, thus reduce the friction. Moreover, the
natural reaction of a atom/molecule encounter an obstacle would be
moving inward(vibration). As it encounter more obstacle on a per
second basis, it would have either higher frequency or magnitude of
the vibration. Vibration is one of the natural way for that
atom/molecule to REDUCE the chance of meeting yet another obstacle,
thus reducing friction. However, if the surface is smooth enough, this
effect would reduce considerable, so paradoxically, a rough surface
may ‘prepare’ the moving object better than a smooth surface.
Another explanation is to think the contacting side of the moving
object as many little wheels. As we drag the object moving, these
little wheels rotate much like another circular object. Imagine riding
a bicycle in a rough terrain, the faster your ride, the easier for
your to overcome the peculiarities. As the horizontal velocity of that
object directly translate to the angular momentum of the
atoms/molecules at the contacting surface, and each atom/molecule has
its own natural rotation frequency, increasing the horizontal velocity
would help us transfer the kinetic energy more efficiently to the
rolling atom/molecules. Thus reducing the energy waste in the process
for applying energy at the wrong frequency. The increase efficiency is
translated to us as reduce friction.

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