Re: Friction

[Doug Craigen <dcc@ESCAPE.CA>]

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

Couldn't the same be said about a block resting on flat ground?  And,
shouldn't the boat be addressed as a viscosity problem rather than a
friction problem with the resisting force being related to velocity
rather than applied force?

I think the question has to do with the classic diagram of frictional
force vs applied force:

    /
   / ___________
  /
 /
/
u_s    u_k

Are there situations in which u_k > u_s?

for this kind of picture, as far as I can see we cannot have u_k > u_s.
Imagine what happens as you apply greater and greater force to a block
sitting on a level plane.  In the static regime the frictional force
equals the applied force.  We now get to the "kinetic" regime.  If u_k >
u_s then just after entering the "kinetic" regime the frictional force
will be greater than the applied force????  Clearly the block will be
stopped and we will have not entered the kinetic regime after all.  The
frictional force after the block starts moving must be less than the
frictional force just before it starts moving in order for there to be a
net force.

Now we all know such sharp discontunuities don't really exist.  If we
allow for some transition regime which is concave downwards, then my
argument above wouldn't hold.  I'm sure that there are other "real
world" considerations which may be brought up, but there is the usual
question "at what point we are confusing our intro classes with these?".

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

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