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Re: Friction



At 07:13 AM 10/17/00 -0400, David Abineri asked:
Are there any situations in which the coefficient of kinetic friction is
greater than that of static friction for particular surfaces?

Then Doug Craigen wrote:
> >I think the question has to do with the classic diagram of frictional
> >force vs applied force:
> >
> > /
> > / ___________
> > /
> > /
> >/
> >u_s u_k

Then I wrote:

> That's stated to be a force-versus-force diagram.
> The original question asked about the force-versus-velocity relationship.


The diagram I give above is the classic textbook diagram for this
question dealing with the case of applying greater and greater force
until an object starts to move. The velocity is irrelevant except that
v>0 defines the kinetic case. This diagram doesn't anticipate the case
where sliding continues without a continuing applied force. In this
case the frictional force is still u_k*N while the block has any v>0.

But but but ... note the use of the word "coefficient" in the original
question. The actual frictional force depends on many things, including:
-- the normal force,
-- the sideways force,
-- the velocity, and
-- the nature of the materials.
but (!) the frictional coefficient, last I knew, was defined to be
independent of the sideways force.

Therefore the above diagram, whatever its intrinsic strengths or
weaknesses, cannot be directly responsive to the original question. Even
mentioning the sideways force seems like a red herring.

But since the herring has jumped into our boat, we need to pickle it:

For any given materials, and any given normal force, if we were to graph
the coefficients as a function of sideways force, the graph would have to
look something like this:

(coeff)
|
|========* static
|
|================================...========== dynamic
|
|________________________________...__________ (sideways force)


where the dynamic coefficient is everywhere independent of sideways force,
and the static coefficient is also independent of sideways force, up to
some point where the sideways force exceeds the static coefficient times
the normal force, whereupon the static curve ceases to be very meaningful
(because the situation cannot remain static).

Of course the real physics is much more complicated than this, but in the
simplified world implied by the form of the original question, that's the
story.