Re: [Phys-l] Static vs kinetic friction

["John Clement" <clement@hal-pc.org>]

Yes, I know it is complex, and there are all kinds of possibilities.  After
all whole theses are written about it.  So I would say the consensus is that
it is not worth worrying about, especially since the variability may be more
than the difference in values.  After all friction is an empirical law and
is only obeyed well in simple physics demos.

I have found that when pulling a simple wood block on a Formica desk that
the difference between the static and kinetic value can be quite large as
long as the force is increased very slowly.  One of the most convincing
demos is to take an ordinary 2x4 about a ft long or so and show students
that the frictional force is the same no matter which side it is laying on.
The area varies by a factor of 2&6.  This can easily be done with a force
probe.  Cleaning the desk is helpful, but that only insures uniform friction
with changes in position.  The wood block is left au natural after the
initial sanding and screw hood attachments.

John M. Clement
Houston, TX

> John Clement wrote:
> > ...I have a cart on a ramp which is given
> > an initial push and then is sometimes caught at the end of the ramp.  I
> have
> > included friction, but only kinetic friction....
> > Does static friction come into play during the instant when a cart
> reverses
> > direction after being kicked up the ramp?
> > John M. Clement
> > Houston, TX
> In a cursory effort to test this question, I waxed the surface of an
> anodized
> aluminum 60 cm vernier caliper, released the fixing screw on the movable
> jaw, and
> inclined the scale to find the angle at which the jaw slid, then
> decreased the angle
>  and pushed the jaw up the scale.
> I found the difference in coefficients was rather small, and the
> variability
>  comparatively large, so I was unable to provide a definitive answer for
> this setup.
> Sorry!