Re: Friction

[Doug Craigen <dcc@ESCAPE.CA>]

"John S. Denker" wrote:
> > John Denker 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 10:04 AM 10/17/00 -0500, Doug Craigen wrote:
> > Couldn't the same be said about a block resting on flat ground?
>
> No.

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?

> > I think the question has to do with the classic diagram of frictional
> > force vs applied force:
> >
> >    /
> >   / ___________
> >  /
> > /
> > /
> > u_s    u_k
>
> That's stated to be a force-versus-force diagram.
> The original question asked about the force-versus-velocity relationship.

The original question was "Are there any situations in which the
coefficient of kinetic friction is greater than that of static friction
for particular surfaces?".

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.

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

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