Re: Friction

[Doug Craigen <dcc@ESCAPE.CA>]

John Denker wrote:
> > I wrote:
> > > > Example:  A boat.  The static friction between the surface of the hull and
> > > > the surface of the water is zero.  At any nonzero velocity, the dynamic
> > > > friction is greater than zero.
>
> At 09:30 AM 10/18/00 -0500, Doug Craigen wrote:
>
> > Example:  A block on flat ground with no horizontal forces applied.  The
> > static friction between the surface of the block and the surface of the
> > plane is zero.  At any nonzero velocity, the dynamic friction is greater
> > than zero.
> >
> > Is there a problem with this statement?
>
> The problem is that it is non-responsive to the original question, which

Exactly.  I was asking why this is any different than your response to
the question.

> asked about the _coefficient_.

So your comments were intended to address the _coefficients_ of
friction.  Understanding that much makes your writing clearer.

>   -- The block has zero friction in response to zero sideways force;  if we
> try to calculate the coefficient we get 0/0, which is not very informative.

0/0????  I simply don't follow.  You are saying that the coefficient due
to friction is not defined when there is no sideways force?  This
coefficient is a property of the surfaces and has nothing to do with the
presence of a sideways force.

>   -- In contrast, the boat has zero static friction even in the presence of
> a sideways force;  from this we calculate with confidence that the
> coefficient is zero, which is what the original question was asking about.

ie. 0/something = 0

I think that what you are aiming for is along these lines...
coefficent due to friction = frictional force/normal force

if so:
1) the sideways force doesn't have anything to do with it
2) this only applies to the kinetic case, you are presenting it for both
cases

In fact, it is not clear to me why one would even talk about "static"
and "kinetic" cases for a boat since there is no discontinuity in any
"coefficient of friction" that might be calculated (and the static case
is a point not a regime).  Such a coefficient would be zero in the
static case not because of any sideways force case, but because
mu_static=F_t / F_n
where:
  F_t is the threshold force which will begin accelerating the object if
it begins with v=0
  F_n is the normal force

For a boat F_t = 0 & F_n (bouyant force?)>0, so mu_static=0
For a block F_t / F_n is not 0/0, it is perfectly well defined.

***

Knowing that you meant coefficient due to friction, I will readily agree
that if one calculates some mu(v) for a boat on water, that mu(0) <
mu(v>0) so that there is a sense in which the boat on water is a
solution to the question.

\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\

Doug Craigen
          http://www.dctech.com/physics/about_dc.html