Re: Friction

[brian whatcott <inet@INTELLISYS.NET>]

At 06:23 10/17/00 -0700, you wrote:
> > Are there any situations in which the coefficient of kinetic
> > friction is greater than that of static friction for
> > particular surfaces?
>
> I was once told that ground cast iron on Teflon had this
> property, but I don't know of its being used anywhere.
> /// Telescopes are floated in mercury or on pumped oil to
> achieve this condition.
>
> Leigh

It is indeed helpful to come at length upon a physical system
where we see a contrast between idealized math models and physical
 models which can demonstrate something of the cause and effect
observables which created so much debate in the preceding thread.

It is this issue of heat production which is a consequence of
motion in a physical medium that is a prime marker of an "effect".

 If one associates heating with an effect arising when an object
moves in a physical medium - as it might be a boat moving on water,
 a telescope floating on a mercury bearing, a slinky falling through
the air for instance; there is no doubt that it is some relative
motion which is linked with the heating effect. Though the equation be
divided by an equals sign in the usual manner - there is no doubt
about the lack of reversibility here. Heating, in the general case,
 cannot return the motion in its pristine quantity.

People who are imbedded in the realist or physical approach
when faced with a formulation like f = m.a might ask which term is
associated with motion? - and accordingly prefer the right hand side
of the equation as an effect. The equation for this group,
is effectively f = m.a + heating.

Considering the counter example, where an offset mass is associated
with a vibration force in a rotating wheel, we see that the motion
 is associated with the left hand side of the equation m.a = f

NOW where should the heating effect be placed?
It is evident there is heating associated with the wheel's rotation
in air, eccentric mass or no - but there is an extra heating effect
associated with the offset mass, if the wheel's rotation allows this
degree of freedom.
In this compliant situation, one might represent the situation as
m.a + heating by rotation = force + heating by vibration.

Here a subconscious preference for heating as associated with 'effect'
is defeated by its ubiquity (to both sides) - and it would be
necessary to arrange terms in the arithmetician's manner (expecting
vibratory heat to be out weighed by rotatory heating) to decide in favor
of the usual candidate - but with this example of mixed contributions,
it is probably better to go with Hume, in deciding that decisions of
cause and effect, vital though they are to scientific endeavor, are
not to be found by inspection of math equations, but rather are
decided by experience, by consensus in fact of the users of
scientific models.


brian whatcott <inet@intellisys.net>  Altus OK
                Eureka!